1.
Nesosilicate
–
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of rock-forming minerals and they are classified based on the structure of their silicate groups, which contain different ratios of silicon and oxygen. Nesosilicates, or orthosilicates, have the orthosilicate ion, which constitute isolated 4− tetrahedra that are connected only by interstitial cations and these exist as 3-member 6− and 6-member 12− rings, where T stands for a tetrahedrally coordinated cation. Inosilicates, or chain silicates, have interlocking chains of silicate tetrahedra with either SiO3,1,3 ratio, for single chains or Si4O11,4,11 ratio, for double chains. Nickel–Strunz classification,09. D Pyroxene group Enstatite – orthoferrosilite series Enstatite – MgSiO3 Ferrosilite – FeSiO3 Pigeonite – Ca0.251, all phyllosilicate minerals are hydrated, with either water or hydroxyl groups attached. Serpentine subgroup Antigorite – Mg3Si2O54 Chrysotile – Mg3Si2O54 Lizardite – Mg3Si2O54 Clay minerals group Halloysite – Al2Si2O54 Kaolinite – Al2Si2O54 Illite – 24O10 Montmorillonite –0 and this group comprises nearly 75% of the crust of the Earth. Tectosilicates, with the exception of the group, are aluminosilicates. Nickel–Strunz classification,09. F and 09. G,04. A, an introduction to the rock-forming minerals. Wise, W. S. Zussman, J. Rock-forming minerals, P.982 pp. Hurlbut, Cornelius S. Danas Manual of Mineralogy. Mindat. org, Dana classification Webmineral, Danas New Silicate Classification Media related to Silicates at Wikimedia Commons
2.
Chemical formula
–
These are limited to a single typographic line of symbols, which may include subscripts and superscripts. A chemical formula is not a name, and it contains no words. Although a chemical formula may imply certain simple chemical structures, it is not the same as a full chemical structural formula. Chemical formulas can fully specify the structure of only the simplest of molecules and chemical substances, the simplest types of chemical formulas are called empirical formulas, which use letters and numbers indicating the numerical proportions of atoms of each type. Molecular formulas indicate the numbers of each type of atom in a molecule. For example, the formula for glucose is CH2O, while its molecular formula is C6H12O6. This is possible if the relevant bonding is easy to show in one dimension, an example is the condensed molecular/chemical formula for ethanol, which is CH3-CH2-OH or CH3CH2OH. For reasons of structural complexity, there is no condensed chemical formula that specifies glucose, chemical formulas may be used in chemical equations to describe chemical reactions and other chemical transformations, such as the dissolving of ionic compounds into solution. A chemical formula identifies each constituent element by its chemical symbol, in empirical formulas, these proportions begin with a key element and then assign numbers of atoms of the other elements in the compound, as ratios to the key element. For molecular compounds, these numbers can all be expressed as whole numbers. For example, the formula of ethanol may be written C2H6O because the molecules of ethanol all contain two carbon atoms, six hydrogen atoms, and one oxygen atom. Some types of compounds, however, cannot be written with entirely whole-number empirical formulas. An example is boron carbide, whose formula of CBn is a variable non-whole number ratio with n ranging from over 4 to more than 6.5. When the chemical compound of the consists of simple molecules. These types of formulas are known as molecular formulas and condensed formulas. A molecular formula enumerates the number of atoms to reflect those in the molecule, so that the formula for glucose is C6H12O6 rather than the glucose empirical formula. However, except for very simple substances, molecular chemical formulas lack needed structural information, for simple molecules, a condensed formula is a type of chemical formula that may fully imply a correct structural formula. For example, ethanol may be represented by the chemical formula CH3CH2OH
3.
Crystal system
–
In crystallography, the terms crystal system, crystal family and lattice system each refer to one of several classes of space groups, lattices, point groups or crystals. Informally, two crystals are in the crystal system if they have similar symmetries, though there are many exceptions to this. Space groups and crystals are divided into seven crystal systems according to their point groups, five of the crystal systems are essentially the same as five of the lattice systems, but the hexagonal and trigonal crystal systems differ from the hexagonal and rhombohedral lattice systems. The six crystal families are formed by combining the hexagonal and trigonal crystal systems into one hexagonal family, a lattice system is a class of lattices with the same set of lattice point groups, which are subgroups of the arithmetic crystal classes. The 14 Bravais lattices are grouped into seven lattice systems, triclinic, monoclinic, orthorhombic, tetragonal, rhombohedral, hexagonal, in a crystal system, a set of point groups and their corresponding space groups are assigned to a lattice system. Of the 32 point groups that exist in three dimensions, most are assigned to only one system, in which case both the crystal and lattice systems have the same name. However, five point groups are assigned to two systems, rhombohedral and hexagonal, because both exhibit threefold rotational symmetry. These point groups are assigned to the crystal system. In total there are seven crystal systems, triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, a crystal family is determined by lattices and point groups. It is formed by combining crystal systems which have space groups assigned to a lattice system. In three dimensions, the families and systems are identical, except the hexagonal and trigonal crystal systems. In total there are six families, triclinic, monoclinic, orthorhombic, tetragonal, hexagonal. Spaces with less than three dimensions have the number of crystal systems, crystal families and lattice systems. In one-dimensional space, there is one crystal system, in 2D space, there are four crystal systems, oblique, rectangular, square and hexagonal. The relation between three-dimensional crystal families, crystal systems and lattice systems is shown in the table, Note. To avoid confusion of terminology, the term trigonal lattice is not used, if the original structure and inverted structure are identical, then the structure is centrosymmetric. Still, even for non-centrosymmetric case, inverted structure in some cases can be rotated to align with the original structure and this is the case of non-centrosymmetric achiral structure. If the inverted structure cannot be rotated to align with the structure, then the structure is chiral
4.
Orthorhombic crystal system
–
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. All three bases intersect at 90° angles, so the three lattice vectors remain mutually orthogonal, there are two orthorhombic Bravais lattices in two dimensions, Primitive rectangular and centered rectangular. The primitive rectangular lattice can also be described by a centered rhombic unit cell, there are four orthorhombic Bravais lattices, primitive orthorhombic, base-centered orthorhombic, body-centered orthorhombic, and face-centered orthorhombic. In this axis setting, the primitive and base-centered lattices interchange in centering type, crystal structure Overview of all space groups Hurlbut, Cornelius S. Klein, Cornelis. Hahn, Theo, ed. International Tables for Crystallography, Volume A, Space Group Symmetry
5.
Crystallographic point group
–
For a periodic crystal, the group must also be consistent with maintenance of the three-dimensional translational symmetry that defines crystallinity. The macroscopic properties of a crystal would look exactly the same before, in the classification of crystals, each point group is also known as a crystal class. There are infinitely many three-dimensional point groups, however, the crystallographic restriction of the infinite families of general point groups results in there being only 32 crystallographic point groups. These 32 point groups are one-and-the same as the 32 types of morphological crystalline symmetries derived in 1830 by Johann Friedrich Christian Hessel from a consideration of observed crystal forms, the point groups are denoted by their component symmetries. There are a few standard notations used by crystallographers, mineralogists, for the correspondence of the two systems below, see crystal system. In Schoenflies notation, point groups are denoted by a symbol with a subscript. The symbols used in crystallography mean the following, Cn indicates that the group has a rotation axis. Cnh is Cn with the addition of a plane perpendicular to the axis of rotation. Cnv is Cn with the addition of n mirror planes parallel to the axis of rotation, s2n denotes a group that contains only a 2n-fold rotation-reflection axis. Dn indicates that the group has a rotation axis plus n twofold axes perpendicular to that axis. Dnh has, in addition, a plane perpendicular to the n-fold axis. Dnd has, in addition to the elements of Dn, mirror planes parallel to the n-fold axis, the letter T indicates that the group has the symmetry of a tetrahedron. Td includes improper rotation operations, T excludes improper rotation operations, the letter O indicates that the group has the symmetry of an octahedron, with or without improper operations. Due to the crystallographic restriction theorem, n =1,2,3,4, d4d and D6d are actually forbidden because they contain improper rotations with n=8 and 12 respectively. The 27 point groups in the table plus T, Td, Th, O, an abbreviated form of the Hermann–Mauguin notation commonly used for space groups also serves to describe crystallographic point groups. Group names are Molecular symmetry Point group Space group Point groups in three dimensions Crystal system Point-group symbols in International Tables for Crystallography,12.1, pp. 818-820 Names and symbols of the 32 crystal classes in International Tables for Crystallography. 10.1, p.794 Pictorial overview of the 32 groups Point Groups - Flow Chart Inorganic Chemistry Group Theory Practice Problems
6.
H-M symbol
–
In geometry, Hermann–Mauguin notation is used to represent the symmetry elements in point groups, plane groups and space groups. It is named after the German crystallographer Carl Hermann and the French mineralogist Charles-Victor Mauguin and this notation is sometimes called international notation, because it was adopted as standard by the International Tables For Crystallography since their first edition in 1935. Rotation axes are denoted by a number n —1,2,3,4,5,6,7,8, for improper rotations, Hermann–Mauguin symbols show rotoinversion axes, unlike Schoenflies and Shubnikov notations, where the preference is given to rotation-reflection axes. The rotoinversion axes are represented by the number with a macron. The symbol for a plane is m. The direction of the plane is defined as the direction of perpendicular to the face. Hermann–Mauguin symbols show symmetrically non-equivalent axes and planes, the direction of a symmetry element is represented by its position in the Hermann–Mauguin symbol. If a rotation axis n and a mirror plane m have the same direction, if two or more axes have the same direction, the axis with higher symmetry is shown. Higher symmetry means that the axis generates a pattern with more points, for example, rotation axes 3,4,5,6,7,8 generate 3-, 4-, 5-, 6-, 7-, 8-point patterns, respectively. Improper rotation axes 3,4,5,6,7,8 generate 6-, 4-, 10-, 6-, 14-, 8-point patterns, if both, the rotation and rotoinversion axes satisfy the previous rule, the rotation axis should be chosen. For example, 3/m combination is equivalent to 6, since 6 generates 6 points, and 3 generates only 3,6 should be written instead of 3/m. Analogously, in the case when both 3 and 3 axes are present,3 should be written, however we write 4/m, not 4/m, because both 4 and 4 generate four points. Finally, the Hermann–Mauguin symbol depends on the type of the group and these groups may contain only two-fold axes, mirror planes, and inversion center. These are the point groups 1 and 1,2, m, and 2/m, and 222, 2/m2/m2/m. If the symbol contains three positions, then they denote symmetry elements in the x, y, z direction, First position — primary direction — z direction, assigned to the higher-order axis. Second position — symmetrically equivalent secondary directions, which are perpendicular to the z-axis and these can be 2, m, or 2/m. Third position — symmetrically equivalent tertiary directions, passing between secondary directions and these can be 2, m, or 2/m. These are the crystallographic groups 3,32, 3m,3, and 32/m,4,422, 4mm,4, 42m, 4/m, and 4/m2/m2/m, and 6,622, 6mm,6, 6m2, 6/m, and 6/m2/m2/m
7.
Space group
–
In mathematics, physics and chemistry, a space group is the symmetry group of a configuration in space, usually in three dimensions. In three dimensions, there are 219 distinct types, or 230 if chiral copies are considered distinct, Space groups are also studied in dimensions other than 3 where they are sometimes called Bieberbach groups, and are discrete cocompact groups of isometries of an oriented Euclidean space. In crystallography, space groups are called the crystallographic or Fedorov groups. A definitive source regarding 3-dimensional space groups is the International Tables for Crystallography, in 1879 Leonhard Sohncke listed the 65 space groups whose elements preserve the orientation. More accurately, he listed 66 groups, but Fedorov and Schönflies both noticed that two of them were really the same, the space groups in 3 dimensions were first enumerated by Fedorov, and shortly afterwards were independently enumerated by Schönflies. The correct list of 230 space groups was found by 1892 during correspondence between Fedorov and Schönflies, burckhardt describes the history of the discovery of the space groups in detail. The space groups in three dimensions are made from combinations of the 32 crystallographic point groups with the 14 Bravais lattices, the combination of all these symmetry operations results in a total of 230 different space groups describing all possible crystal symmetries. The elements of the space group fixing a point of space are rotations, reflections, the identity element, the translations form a normal abelian subgroup of rank 3, called the Bravais lattice. There are 14 possible types of Bravais lattice, the quotient of the space group by the Bravais lattice is a finite group which is one of the 32 possible point groups. Translation is defined as the moves from one point to another point. A glide plane is a reflection in a plane, followed by a parallel with that plane. This is noted by a, b or c, depending on which axis the glide is along. There is also the n glide, which is a glide along the half of a diagonal of a face, and the d glide, the latter is called the diamond glide plane as it features in the diamond structure. In 17 space groups, due to the centering of the cell, the glides occur in two directions simultaneously, i. e. the same glide plane can be called b or c, a or b. For example, group Abm2 could be also called Acm2, group Ccca could be called Cccb, in 1992, it was suggested to use symbol e for such planes. The symbols for five groups have been modified, A screw axis is a rotation about an axis. These are noted by a number, n, to describe the degree of rotation, the degree of translation is then added as a subscript showing how far along the axis the translation is, as a portion of the parallel lattice vector. So,21 is a rotation followed by a translation of 1/2 of the lattice vector
8.
Crystal structure
–
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter. The smallest group of particles in the material that constitutes the pattern is the unit cell of the structure. The unit cell completely defines the symmetry and structure of the crystal lattice. The repeating patterns are said to be located at the points of the Bravais lattice, the lengths of the principal axes, or edges, of the unit cell and the angles between them are the lattice constants, also called lattice parameters. The symmetry properties of the crystal are described by the concept of space groups, all possible symmetric arrangements of particles in three-dimensional space may be described by the 230 space groups. The crystal structure and symmetry play a role in determining many physical properties, such as cleavage, electronic band structure. The crystal structure of a material can be described in terms of its unit cell, the unit cell is a box containing one or more atoms arranged in three dimensions. The unit cells stacked in three-dimensional space describe the arrangement of atoms of the crystal. Commonly, atomic positions are represented in terms of fractional coordinates, the atom positions within the unit cell can be calculated through application of symmetry operations to the asymmetric unit. The asymmetric unit refers to the smallest possible occupation of space within the unit cell and this does not, however imply that the entirety of the asymmetric unit must lie within the boundaries of the unit cell. Symmetric transformations of atom positions are calculated from the group of the crystal structure. Vectors and planes in a lattice are described by the three-value Miller index notation. It uses the indices ℓ, m, and n as directional parameters, which are separated by 90°, by definition, the syntax denotes a plane that intercepts the three points a1/ℓ, a2/m, and a3/n, or some multiple thereof. That is, the Miller indices are proportional to the inverses of the intercepts of the plane with the unit cell, if one or more of the indices is zero, it means that the planes do not intersect that axis. A plane containing a coordinate axis is translated so that it no longer contains that axis before its Miller indices are determined, the Miller indices for a plane are integers with no common factors. Negative indices are indicated with horizontal bars, as in, in an orthogonal coordinate system for a cubic cell, the Miller indices of a plane are the Cartesian components of a vector normal to the plane. Likewise, the planes are geometric planes linking nodes
9.
Crystal habit
–
In mineralogy, crystal habit is the characteristic external shape of an individual crystal or crystal group. A single crystals habit is a description of its shape and its crystallographic forms. Recognizing the habit may help in identifying a mineral, when the faces are well-developed due to uncrowded growth a crystal is called euhedral, one with partially developed faces is subhedral, and one with undeveloped crystal faces is called anhedral. The long axis of a quartz crystal typically has a six-sided prismatic habit with parallel opposite faces. Aggregates can be formed of individual crystals with euhedral to anhedral grains, the arrangement of crystals within the aggregate can be characteristic of certain minerals. For example, minerals used for asbestos insulation often grow in a fibrous habit, the terms used by mineralogists to report crystal habits describe the typical appearance of an ideal mineral. Recognizing the habit can aid in identification as some habits are characteristic, most minerals, however, do not display ideal habits due to conditions during crystallization. Minerals belonging to the crystal system do not necessarily exhibit the same habit. Some habits of a mineral are unique to its variety and locality, For example, while most sapphires form elongate barrel-shaped crystals, ordinarily, the latter habit is seen only in ruby. Sapphire and ruby are both varieties of the mineral, corundum. Some minerals may replace other existing minerals while preserving the originals habit, a classic example is tigers eye quartz, crocidolite asbestos replaced by silica. While quartz typically forms prismatic crystals, in tigers eye the original fibrous habit of crocidolite is preserved, the names of crystal habits are derived from, Predominant crystal faces. Abnormal grain growth Grain growth Crystallization
10.
Cleavage (crystal)
–
Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. Cleavage forms parallel to planes, Basal or pinacoidal cleavage occurs when there is only one cleavage plane. Mica also has basal cleavage, this is why mica can be peeled into thin sheets, cubic cleavage occurs on when there are three cleavage planes intersecting at 90 degrees. Halite has cubic cleavage, and therefore, when halite crystals are broken, octahedral cleavage occurs when there are four cleavage planes in a crystal. Octahedral cleavage is common for semiconductors, rhombohedral cleavage occurs when there are three cleavage planes intersecting at angles that are not 90 degrees. Prismatic cleavage occurs when there are two planes in a crystal. Dodecahedral cleavage occurs when there are six cleavage planes in a crystal, crystal parting occurs when minerals break along planes of structural weakness due to external stress or along twin composition planes. Parting breaks are very similar in appearance to cleavage, but only due to stress. Examples include magnetite which shows octahedral parting, the parting of corundum. Cleavage is a property traditionally used in mineral identification, both in hand specimen and microscopic examination of rock and mineral studies. As an example, the angles between the cleavage planes for the pyroxenes and the amphiboles are diagnostic. Crystal cleavage is of importance in the electronics industry and in the cutting of gemstones. Precious stones are generally cleaved by impact, as in diamond cutting, synthetic single crystals of semiconductor materials are generally sold as thin wafers which are much easier to cleave. Elemental semiconductors are diamond cubic, a group for which octahedral cleavage is observed. This means that some orientations of wafer allow near-perfect rectangles to be cleaved, most other commercial semiconductors can be made in the related zinc blende structure, with similar cleavage planes. Cleavage Mineral galleries, Mineral properties – Cleavage
11.
Fracture (mineralogy)
–
In the field of mineralogy, fracture is the texture and shape of a rocks surface formed when a mineral is fractured. Minerals often have a highly distinctive fracture, making it a feature used in their identification. Fracture differs from cleavage in that the latter involves clean splitting along the planes of the minerals crystal structure. All minerals exhibit fracture, but when very strong cleavage is present, conchoidal fracture breakage that resembles the concentric ripples of a mussel shell. It often occurs in amorphous or fine-grained minerals such as flint, opal or obsidian, subconchoidal fracture is similar to conchoidal fracture, but with less significant curvature. Earthy fracture is reminiscent of freshly broken soil and it is frequently seen in relatively soft, loosely bound minerals, such as limonite, kaolinite and aluminite. Hackly fracture is jagged, sharp and not even and it occurs when metals are torn, and so is often encountered in native metals such as copper and silver. Splintery fracture comprises sharp elongated points and it is particularly seen in fibrous minerals such as chrysotile, but may also occur in non-fibrous minerals such as kyanite. Uneven fracture is a surface or one with random irregularities. It occurs in a range of minerals including arsenopyrite, pyrite and magnetite. Rudolf Duda and Lubos Rejl, Minerals of the World http, //www. galleries. com/minerals/property/fracture. htm
12.
Mohs scale of mineral hardness
–
The Mohs scale of mineral hardness is a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. Created in 1812 by German geologist and mineralogist Friedrich Mohs, it is one of several definitions of hardness in materials science, while greatly facilitating the identification of minerals in the field, the Mohs scale does not show how well hard materials perform in an industrial setting. Despite its lack of precision, the Mohs scale is highly relevant for field geologists, the Mohs scale hardness of minerals can be commonly found in reference sheets. Reference materials may be expected to have a uniform Mohs hardness, the Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly. The samples of matter used by Mohs are all different minerals, Minerals are pure substances found in nature. Rocks are made up of one or more minerals, as the hardest known naturally occurring substance when the scale was designed, diamonds are at the top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the material can scratch. For example, if material is scratched by apatite but not by fluorite. Scratching a material for the purposes of the Mohs scale means creating non-elastic dislocations visible to the naked eye, frequently, materials that are lower on the Mohs scale can create microscopic, non-elastic dislocations on materials that have a higher Mohs number. The Mohs scale is an ordinal scale. For example, corundum is twice as hard as topaz, the table below shows the comparison with the absolute hardness measured by a sclerometer, with pictorial examples. On the Mohs scale, a streak plate has a hardness of 7.0, using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale. The table below incorporates additional substances that may fall between levels, Comparison between Hardness and Hardness, Mohs hardness of elements is taken from G. V, samsonov in Handbook of the physicochemical properties of the elements, IFI-Plenum, New York, USA,1968. The Hardness of Minerals and Rocks
13.
Lustre (mineralogy)
–
Lustre or luster is the way light interacts with the surface of a crystal, rock, or mineral. The word traces its origins back to the latin lux, meaning light, a range of terms are used to describe lustre, such as earthy, metallic, greasy, and silky. Similarly, the term refers to a glassy lustre. A list of terms is given below. Lustre varies over a continuum, and so there are no rigid boundaries between the different types of lustre. The terms are frequently combined to describe types of lustre. Some minerals exhibit unusual optical phenomena, such as asterism or chatoyancy, a list of such phenomena is given below. Adamantine minerals possess a superlative lustre, which is most notably seen in diamond, such minerals are transparent or translucent, and have a high refractive index. Minerals with an adamantine lustre are uncommon, with examples being cerussite. Minerals with a degree of lustre are referred to as subadamantine, with some examples being garnet. Dull minerals exhibit little to no lustre, due to coarse granulations which scatter light in all directions, a distinction is sometimes drawn between dull minerals and earthy minerals, with the latter being coarser, and having even less lustre. Greasy minerals resemble fat or grease, a greasy lustre often occurs in minerals containing a great abundance of microscopic inclusions, with examples including opal and cordierite. Many minerals with a greasy lustre also feel greasy to the touch, metallic minerals have the lustre of polished metal, and with ideal surfaces will work as a reflective surface. Examples include galena, pyrite and magnetite, pearly minerals consist of thin transparent co-planar sheets. Light reflecting from these layers give them a lustre reminiscent of pearls, such minerals possess perfect cleavage, with examples including muscovite and stilbite. Resinous minerals have the appearance of resin, chewing gum or plastic, a principal example is amber, which is a form of fossilized resin. Silky minerals have an arrangement of extremely fine fibres, giving them a lustre reminiscent of silk. Examples include asbestos, ulexite and the satin spar variety of gypsum, a fibrous lustre is similar, but has a coarser texture
14.
Streak (mineralogy)
–
The streak of a mineral is the color of the powder produced when it is dragged across an un-weathered surface. If no streak seems to be made, the streak is said to be white or colorless. Streak is particularly important as a diagnostic for opaque and colored materials and it is less useful for silicate minerals, most of which have a white streak or are too hard to powder easily. The apparent color of a mineral can vary widely because of impurities or a disturbed macroscopic crystal structure. Small amounts of an impurity that strongly absorbs a particular wavelength can radically change the wavelengths of light that are reflected by the specimen, and thus change the apparent color. However, when the specimen is dragged to produce a streak, it is broken into randomly oriented microscopic crystals, the surface across which the mineral is dragged is called a streak plate, and is generally made of unglazed porcelain tile. In the absence of a plate, the unglazed underside of a porcelain bowl or vase or the back of a glazed tile will work. Sometimes a streak is more easily or accurately described by comparing it with the streak made by another streak plate. Because the trail left behind results from the mineral being crushed into powder, in case of harder minerals, the color of the powder can be determined by filing or crushing with a hammer a small sample, which is then usually rubbed on a streak plate. Most minerals that are harder have a white streak. Some minerals leave a similar to their natural color, such as cinnabar. Other minerals leave surprising colors, such as fluorite, which always has a streak, although it can appear in purple, blue, yellow. Hematite, which is black in appearance, leaves a red streak which accounts for its name, galena, which can be similar in appearance to hematite, is easily distinguished by its gray streak. Hamilton, W. R. Cambridge Guide to Minerals, Rocks, the Encyclopedia of Gemstones and Minerals. New York, Facts on File. p.251, physical Characteristics of Minerals, at Introduction to Mineralogy by Andrea Bangert What is Streak
15.
Transparency and translucency
–
In the field of optics, transparency is the physical property of allowing light to pass through the material without being scattered. On a macroscopic scale, the photons can be said to follow Snells Law, in other words, a translucent medium allows the transport of light while a transparent medium not only allows the transport of light but allows for image formation. The opposite property of translucency is opacity, transparent materials appear clear, with the overall appearance of one color, or any combination leading up to a brilliant spectrum of every color. When light encounters a material, it can interact with it in different ways. These interactions depend on the wavelength of the light and the nature of the material, photons interact with an object by some combination of reflection, absorption and transmission. Some materials, such as glass and clean water, transmit much of the light that falls on them and reflect little of it. Many liquids and aqueous solutions are highly transparent, absence of structural defects and molecular structure of most liquids are mostly responsible for excellent optical transmission. Materials which do not transmit light are called opaque, many such substances have a chemical composition which includes what are referred to as absorption centers. Many substances are selective in their absorption of light frequencies. They absorb certain portions of the spectrum while reflecting others. The frequencies of the spectrum which are not absorbed are either reflected or transmitted for our physical observation and this is what gives rise to color. The attenuation of light of all frequencies and wavelengths is due to the mechanisms of absorption. Transparency can provide almost perfect camouflage for animals able to achieve it and this is easier in dimly-lit or turbid seawater than in good illumination. Many marine animals such as jellyfish are highly transparent, at the atomic or molecular level, physical absorption in the infrared portion of the spectrum depends on the frequencies of atomic or molecular vibrations or chemical bonds, and on selection rules. Nitrogen and oxygen are not greenhouse gases because there is no absorption because there is no molecular dipole moment. With regard to the scattering of light, the most critical factor is the scale of any or all of these structural features relative to the wavelength of the light being scattered. Primary material considerations include, Crystalline structure, whether or not the atoms or molecules exhibit the long-range order evidenced in crystalline solids, glassy structure, scattering centers include fluctuations in density or composition. Microstructure, scattering centers include internal surfaces such as boundaries, crystallographic defects
16.
Specific gravity
–
This page is about the measurement using water as a reference. For a general use of gravity, see relative density. See intensive property for the property implied by specific, Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is always water at its densest for liquids. Nonetheless, the temperature and pressure must be specified for both the sample and the reference, pressure is nearly always 1 atm. Temperatures for both sample and reference vary from industry to industry, in British beer brewing, the practice for specific gravity as specified above is to multiply it by 1000. Being a ratio of densities, specific gravity is a dimensionless quantity, Specific gravity varies with temperature and pressure, reference and sample must be compared at the same temperature and pressure or be corrected to a standard reference temperature and pressure. Substances with a gravity of 1 are neutrally buoyant in water. Those with SG greater than 1 are denser than water and will, disregarding surface tension effects and those with an SG less than 1 are less dense than water and will float on it. In scientific work, the relationship of mass to volume is expressed directly in terms of the density of the substance under study. It is in industry where specific gravity finds wide application, often for historical reasons. True specific gravity can be expressed mathematically as, S G true = ρ sample ρ H2 O where ρ sample is the density of the sample, the density of water varies with temperature and pressure as does the density of the sample. So it is necessary to specify the temperatures and pressures at which the densities or weights were determined and it is nearly always the case that measurements are made at 1 nominal atmosphere. For true specific gravity calculations, air pressure must be considered, temperatures are specified by the notation with T s representing the temperature at which the samples density was determined and T r the temperature at which the reference density is specified. For example, SG would be understood to mean that the density of the sample was determined at 20°C and of the water at 4°C. Taking into account different sample and reference temperatures, we note that, while S G H2 O =1.000000, it is also the case that S G H2 O =0.998203 /0.999840 =0.998363. Here, temperature is being specified using the current ITS-90 scale, on the previous IPTS-68 scale, the densities at 20 °C and 4 °C are 0.9982071 and 0.9999720 respectively, resulting in an SG value for water of 0.9982343. For example, in the industry, the Plato table lists sucrose concentration by weight against true SG
17.
Refractive index
–
In optics, the refractive index or index of refraction n of a material is a dimensionless number that describes how light propagates through that medium. It is defined as n = c v, where c is the speed of light in vacuum, for example, the refractive index of water is 1.333, meaning that light travels 1.333 times faster in a vacuum than it does in water. The refractive index determines how light is bent, or refracted. The refractive indices also determine the amount of light that is reflected when reaching the interface, as well as the angle for total internal reflection. This implies that vacuum has a index of 1. The refractive index varies with the wavelength of light and this is called dispersion and causes the splitting of white light into its constituent colors in prisms and rainbows, and chromatic aberration in lenses. Light propagation in absorbing materials can be described using a refractive index. The imaginary part then handles the attenuation, while the real part accounts for refraction, the concept of refractive index is widely used within the full electromagnetic spectrum, from X-rays to radio waves. It can also be used with wave phenomena such as sound, in this case the speed of sound is used instead of that of light and a reference medium other than vacuum must be chosen. Thomas Young was presumably the person who first used, and invented, at the same time he changed this value of refractive power into a single number, instead of the traditional ratio of two numbers. The ratio had the disadvantage of different appearances, newton, who called it the proportion of the sines of incidence and refraction, wrote it as a ratio of two numbers, like 529 to 396. Hauksbee, who called it the ratio of refraction, wrote it as a ratio with a fixed numerator, hutton wrote it as a ratio with a fixed denominator, like 1.3358 to 1. Young did not use a symbol for the index of refraction, in the next years, others started using different symbols, n, m, and µ. For visible light most transparent media have refractive indices between 1 and 2, a few examples are given in the adjacent table. These values are measured at the yellow doublet D-line of sodium, with a wavelength of 589 nanometers, gases at atmospheric pressure have refractive indices close to 1 because of their low density. Almost all solids and liquids have refractive indices above 1.3, aerogel is a very low density solid that can be produced with refractive index in the range from 1.002 to 1.265. Moissanite lies at the end of the range with a refractive index as high as 2.65. Most plastics have refractive indices in the range from 1.3 to 1.7, for infrared light refractive indices can be considerably higher
18.
Birefringence
–
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent, the birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress and this effect was first described by the Danish scientist Rasmus Bartholin in 1669, who observed it in calcite, a crystal having one of the strongest birefringences. A mathematical description of wave propagation in a birefringent medium is presented below, following is a qualitative explanation of the phenomenon. Thus rotating the material around this axis does not change its optical behavior and this special direction is known as the optic axis of the material. Light whose polarization is perpendicular to the axis is governed by a refractive index no. Light whose polarization is in the direction of the optic axis sees an optical index ne, for any ray direction there is a linear polarization direction perpendicular to the optic axis, and this is called an ordinary ray. The magnitude of the difference is quantified by the birefringence, Δ n = n e − n o, the propagation of the ordinary ray is simply described by no as if there were no birefringence involved. However the extraordinary ray, as its name suggests, propagates unlike any wave in an optical material. Its refraction at a surface can be using the effective refractive index. However it is in fact an inhomogeneous wave whose power flow is not exactly in the direction of the wave vector and this causes an additional shift in that beam, even when launched at normal incidence, as is popularly observed using a crystal of calcite as photographed above. Rotating the calcite crystal will cause one of the two images, that of the ray, to rotate slightly around that of the ordinary ray. When the light propagates either along or orthogonal to the optic axis, in the first case, both polarizations see the same effective refractive index, so there is no extraordinary ray. In the second case the extraordinary ray propagates at a different phase velocity but is not an inhomogeneous wave, for instance, a quarter-wave plate is commonly used to create circular polarization from a linearly polarized source. The case of so-called biaxial crystals is substantially more complex and these are characterized by three refractive indices corresponding to three principal axes of the crystal. For most ray directions, both polarizations would be classified as extraordinary rays but with different effective refractive indices, being extraordinary waves, however, the direction of power flow is not identical to the direction of the wave vector in either case. The two refractive indices can be determined using the index ellipsoids for given directions of the polarization, note that for biaxial crystals the index ellipsoid will not be an ellipsoid of revolution but is described by three unequal principle refractive indices nα, nβ and nγ. Thus there is no axis around which a rotation leaves the optical properties invariant, for this reason, birefringent materials with three distinct refractive indices are called biaxial
19.
Pleochroism
–
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles, especially with polarized light. Anisotropic crystals will have properties that vary with the direction of light. The polarization of light determines the direction of the electric field and these kinds of crystals have one or two optical axes. If absorption of light varies with the relative to the optical axis in a crystal then pleochroism results. Anisotropic crystals have double refraction of light where light of different polarizations is bent different amounts by the crystal, the components of a divided light beam follow different paths within the mineral and travel at different speeds. When the mineral is observed at some angle, light following some combination of paths and polarizations will be present, at another angle, the light passing through the crystal will be composed of another combination of light paths and polarizations, each with their own color. The light passing through the mineral will therefore have different colors when it is viewed from different angles, tetragonal, trigonal and hexagonal minerals can only show two colors and are called dichroic. Orthorhombic, monoclinic and triclinic crystals can show three and are trichroic, for example, hypersthene, with two optical axes, can have red, yellow or blue appearance when oriented in three different ways in three-dimensional space. Tourmaline is notable for exhibiting strong pleochroism, gems are sometimes cut and set either to display pleochroism or to hide it, depending on the colors and their attractiveness. The pleochroic colours are at their maximum when light is polarized parallel with a crystallographic axis, the axes are designated X, Y and Z. These axes can be determined from the appearance of a crystal in an interference pattern. Where there are two axes, the acute bisection of the axes gives Z for positive minerals and X for negative minerals. Perpendicular to these is the Y axis, the colour is measured with the polarization parallel to each direction. An absorption formula records the amount of parallel to each axis in the form of X < Y < Z with the left most having the least absorption. Minerals that are very similar often have very different pleochroic color schemes. In such cases, a section of the mineral is used and examined under polarized transmitted light with a petrographic microscope. Another device using this property to identify minerals is the dichroscope
20.
Kyanite
–
Kyanite is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than four kilobars, kyanite is also known as disthene, rhaeticite and cyanite. Kyanite is a member of the series, which also includes the polymorph andalusite. Kyanite is strongly anisotropic, in that its hardness varies depending on its crystallographic direction, in kyanite, this anisotropism can be considered an identifying characteristic. At temperatures above 1100 °C kyanite decomposes into mullite and vitreous silica via the reaction,3 → 3Al2O3·2SiO2 + SiO2. This transformation results in an expansion and its name comes from the same origin as that of the color cyan, being derived from the Ancient Greek word κύανος. This is generally rendered into English as kyanos or kuanos and means dark blue, kyanite is used primarily in refractory and ceramic products, including porcelain plumbing fixtures and dishware. It is also used in electronics, electrical insulators and abrasives, kyanite has been used as a semiprecious gemstone, which may display cats eye chatoyancy, though this use is limited by its anisotropism and perfect cleavage. Color varieties include recently discovered orange kyanite from Tanzania, the orange color is due to inclusion of small amounts of manganese in the structure. Kyanite is one of the minerals that are used to estimate the temperature, depth. Kyanites elongated, columnar crystals are usually a good first indication of the mineral, associated minerals are useful as well, especially the presence of the polymorphs of staurolite, which occur frequently with kyanite. However, the most useful characteristic in identifying kyanite is its anisotropism, kyanite occurs in gneiss, schist, pegmatite, and quartz veins resulting from high pressure regional metamorphism of principally pelitic rocks. It occurs as detrital grains in sedimentary rocks and it occurs associated with staurolite, andalusite, sillimanite, talc, hornblende, gedrite, mullite and corundum. Kyanite occurs in Manhattan schist, formed under extreme pressure as a result of the two landmasses that formed supercontinent Pangaea
21.
Andalusite
–
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. Andalusite is trimorphic with kyanite and sillimanite, being the lower pressure mid temperature polymorph, at higher temperatures and pressures, andalusite may convert to sillimanite. Thus, as with its other polymorphs, andalusite is an index mineral, providing clues to depth. The variety chiastolite commonly contains dark inclusions of carbon or clay which form a pattern when shown in cross-section. A clear variety first found in Andalusia, Spain can be cut into a gemstone, faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of iridescence, although the colors are actually the result of unusually strong pleochroism. It is associated with mica schist which increases alkali content in ultimate product, andalusite is an aluminium compound with high heat resistance used in furnaces, kilns and other industrial processes. South Africa possesses by far the largest portion of the world’s known andalusite deposits, andalusite is a common metamorphic mineral which forms under low pressure and low to high temperatures. The minerals kyanite and sillimanite are polymorphs of andalusite, each occurring under different temperature-pressure regimes and are rarely found together in the same rock. Because of this the three minerals are a tool to help identify the pressure-temperature paths of the host rock in which they are found. It was first described and named after the locality in the Ronda Massif, Málaga, Andalusia. List of minerals http, //www. gemstone. org/gem-by-gem/english/andalusite. html http, //s804. photobucket. com/user/mutantnoggins/media/AndalusiteAND10x5mag_zpsefd120ed. jpg. html Source, Andalusia, Spain
22.
Aluminosilicate
–
Aluminosilicate minerals are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a component of kaolin and other clay minerals. Andalusite, kyanite, and sillimanite are naturally occurring minerals that have the composition Al2SiO5. The triple point of the three polymorphs is located at a temperature of 500 °C and a pressure of 0.4 GPa and these three minerals are commonly used as index minerals in metamorphic rocks. Hydrated aluminosilicate minerals are referred to as zeolites and are porous structures that are naturally occurring materials, the catalyst silica-alumina is an amorphous substance which is not an aluminosilicate compound. There exist a variety of glass types. The characteristics of different types depend on the chemical composition of the glass with a special focus on the oxide composition. Glasses can be categorized into different groups, one of them including the so-called aluminosilicate glasses. This glass type can be differentiated, Characteristically, these glasses are free of alkali oxides and contain 15-25% AI2O3, 52-60% SiO2. Very high transformation temperatures and softening points are typical features, main fields of application are glass bulbs for halogen lamps, high-temperature thermometers and thermally and electrically highly loadable film resistors. The AI2O3 content of alkali aluminosilicate glasses is typically 10-25% and the content over 10%. The high alkali content prepares the glass for ion exchange with bigger alkali ions in order to improve the surface compressive strength, due to this particular feature, this glass type is especially suitable for the use in touch displays, solar cells cover glass and laminated safety glass. High transformation temperatures and outstanding properties, e. g. hardness. Aluminium silicate Silicate minerals Calcium aluminosilicate Sodium aluminosilicate
23.
Mineral
–
A mineral is a naturally occurring chemical compound, usually of crystalline form and abiogenic in origin. A mineral has one specific chemical composition, whereas a rock can be an aggregate of different minerals or mineraloids, the study of minerals is called mineralogy. There are over 5,300 known mineral species, over 5,070 of these have been approved by the International Mineralogical Association, the silicate minerals compose over 90% of the Earths crust. The diversity and abundance of species is controlled by the Earths chemistry. Silicon and oxygen constitute approximately 75% of the Earths crust, which translates directly into the predominance of silicate minerals, minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various species, changes in the temperature, pressure, or bulk composition of a rock mass cause changes in its minerals. Minerals can be described by their various properties, which are related to their chemical structure. Common distinguishing characteristics include crystal structure and habit, hardness, lustre, diaphaneity, colour, streak, tenacity, cleavage, fracture, parting, more specific tests for describing minerals include magnetism, taste or smell, radioactivity and reaction to acid. Minerals are classified by key chemical constituents, the two dominant systems are the Dana classification and the Strunz classification, the silicate class of minerals is subdivided into six subclasses by the degree of polymerization in the chemical structure. All silicate minerals have a unit of a 4− silica tetrahedron—that is, a silicon cation coordinated by four oxygen anions. These tetrahedra can be polymerized to give the subclasses, orthosilicates, disilicates, cyclosilicates, inosilicates, phyllosilicates, other important mineral groups include the native elements, sulfides, oxides, halides, carbonates, sulfates, and phosphates. The first criterion means that a mineral has to form by a natural process, stability at room temperature, in the simplest sense, is synonymous to the mineral being solid. More specifically, a compound has to be stable or metastable at 25 °C, modern advances have included extensive study of liquid crystals, which also extensively involve mineralogy. Minerals are chemical compounds, and as such they can be described by fixed or a variable formula, many mineral groups and species are composed of a solid solution, pure substances are not usually found because of contamination or chemical substitution. Finally, the requirement of an ordered atomic arrangement is usually synonymous with crystallinity, however, crystals are also periodic, an ordered atomic arrangement gives rise to a variety of macroscopic physical properties, such as crystal form, hardness, and cleavage. There have been recent proposals to amend the definition to consider biogenic or amorphous substances as minerals. The formal definition of an approved by the IMA in 1995, A mineral is an element or chemical compound that is normally crystalline. However, if geological processes were involved in the genesis of the compound, Mineral classification schemes and their definitions are evolving to match recent advances in mineral science
24.
Benjamin Silliman
–
Benjamin Silliman was an early American chemist and science educator. The British forces had taken his father, General Gold Selleck Silliman, Silliman was educated at Yale, receiving a B. A. degree in 1796 and a M. A. in 1799. He studied law with Simeon Baldwin from 1798 to 1799 and became a tutor at Yale from 1799 to 1802 and he was admitted to the bar in 1802. President Timothy Dwight IV of Yale proposed that he himself to teach in chemistry and natural history. Silliman studied chemistry with Professor James Woodhouse at the University of Pennsylvania in Philadelphia and these lectures were the first science lectures ever given at Yale. In 1805, he traveled to Edinburgh for further study, returning to New Haven, he studied its geology, and made a chemical analysis of the meteorite that fell near Weston, Connecticut, publishing the first scientific account of any American meteorite. He lectured publicly at New Haven in 1808 and came to many of the constituent elements of many minerals. In 1837, the first prismatic barite ore of tungsten in the United States was discovered at the mine, the mineral sillimanite was named after Silliman in 1850. Upon the founding of the Medical School, he taught there as one of the founding faculty members. Silliman was a supporter of coeducation in the Ivy League. Although Yale wouldnt admit women as students until over 100 years later and his efforts convinced Frederick Barnard, later President of Columbia College, that women ought to be admitted as students. In that institution the undersigned had an opportunity to observe, as a student, mcCullough also relates that Silliman would later become president of the college. Like his son-in-law James Dana, Silliman was a Christian, in the same line of thought, he posed arguments against atheism and materialism. At 6,30 in the morning of December 14,1807, three loud explosions were heard over the town of Weston in Fairfield County, Connecticut. Stone fragments fell in at least 6 places, the largest and only unbroken stone of the Weston fall, which weighed 36.5 pounds, was found some days after Silliman and Kingsley had spent several fruitless hours hunting for it. The owner, a Trumbull farmer named Elijah Seeley, was urged to present it to Yale by local people who had met the professors during their investigation, but he insisted on putting it up for sale. His son Benjamin Silliman Jr. also a professor of chemistry at Yale and his second marriage was in 1851 to Mrs. Sarah Isabella Webb, daughter of John McClellan. Silliman died at New Haven and is buried in Grove Street Cemetery, Silliman deemed slavery an enormous evil
25.
Chester, Connecticut
–
Chester is a town in Middlesex County, Connecticut, United States. The population was 3,743 at the 2000 census, the town center is also defined by the U. S. Census Bureau as a census-designated place. The Wangunks, a tribe of Native Americans, occupied the land called Pattaconk prior to English settlement of the area in 1692. The town was formed from the quarter of Saybrook and incorporated in 1836. Its location is currently a historical landmark. According to the United States Census Bureau, the town has an area of 16.8 square miles. The CDP has an area of 2.1 square miles of which 1. 46% is water. As of the census of 2000, there were 3,743 people,1,510 households, the population density was 233.5 people per square mile. There were 1,613 housing units at a density of 100.6 per square mile. The racial makeup of the town was 96. 79% White,0. 85% African American,0. 35% Native American,0. 83% Asian,0. 05% Pacific Islander,0. 24% from other races, and 0. 88% from two or more races. Hispanic or Latino of any race were 1. 71% of the population,28. 1% of all households were made up of individuals and 13. 0% had someone living alone who was 65 years of age or older. The average household size was 2.38 and the family size was 2.93. In the town, the population was out with 22. 3% under the age of 18,4. 4% from 18 to 24,30. 1% from 25 to 44,26. 1% from 45 to 64. The median age was 42 years, for every 100 females there were 91.6 males. For every 100 females age 18 and over, there were 91.6 males, the median income for a household in the town was $65,156, and the median income for a family was $79,941. Males had an income of $45,515 versus $40,444 for females. The per capita income for the town was $32,191, none of the families and 1. 3% of the population were living below the poverty line, including no under eighteens and 2. 6% of those over 64. As of the census of 2000, there were 1,546 people,632 households, the population density was 762.1 inhabitants per square mile
26.
Middlesex County, Connecticut
–
Middlesex County is a county in the south central part of the U. S. state of Connecticut. As of the 2010 census, the population was 165,676, the county was created in May 1785 from portions of Hartford County and New London County. Middlesex County is included in the Hartford-West Hartford-East Hartford metropolitan statistical area known as Greater Hartford, as with all eight of Connecticuts counties, there is now no county government and no county seat. In Connecticut, towns are responsible for all government activities, including local police, fire and rescue, snow removal. In a few cases, neighboring towns will share resources, e. g. water, gas. Middlesex County is merely a group of towns on a map, middletown was the county seat of Middlesex County from its creation in 1785 until the elimination of county government in 1960. There is no government in Middlesex County other than the Middlesex County Judicial District, joseph E. Bibisi was the last person to serve as high sheriff of Middlesex County. The county has an area of 439 square miles, of which 369 square miles is land and 70 square miles is water. It is the smallest county in Connecticut by land area and second-smallest by total area, the terrain trends from mostly level along the Connecticut River and Atlantic coast to gently rolling uplands away from them. The highest elevation is close to a station in Meshomasic State Forest, at 916 feet above sea level. Middlesex County is also the home of Wadsworth Falls, the population density was 420 people per square mile. There were 67,285 housing units at a density of 182 per square mile. 4. 73% of the population were Hispanic or Latino of any race,20. 4% were of Italian,14. 3% Irish,11. 1% English,8. 9% Polish and 8. 2% German ancestry according to Census 2000. 91. 1% spoke English,2. 6% Spanish,1. 9% Italian,1. 2% French and 1. 0% Polish as their first language. 27. 20% of all households were made up of individuals and 10. 00% had someone living alone who was 65 years of age or older, the average household size was 2.43 and the average family size was 2.98. In the county, the population was out with 23. 20% under the age of 18,7. 30% from 18 to 24,31. 10% from 25 to 44,24. 80% from 45 to 64. The median age was 39 years, for every 100 females there were 95.10 males. For every 100 females age 18 and over, there were 92.10 males, the median income for a household in the county was $59,175, and the median income for a family was $71,319
27.
Sri Lanka
–
Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka, is an island country in South Asia near south-east India. Sri Lanka has maritime borders with India to the northwest and the Maldives to the southwest, Sri Lankas documented history spans 3,000 years, with evidence of pre-historic human settlements dating back to at least 125,000 years. Its geographic location and deep harbours made it of strategic importance from the time of the ancient Silk Road through to World War II. Sri Lanka was known from the beginning of British colonial rule until 1972 as Ceylon, Sri Lankas recent history has been marred by a thirty-year civil war which decisively ended when the Sri Lankan military defeated the Liberation Tigers of Tamil Eelam in 2009. A diverse and multicultural country, Sri Lanka is home to many religions, ethnic groups, in addition to the majority Sinhalese, it is home to large groups of Sri Lankan and Indian Tamils, Moors, Burghers, Malays, Kaffirs and the aboriginal Vedda. Sri Lanka has a rich Buddhist heritage, and the first known Buddhist writings of Sri Lanka, Sri Lanka is a republic and a unitary state governed by a semi-presidential system. The legislative capital, Sri Jayawardenepura Kotte, is a suburb of the capital and largest city. Along with the Maldives, Sri Lanka is one of the two countries in South Asia that are rated among high human development on the Human Development Index. In antiquity, Sri Lanka was known to travellers by a variety of names, according to the Mahavamsa, the legendary Prince Vijaya named the land Tambapanni, because his followers hands were reddened by the red soil of the area. In Hindu mythology, such as the Mahabharata, the island was referred to as Lankā, in Tamil, the island is referred to as Eelam. Ancient Greek geographers called it Taprobanā or Taprobanē from the word Tambapanni, as a British crown colony, the island was known as Ceylon, it achieved independence as the Dominion of Ceylon in 1948. The country is known in Sinhalese as Śrī Laṃkā and in Tamil as Ilaṅkai, in 1972, its formal name was changed to Free, Sovereign and Independent Republic of Sri Lanka. Later in 1978 it was changed to the Democratic Socialist Republic of Sri Lanka, as the name Ceylon still appears in the names of a number of organisations, the Sri Lankan government announced in 2011 a plan to rename all those over which it has authority. The pre-history of Sri Lanka goes back 125,000 years, the era spans the Palaeolithic, Mesolithic and early Iron Ages. Among the Paleolithic human settlements discovered in Sri Lanka, Pahiyangala and it is said that Kubera was overthrown by his demon stepbrother Ravana, the powerful emperor who built a mythical flying machine named Dandu Monara. The modern city of Wariyapola is described as Ravanas airport, early inhabitants of Sri Lanka were probably ancestors of the Vedda people, an indigenous people numbering approximately 2,500 living in modern-day Sri Lanka. According to the Mahāvamsa, a written in Pāḷi, the original inhabitants of Sri Lanka are the Yakshas and Nagas. Ancient cemeteries that were used before 600BC and other signs of advanced civilization has also discovered in Sri Lanka
28.
Polymorphism (materials science)
–
In materials science, polymorphism is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, the complete morphology of a material is described by polymorphism and other variables such as crystal habit, amorphous fraction or crystallographic defects. Polymorphism is relevant to the fields of pharmaceuticals, agrochemicals, pigments, dyestuffs, foods, when polymorphism exists as a result of a difference in crystal packing, it is called packing polymorphism. Polymorphism can also result from the existence of different conformers of the molecule in conformational polymorphism. In pseudopolymorphism the different crystal types are the result of hydration or solvation and this is more correctly referred to as solvomorphism as different solvates have different chemical formulae. An example of an organic polymorph is glycine, which is able to form monoclinic, silica is known to form many polymorphs, the most important of which are, α-quartz, β-quartz, tridymite, cristobalite, coesite, and stishovite. A classical example is the pair of minerals, calcite and aragonite, an analogous phenomenon for amorphous materials is polyamorphism, when a substance can take on several different amorphous modifications. In terms of thermodynamics, there are two types of polymorphic behaviour, for an enantiotropic system, a plot of the free energy against temperature shows a crossing point threshold before the various melting points. It may also be possible to revert interchangeably between the two polymorphs by heating or cooling, or through contact with a lower energy polymorph. Present-day analysis identifies three polymorphs for benzamide, the least stable one, formed by flash cooling is the orthorhombic form II and this type is followed by the monoclinic form III. The most stable form is monoclinic form I, the hydrogen bonding mechanisms are the same for all three phases, however, they differ strongly in their pi-pi interactions. Polymorphs have different stabilities and may spontaneously convert from a form to the stable form at a particular temperature. Most polymorphs of organic molecules only differ by a few kJ/mol in lattice energy, approximately 50% of known polymorph pairs differ by less than 2 kJ/mol and stability differences of more than 10 kJ/mol are rare. They also exhibit different melting points, solubilities, X-ray crystal, various conditions in the crystallisation process is the main reason responsible for the development of different polymorphic forms. In 2006 a new form of maleic acid was discovered 124 years after the first crystal form was studied. Maleic acid is a manufactured on a very large scale in the chemical industry and is a salt forming component in medicine. The new crystal type is produced when a co-crystal of caffeine and maleic acid is dissolved in chloroform, whereas form I has monoclinic space group P21/c, the new form has space group Pc. 1,3, 5-Trinitrobenzene is more than 125 years old and was used as an explosive before the arrival of the safer 2,4, only one crystal form of 1,3, 5-trinitrobenzene was known in the space group Pbca
29.
Thin section
–
A thin sliver of rock is cut from the sample with a diamond saw and ground optically flat. It is then mounted on a slide and then ground smooth using progressively finer abrasive grit until the sample is only 30 μm thick. The method involved using the Michel-Lévy interference colour chart, typically quartz is used as the gauge to determine thickness as it is one of the most abundant minerals. As different minerals have different optical properties, most rock forming minerals can be easily identified, plagioclase for example can be seen in the photo on the right as a clear mineral with multiple parallel twinning planes. The large blue-green minerals are clinopyroxene with some exsolution of orthopyroxene, thin sections are prepared in order to investigate the optical properties of the minerals in the rock. This work is a part of petrology and helps to reveal the origin, a photograph of a rock in thin section is often referred to as a photomicrograph. Fine-grained rocks, particularly those containing minerals of high birefringence, such as calcite, are prepared as ultra-thin sections. An ordinary 30 μm thin section is prepared as described above, the section is then polished on both sides using a fine diamond paste until it has a thickness in the range of 2-12 μm. This technique has been used to study the microstructure of fine-grained carbonates such as the Lochseitenkalk mylonite in which the grains are less than 5 μm in size. Ceramography, thin sections of ceramics Shelley, D
30.
Metamorphism
–
Metamorphism is the change of minerals or geologic texture in pre-existing rocks, without the protolith melting into liquid magma. The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids, the chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earths surface due to weathering and/or diagenesis are not classified as metamorphism, Metamorphism typically occurs between diagenesis, and melting. Three types of metamorphism exist, contact, dynamic, and regional, Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism, Metamorphic rocks can change without melting. When pressure is applied, somewhat flattened grains that orient in the same direction have a stable configuration. The upper boundary of metamorphic conditions is related to the onset of melting processes in the rock, the maximum temperature for metamorphism is typically 700 –900 °C, depending on the pressure and on the composition of the rock. Migmatites are rocks formed at this limit, which contain pods. Since the 1980s it has recognized that rocks are rarely dry enough. Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event, the collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism. These orogenic mountains are eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the slab as it plunges toward the mantle in a subduction zone also produce regional metamorphic effects. The techniques of structural geology are used to unravel the collisional history, regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane. Contact metamorphism occurs typically around intrusive igneous rocks as a result of the increase caused by the intrusion of magma into cooler country rock. The area surrounding the intrusion where the contact metamorphism effects are present is called the metamorphic aureole, contact metamorphic rocks are usually known as hornfels. Rocks formed by contact metamorphism may not present signs of deformation and are often fine-grained. Contact metamorphism is greater adjacent to the intrusion and dissipates with distance from the contact, the size of the aureole depends on the heat of the intrusion, its size, and the temperature difference with the wall rocks. Dikes generally have small aureoles with minimal metamorphism whereas large ultramafic intrusions can have significantly thick, the metamorphic grade of an aureole is measured by the peak metamorphic mineral which forms in the aureole
31.
Sedimentary rock
–
Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of that material at the Earths surface and within bodies of water. Sedimentation is the name for processes that cause mineral and/or organic particles to settle in place. The particles that form a rock by accumulating are called sediment. Sedimentation may also occur as minerals precipitate from solution or shells of aquatic creatures settle out of suspension. The sedimentary rock cover of the continents of the Earths crust is extensive, sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata, forming a structure called bedding, sedimentary rocks are also important sources of natural resources like coal, fossil fuels, drinking water or ores. The study of the sequence of rock strata is the main source for an understanding of the Earths history, including palaeogeography, paleoclimatology. The scientific discipline that studies the properties and origin of rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps partly with other disciplines in the Earth sciences, such as pedology, geomorphology, geochemistry, sedimentary rocks have also been found on Mars. Clastic sedimentary rocks are composed of rock fragments that were cemented by silicate minerals. Clastic rocks are composed largely of quartz, feldspar, rock fragments, clay minerals, and mica, any type of mineral may be present, clastic sedimentary rocks, are subdivided according to the dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions, gravel, sand, and mud and this tripartite subdivision is mirrored by the broad categories of rudites, arenites, and lutites, respectively, in older literature. The subdivision of these three categories is based on differences in clast shape, conglomerates and breccias), composition. Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel, composition of framework grains The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks, all other minerals are considered accessories and not used in the naming of the rock, regardless of abundance. Clean sandstones with open space are called arenites. Muddy sandstones with abundant muddy matrix are called wackes, six sandstone names are possible using the descriptors for grain composition and the amount of matrix. Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles and these relatively fine-grained particles are commonly transported by turbulent flow in water or air, and deposited as the flow calms and the particles settle out of suspension
32.
Gneiss
–
Gneiss is a common distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. The foliations are characterized by alternating darker and lighter colored bands, the word gneiss comes from the Middle High German verb gneist. It has occurred in English since at least 1757, gneissic rocks are usually medium- to coarse-foliated, they are largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, gneiss rocks may also be named after a characteristic component such as garnet gneiss, biotite gneiss, albite gneiss, etc. Orthogneiss designates a gneiss derived from a rock, and paragneiss is one from a sedimentary rock. Gneissose rocks have properties similar to gneiss, gneiss appears to be striped in bands, called gneissic banding. The banding is developed under high temperature and pressure conditions, the minerals are arranged into layers that appear as bands in cross section. The appearance of layers, called compositional banding, occurs because the layers, the darker bands have relatively more mafic minerals. The lighter bands contain relatively more felsic and these forces stretch out the rock like a plastic, and the original material is spread out into sheets. Another cause of banding is metamorphic differentiation, which separates different materials into different layers through chemical reactions, not all gneiss rocks have detectable banding. In kyanite gneiss, crystals of kyanite appear as random clumps in what is mainly a plagioclase matrix, henderson gneiss is found in North Carolina and South Carolina, US, east of the Brevard Shear Zone. It has deformed into two sequential forms, the second, more warped, form is associated with the Brevard Fault, and the first deformation results from displacement to the southwest. Most of the Outer Hebrides of Scotland have a formed from Lewisian gneiss. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree. These rocks are igneous in origin, mixed with metamorphosed marble, quartzite and mica schist with later intrusions of basaltic dikes. Gneisses of Archean and Proterozoic age occur in the Baltic Shield, in antiquity, gneisses were also utilized in architectural construction. They were used to erect the Sphinx of Taharqo in the Nile Valley, list of rock types Blatt, Harvey and Robert J. Tracy. Petrology, Igneous, Sedimentary and Metamorphic, 2nd ed. Freeman, mcKirdy, Alan, Roger Crofts and John Gordon
33.
Granulite
–
Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are of particular interest to geologists because many granulites represent samples of the continental crust. Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature, the minerals present in a granulite will vary depending on the parent rock of the granulite and the temperature and pressure conditions experienced during metamorphism. A common type of granulite found in metamorphic rocks of the continents contains pyroxene, plagioclase feldspar and accessory garnet, oxides. Both clinopyroxene and orthopyroxene may be present, and in fact, a granulite may be visually quite distinct with abundant small pink or red pyralspite garnets in a granular holocrystalline matrix. Concentrations of garnets, micas, or amphiboles may form along a linear pattern resembling gneiss or migmatite banding, granulites form at high-temperature conditions at a range of pressure conditions, typically during regional metamorphism. In some cases, the temperatures are difficult to account for at the inferred depths at typical geothermal gradients. In extreme cases, granulites may form at temperatures in excess of 1000 °C, in continental crustal rocks, biotite may break down at high temperatures to form orthopyroxene + potassium feldspar + water, producing a granulite. Other possible minerals formed at dry conditions include sapphirine, spinel, sillimanite, some assemblages such as sapphirine + quartz indicate very high temperatures. The granulite facies is determined by the lower boundary of 700 +/−50 °C. The most common mineral assemblage of granulite facies consists of antiperthitic plagioclase, alkali feldspar containing up to 50% albite, transition between amphibolite and granulite facies is defined by these reaction isograds, amphibole -> pyroxene + H2O biotite -> K-feldspar + garnet + orthopyroxene + H2O. Granulite is a used by petrographers to designate two distinct classes of rocks. According to the terminology of the French school it signifies a granite in which kinds of mica occur, and corresponds to the German Granit, or to the English muscovite biotite granite. This application has not been accepted generally, among English and American geologists the term is generally employed in this sense. In the rocks of this group the minerals, as seen in a microscopic slide, the individual crystals never have perfect form, and indeed traces of it are rare. In some granulites they interlock, with borders, in others they have been drawn out. In most cases they are rounded with smaller grains between the larger. This is especially true of the quartz and feldspar which are the predominant minerals, the garnets are very generally larger than the above-mentioned ingredients, and easily visible with the eye as pink spots on the broken surfaces of the rock
34.
Almandine
–
Almandine /ˈælməndɪn/, also known incorrectly as almandite, is a species of mineral belonging to the garnet group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, Almandine is an iron alumina garnet, of deep red color, inclining to purple. It is frequently cut with a face, or en cabochon. Viewed through the spectroscope in a light, it generally shows three characteristic absorption bands. Almandine is one end-member of a solid solution series, with the other end member being the garnet pyrope. The almandine crystal formula is, Fe3Al23, magnesium substitutes for the iron with increasingly pyrope-rich composition. Almandine crystallizes in the space group Ia3d, with unit-cell parameter a ≈11.512 Å at 100 K. Almandine is antiferromagnetic with the Néel temperature of 7.5 K. It contains two equivalent magnetic sublattices, Almandine occurs rather abundantly in the gem-gravels of Sri Lanka, whence it has sometimes been called Ceylon-ruby. When the color inclines to a violet tint, the stone is often called Syriam garnet, a said to be taken from Syriam. Fine rhombic dodecahedra occur in the rocks of the Zillertal, in Tyrol. An almandine in which the oxide is replaced partly by magnesia is found at Luisenfeld in German East Africa. In the United States there are many localities which yield almandine, fine crystals of almandine embedded in mica-schist occur near Wrangell in Alaska. The coarse varieties of almandine are often crushed for use as an abrasive agent
35.
Cordierite
–
For the green variety of quartz see prasiolite. Cordierite or iolite is an iron aluminium cyclosilicate. Iron is almost always present and a solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula, 2Al3 to 2Al3. A high-temperature polymorph exists, indialite, which is isostructural with beryl and has a distribution of Al in the 6O18 rings. Cordierite, which was discovered in 1813, is named after the French geologist Louis Cordier, cordierite typically occurs in contact or regional metamorphism of pelitic rocks. It is especially common in hornfels produced by metamorphism of pelitic rocks. Two common metamorphic mineral assemblages include sillimanite-cordierite-spinel and cordierite-spinel-plagioclase-orthopyroxene, other associated minerals include garnet and anthophyllite. Cordierite also occurs in some granites, pegmatites, and norites in gabbroic magmas, alteration products include mica, chlorite, and talc. Cordierite occurs, for example, in the contact zone at Geevor Tin Mine in Cornwall. Catalytic converters are made from ceramics containing a large proportion of synthetic cordierite. The manufacturing process aligns the cordierite crystals to make use of the very low thermal expansion along one axis. This prevents thermal shock cracking from taking place when the catalytic converter is used, as the transparent variety iolite, it is often used as a gemstone. The name iolite comes from the Greek word for violet, another old name is dichroite, a Greek word meaning two-colored rock, a reference to cordierites strong pleochroism. It has also been called water-sapphire and Vikings Compass because of its usefulness in determining the direction of the sun on overcast days and this works by determining the direction of polarization of the sky overhead. Gem quality iolite varies in color from blue to blue violet to yellowish gray to light blue as the light angle changes. Iolite is sometimes used as a substitute for sapphire. It is much softer than sapphires and is found in Australia, Brazil, Burma, Canada, India, Madagascar, Namibia, Sri Lanka, Tanzania. The largest iolite crystal found weighed more than 24,000 carats, another name for blue iolite is steinheilite, after Fabian Steinheil, the Russian military governor of Finland who observed that it was a different mineral from quartz
36.
Biotite
–
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K 3AlSi 3O102. Hausmann in 1847 in honor of the French physicist Jean-Baptiste Biot, iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound together by potassium ions. It is sometimes called iron mica because it is more iron-rich than phlogopite and it is also sometimes called black mica as opposed to white mica – both form in some rocks, and in some instances side-by-side. Like other mica minerals, biotite has a perfect basal cleavage, and consists of flexible sheets, or lamellae. It has a crystal system, with tabular to prismatic crystals with an obvious pinacoid termination. It has four faces and two pinacoid faces to form a pseudohexagonal crystal. Although not easily seen because of the cleavage and sheets, fracture is uneven and it appears greenish to brown or black, and even yellow when weathered. It can be transparent to opaque, has a vitreous to pearly luster, when biotite is found in large chunks, they are called “books” because it resembles a book with pages of many sheets. The color of biotite is usually black and the mineral has a hardness of 2. 5-3 on the Mohs scale of mineral hardness, biotite dissolves in both acid and alkaline aqueous solutions, with the highest dissolution rates at low pH. However, biotite dissolution is highly anisotropic with crystal edge surfaces reacting 45 to 132 times faster than basal surfaces, under cross-polarized light biotite can generally be identified by the gnarled birds eye extinction. Biotite is found in a variety of igneous and metamorphic rocks. For instance, biotite occurs in the lava of Mount Vesuvius, biotite in granite tends to be poorer in magnesium than the biotite found in its volcanic equivalent, rhyolite. Biotite is an essential phenocryst in some varieties of lamprophyre, biotite is occasionally found in large cleavable crystals, especially in pegmatite veins, as in New England, Virginia and North Carolina. Other notable occurrences include Bancroft and Sudbury, Ontario and it is an essential constituent of many metamorphic schists, and it forms in suitable compositions over a wide range of pressure and temperature. It has been estimated that biotite comprises up to 7% of the continental crust. The largest documented single crystals of biotite were approximately 7 m2 sheets found in Iveland, biotite is used extensively to constrain ages of rocks, by either potassium-argon dating or argon-argon dating. Because argon escapes readily from the crystal structure at high temperatures. Biotite is also useful in assessing temperature histories of metamorphic rocks, because the partitioning of iron and magnesium between biotite and garnet is sensitive to temperature
37.
Quartz
–
Quartz is the second most abundant mineral in Earths continental crust, behind feldspar. There are many different varieties of quartz, several of which are semi-precious gemstones, since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Eurasia. The word quartz is derived from the German word Quarz and its Middle High German ancestor twarc, the Ancient Greeks referred to quartz as κρύσταλλος derived from the Ancient Greek κρύος meaning icy cold, because some philosophers apparently believed the mineral to be a form of supercooled ice. Today, the rock crystal is sometimes used as an alternative name for the purest form of quartz. Quartz belongs to the crystal system. The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end, well-formed crystals typically form in a bed that has unconstrained growth into a void, usually the crystals are attached at the other end to a matrix and only one termination pyramid is present. However, doubly terminated crystals do occur where they develop freely without attachment, a quartz geode is such a situation where the void is approximately spherical in shape, lined with a bed of crystals pointing inward. α-quartz crystallizes in the crystal system, space group P3121 and P3221 respectively. β-quartz belongs to the system, space group P6222 and P6422. These space groups are truly chiral, both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks. The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is an identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent or translucent, common colored varieties include citrine, rose quartz, amethyst, smoky quartz, milky quartz, and others. The most important distinction between types of quartz is that of macrocrystalline and the microcrystalline or cryptocrystalline varieties, the cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a form of silica consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite. Other opaque gemstone varieties of quartz, or mixed rocks including quartz, often including contrasting bands or patterns of color, are agate, carnelian or sard, onyx, heliotrope, amethyst is a form of quartz that ranges from a bright to dark or dull purple color. The worlds largest deposits of amethysts can be found in Brazil, Mexico, Uruguay, Russia, France, Namibia, sometimes amethyst and citrine are found growing in the same crystal. It is then referred to as ametrine, an amethyst is formed when there is iron in the area where it was formed
38.
Schist
–
Schist is a medium-grade metamorphic rock with medium to large, flat, sheet-like grains in a preferred orientation. It is defined by having more than 50% platy and elongated minerals, often finely interleaved with quartz and these lamellar minerals include micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a form called quartz schist is produced. Schist forms at a temperature and has larger grains than phyllite. Geological foliation with medium to large grained flakes in a preferred orientation is called schistosity. The names of various schists are derived from their mineral constituents, for example, schists rich in mica are called mica schists and include biotite or muscovite. Most schists are mica schists, but graphite and chlorite schists are also common, Schists are also named for their prominent or perhaps unusual mineral constituents, as in the case of garnet schist, tourmaline schist, and glaucophane schist. The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, Schist is characteristically foliated, meaning that the individual mineral grains split off easily into flakes or slabs. Most schists are derived from clays and muds that have passed through a series of processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists are derived from fine-grained igneous rocks such as basalts, before the mid-18th century, the terms slate, shale and schist were not sharply differentiated by those involved with mining. In the context of underground mining, shale was frequently referred to as slate well into the 20th century. During metamorphism, rocks which were originally sedimentary, igneous or metamorphic are converted into schists, if the composition of the rocks was originally similar, they may be very difficult to distinguish from one another if the metamorphism has been great. A quartz-porphyry, for example, and a fine grained feldspathic sandstone, usually, however, it is possible to distinguish between sedimentary and igneous schists and gneisses. If, for example, the district occupied by these rocks has traces of bedding, clastic structure, or unconformability. In other cases intrusive junctions, chilled edges, contact alteration or porphyritic structure may prove that in its original condition a metamorphic gneiss was an igneous rock. Such rocks as limestones, dolomites, quartzites and aluminous shales have very definite chemical characteristics which distinguish them even when completely recrystallized, the schists are classified principally according to the minerals they consist of and on their chemical composition. For example, many metamorphic limestones, marbles, and calc-schists, with crystalline dolomites, contain silicate minerals such as mica, tremolite, diopside, scapolite, quartz and they are derived from calcareous sediments of different degrees of purity. Another group is rich in quartz, with amounts of white and black mica, garnet, feldspar, zoisite
39.
Hornfels
–
Hornfels is called so because of its exceptional toughness and texture both reminiscent of animal horns. These properties are due to fine grained non-aligned crystals with platy or prismatic habits, Hornfels rocks were referred to by miners in northern England as whetstones. Though they may show banding, due to bedding, etc. they break across this as readily as along it, in fact, the most common hornfels are dark-brown to black with a somewhat velvety luster owing to the abundance of small crystals of shining black mica. The lime hornfels are often white, yellow, pale-green, brown, green and dark-green are the prevalent tints of the hornfels produced by the alteration of igneous rocks. The structure of the hornfels is very characteristic, very rarely do any of the minerals show crystalline form, but the small grains fit closely together like the fragments of a mosaic, they are usually of nearly equal dimensions. This has been called pflaster or pavement structure from the resemblance to rough pavement work, each mineral may also enclose particles of the others, in the quartz, for example, small crystals of graphite, biotite, iron oxides, sillimanite or feldspar may appear in great numbers. Often the whole of the grains are rendered semi-opaque in this way, the minutest crystals may show traces of crystalline outlines, undoubtedly they are of new formation and have originated in situ. The regeneration of the rock has been sufficient to efface most of the original structures, but crystallization has been hampered by the solid condition of the mass and the new minerals are formless and have been unable to reject impurities, but have grown around them. There is also quartz, and often a considerable amount of feldspar, while graphite, tourmaline, the andalusite may be pink and is then often pleochroic in thin sections, or it may be white with the cross-shaped dark enclosures of the matrix that are characteristic of chiastolite. Sillimanite usually forms exceedingly minute needles embedded in quartz, in the rocks of this group cordierite also occurs, not rarely, and may have the outlines of imperfect hexagonal prisms that are divided up into six sectors when seen in polarized light. In biotite hornfels, a faint striping may indicate the original bedding of the unaltered rock, more commonly there is a distinct spotting, visible on the surfaces of the hand specimens. The spots are round or elliptical, and may be paler or darker than the rest of the rock. In some cases they are rich in graphite or carbonaceous matter, in others they are full of brown mica, tourmaline hornfels are found sometimes near the margins of tourmaline granites, they are black with small needles of schorl that under the microscope are dark brown and richly pleochroic. As the tourmaline contains boron, there must have been some permeation of vapors from the granite into the sediments, rocks of this group are often seen in the Cornish tin-mining districts, especially near the ludes. A second great group of hornfels are the calc–silicate hornfels that arise from the alteration of impure limestone. These rocks are fine-grained, and though often banded, are tough and they are excessively variable in their mineralogical composition, and very often alternate in thin seams with biotite hornfels and indurated quartzites. When perfused with boric and fluoric vapors from the granite they may contain much axinite, fluorite and datolite, from diabases, basalts, andesites and other igneous rocks a third type of hornfels is produced. They consist essentially of feldspar with hornblende and pale pyroxene, sphene, biotite and iron oxides are the other common constituents, but these rocks show much variety of composition and structure
40.
Pegmatite
–
A pegmatite is a holocrystalline, intrusive igneous rock composed of interlocking phaneritic crystals usually larger than 2.5 cm in size, such rocks are referred to as pegmatitic. The word pegmatite derives from Homeric Greek, πεγνυμι, which means “to bind together”, in reference to the crystals of quartz. Most pegmatites are composed of quartz, feldspar and mica, having a similar composition as granite. Crystal size is the most striking feature of pegmatites, with usually over 5 cm in size. Individual crystals over 10 metres long have been found, and many of the worlds largest crystals were found within pegmatites and these include, spodumene, microcline, beryl, and tourmaline. Similarly, crystal texture and form within pegmatitic rock may be taken to extreme size, perthite feldspar within a pegmatite often shows gigantic perthitic texture visible to the naked eye. The single feature that is diagnostic to all pegmatites is their large size crystal components, Pegmatite bodies are usually of minor size compared to typical intrusive rock bodies. Pegmatite body size is on the order of magnitude of one to a few hundred meters, compared to typical igneous rocks they are rather inhomogeneous and may show zones with different mineral assemblages. Crystal size and mineral assemblages are usually oriented parallel to the rock or even concentric for pegmatite lenses. Crystal growth rates in pegmatite must be slow to allow gigantic crystals to grow within the confines and pressures of the Earths crust. The mineralogy of a pegmatite is in most cases dominated by some form of feldspar, often with mica and usually with quartz and this is because of the difficulty in counting and sampling mineral grains in a rock which may have crystals from centimeters to meters across. Garnet, commonly almandine or spessartine, is a common mineral within pegmatites intruding mafic, pegmatites associated with granitic domes within the Archaean Yilgarn Craton intruding ultramafic and mafic rocks contain red, orange and brown almandine garnet. Syenite pegmatites are quartz depleted and contain large feldspathoid crystals instead, Pegmatite is difficult to sample representatively due to the large size of the constituent mineral crystals. Often, bulk samples of some 50–60 kg of rock must be crushed to obtain a meaningful, hence, pegmatite is often characterised by sampling the individual minerals which comprise the pegmatite, and comparisons are made according to mineral chemistry. Occasionally, enrichment in the trace elements will result in crystallisation of equally unusual and rare minerals such as beryl, tourmaline, columbite, tantalite, zinnwaldite. Pegmatites are the source of lithium either as spodumene, lithiophyllite or usually from lepidolite. The primary source for caesium is pollucite, a mineral from a zoned pegmatite, the majority of the worlds beryllium is sourced from non-gem quality beryl within pegmatite. Tantalum, niobium, rare-earth elements are sourced from a few pegmatites worldwide, bismuth, molybdenum and tin have been won from pegmatite, but this is not yet an important source of these metals
41.
Glass
–
Glass is a non-crystalline amorphous solid that is often transparent and has widespread practical, technological, and decorative usage in, for example, window panes, tableware, and optoelectronics. The most familiar, and historically the oldest, types of glass are silicate glasses based on the chemical compound silica, the primary constituent of sand. The term glass, in usage, is often used to refer only to this type of material. Many applications of silicate glasses derive from their optical transparency, giving rise to their use as window panes. Glass can be coloured by adding metallic salts, and can also be painted and printed with vitreous enamels and these qualities have led to the extensive use of glass in the manufacture of art objects and in particular, stained glass windows. Although brittle, silicate glass is extremely durable, and many examples of glass fragments exist from early glass-making cultures, because glass can be formed or moulded into any shape, it has been traditionally used for vessels, bowls, vases, bottles, jars and drinking glasses. In its most solid forms it has also used for paperweights, marbles. Some objects historically were so commonly made of glass that they are simply called by the name of the material, such as drinking glasses. Porcelains and many polymer thermoplastics familiar from everyday use are glasses and these sorts of glasses can be made of quite different kinds of materials than silica, metallic alloys, ionic melts, aqueous solutions, molecular liquids, and polymers. For many applications, like glass bottles or eyewear, polymer glasses are a lighter alternative than traditional glass, silica is a common fundamental constituent of glass. In nature, vitrification of quartz occurs when lightning strikes sand, forming hollow, fused quartz is a glass made from chemically-pure SiO2. It has excellent resistance to shock, being able to survive immersion in water while red hot. However, its high melting-temperature and viscosity make it difficult to work with, normally, other substances are added to simplify processing. One is sodium carbonate, which lowers the transition temperature. The soda makes the glass water-soluble, which is undesirable, so lime, some magnesium oxide. The resulting glass contains about 70 to 74% silica by weight and is called a soda-lime glass, soda-lime glasses account for about 90% of manufactured glass. Most common glass contains other ingredients to change its properties, lead glass or flint glass is more brilliant because the increased refractive index causes noticeably more specular reflection and increased optical dispersion. Adding barium also increases the refractive index, iron can be incorporated into glass to absorb infrared energy, for example in heat absorbing filters for movie projectors, while cerium oxide can be used for glass that absorbs UV wavelengths
42.
Aluminium oxide
–
Aluminium oxide or aluminum oxide is a chemical compound of aluminium and oxygen with the chemical formula Al2O3. It is the most commonly occurring of several aluminium oxides, and it is commonly called alumina, and may also be called aloxide, aloxite, or alundum depending on particular forms or applications. It occurs naturally in its crystalline polymorphic phase α-Al2O3 as the mineral corundum, varieties of form the precious gemstones ruby. Al2O3 is significant in its use to produce metal, as an abrasive owing to its hardness. Corundum is the most common naturally occurring form of aluminium oxide. Rubies and sapphires are gem-quality forms of corundum, which owe their characteristic colors to trace impurities, rubies are given their characteristic deep red color and their laser qualities by traces of chromium. Sapphires come in different colors given by various other impurities, such as iron, Al2O3 is an electrical insulator but has a relatively high thermal conductivity for a ceramic material. Aluminium oxide is insoluble in water, in its most commonly occurring crystalline form, called corundum or α-aluminium oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools. Aluminium oxide is responsible for the resistance of aluminium to weathering. Metallic aluminium is very reactive with oxygen, and a thin passivation layer of aluminium oxide forms on any exposed aluminium surface. This layer protects the metal from further oxidation, the thickness and properties of this oxide layer can be enhanced using a process called anodising. A number of alloys, such as bronzes, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance. Aluminium oxide was taken off the United States Environmental Protection Agencys chemicals lists in 1988, aluminium oxide is on EPAs Toxics Release Inventory list if it is a fibrous form. Al2O3 +6 HF →2 AlF3 +3 H2O Al2O3 +2 NaOH +3 H2O →2 NaAl4 The most common form of aluminium oxide is known as corundum. The oxygen ions form a hexagonal close-packed structure with aluminium ions filling two-thirds of the octahedral interstices. In terms of its crystallography, corundum adopts a trigonal Bravais lattice with a group of R-3c. The primitive cell contains two units of aluminium oxide. Each has a crystal structure and properties
43.
Dumortierite
–
Dumortierite is a fibrous variably colored aluminium boro-silicate mineral, Al7BO33O3. Dumortierite crystallizes in the system typically forming fibrous aggregates of slender prismatic crystals. The crystals are vitreous and vary in color from brown, blue, substitution of iron and other tri-valent elements for aluminium result in the color variations. It has a Mohs hardness of 7 and a gravity of 3.3 to 3.4. Crystals show pleochroism from red to blue to violet, Dumortierite quartz is blue colored quartz containing abundant dumortierite inclusions. Dumortierite was first described in 1881 for an occurrence in Chaponost, in the Rhône-Alps of France and it typically occurs in high temperature aluminium rich regional metamorphic rocks, those resulting from contact metamorphism and also in boron rich pegmatites. The most extensive investigation on dumortierite was done on samples from the high grade metamorphic Gfohl unit in Austria by Fuchs et al and it is used in the manufacture of high grade porcelain. It is sometimes mistaken for sodalite and has used as imitation lapis lazuli. Sources of Dumortierite include Austria, Brazil, Canada, France, Italy, Madagascar, Namibia, Nevada, Norway, Peru, Poland, Russia, List of minerals List of minerals named after people Mineral galleries Dumortierite as a Commercial Mineral Dumortierite from Nevada Y. Fuchs, A. Ertl, J. M. Hughes, S. Prowatke, F. Brandstätter, Dumortierite from the Gföhl unit, Lower Austria. Dumortierite from Iran, A First Record, G. F. Claringbull, M. H. Hey
44.
Mullite
–
Mullite or porcelainite is a rare silicate mineral of post-clay genesis. It can form two stoichiometric forms 3Al2O32SiO2 or 2Al2O3 SiO2, unusually, mullite has no charge balancing cations present. As a result, there are three different aluminium sites, two distorted tetrahedral and one octahedral, mullite was first described in 1924 for an occurrence on the Isle of Mull, Scotland. It occurs as argillaceous inclusions in rocks in the Isle of Mull, inclusions in sillimanite within a tonalite at Val Sissone, Italy and with emerylike rocks in Argyllshire. Mullite is present in the form of needles in porcelain and it is produced during various melting and firing processes, and is used as a refractory material, due to its high melting point of 1840°C. The formula for making it was kept a guarded secret. Mullite morphology is also important for its application, in this case, there are two common morphologies for mullite. One is a shape with low aspect ratio and the second is a needle shape with high aspect ratio. The most important condition relates to ceramic chemical composition, if the silica and alumina ratio with low basic materials such as sodium and calcium is adjusted, the needle shape mullite forms at about 1400 °C and the needles will interlock. This mechanical interlocking contributes to the mechanical strength of porcelain. Recent research indicates that a synthetic analogue of mullite can be a replacement for platinum in reducing the amount of pollution generated by diesel engines. Martinón-Torres, Marcos, Thilo Rehren, Ian C, mullite and the mystery of Hessian wares. In Hartmut Schneider and Sridhar Komarneri
