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", implies radiance, gloss, or brilliance. A range of terms are used to describe lustre, such as earthy, metallic and silky; the term vitreous refers to a glassy lustre. A list of these terms is given below. Lustre varies over a wide continuum, so there are no rigid boundaries between the different types of lustre; the terms are combined to describe intermediate 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, most notably seen in diamond; such minerals are transparent or translucent, have a high refractive index. Minerals with a true adamantine lustre are uncommon, with examples being cerussite and cubic zirconia. Minerals with a lesser degree of lustre are referred to as subadamantine, with some examples being garnet and corundum.
Dull minerals exhibit little to no lustre, due to coarse granulations which scatter light in all directions, approximating a Lambertian reflector. An example is kaolinite. A distinction is sometimes drawn between dull minerals and earthy minerals, with the latter being coarser, having less lustre. Greasy minerals resemble grease. A greasy lustre occurs in minerals containing a great abundance of microscopic inclusions, with examples including opal and cordierite, jadeite. Many minerals with a greasy lustre feel greasy to the touch. Metallic minerals have the lustre of polished metal, with ideal surfaces will work as a reflective surface. Examples include galena 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 stilbite. Resinous minerals have the appearance of chewing gum or plastic. A principal example is amber, a form of fossilized resin.
Silky minerals have a parallel arrangement of fine fibres, giving them a lustre reminiscent of silk. Examples include asbestos and the satin spar variety of gypsum. A fibrous lustre has a coarser texture. Submetallic minerals are duller and less reflective. A submetallic lustre occurs in near-opaque minerals with high refractive indices, such as sphalerite and cuprite. Vitreous minerals have the lustre of glass; this type of lustre is one of the most seen, occurs in transparent or translucent minerals with low refractive indices. Common examples include calcite, topaz, beryl and fluorite, among others. Waxy minerals have a lustre resembling wax. Examples include chalcedony. Asterism is the display of a star-shaped luminous area, it is seen in some rubies, where it is caused by impurities of rutile. It can occur in garnet and spinel. Aventurescence is a reflectance effect like that of glitter, it arises from minute, preferentially oriented mineral platelets within the material. These platelets are so numerous that they influence the material's body colour.
In aventurine quartz, chrome-bearing fuchsite makes for a green stone and various iron oxides make for a red stone. Chatoyant minerals display luminous bands; such minerals are composed of parallel fibers, which reflect light into a direction perpendicular to their orientation, thus forming narrow bands of light. The most famous examples are tiger's eye and cymophane, but the effect may occur in other minerals such as aquamarine and tourmaline. Color change is most found in alexandrite, a variety of chrysoberyl gemstones. Other gems occur in color-change varieties, including sapphire, spinel. Alexandrite displays a color change dependent upon light, along with strong pleochroism; the gem results from small-scale replacement of aluminium by chromium oxide, responsible for alexandrite's characteristic green to red color change. Alexandrite from the Ural Mountains in Russia is green by red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light.
The optimum or "ideal" color change would be fine emerald green to fine purplish red, but this is rare. Iridescence is the'play' or'fire' of rainbow-coloured light caused by thin regular structures or layers beneath the surface of a gemstone. Similar to a thin film of oil on water, these layers interfere with the rays of reflected light, reinforcing some colours and cancelling others. Iridescence is seen at its best in precious opal. Schiller, from German for "color play", is the metallic iridescence originating from below the surface of a stone that occurs when light is reflected between layers of minerals, it is seen in moonstone and labradorite and is similar to adularescence and aventurescence
Monoclinic crystal system
In crystallography, the monoclinic crystal system is one of the 7 crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic system, they form a rectangular prism with a parallelogram as its base. Hence two vectors are perpendicular, while the third vector meets the other two at an angle other than 90°. There is only one monoclinic Bravais lattice in two dimensions: the oblique lattice. Two monoclinic Bravais lattices exist: the primitive monoclinic and the base-centered monoclinic lattices. In the monoclinic system there is a used second choice of crystal axes that results in a unit cell with the shape of an oblique rhombic prism. In this axis setting, the primitive and base-centered lattices swap in centering type; the table below organizes the space groups of the monoclinic crystal system by crystal class. It lists the International Tables for Crystallography space group numbers, followed by the crystal class name, its point group in Schoenflies notation, Hermann–Mauguin notation, orbifold notation, Coxeter notation, type descriptors, mineral examples, the notation for the space groups.
Sphenoidal is monoclinic hemimorphic. The three monoclinic hemimorphic space groups are as follows: a prism with as cross-section wallpaper group p2 ditto with screw axes instead of axes ditto with screw axes as well as axes, parallel, in between; the four monoclinic hemihedral space groups include those with pure reflection at the base of the prism and halfway those with glide planes instead of pure reflection planes. Crystal structure Crystallography Crystal Hurlbut, Cornelius S.. Manual of Mineralogy. Pp. 69–73. ISBN 0-471-80580-7. Hahn, Theo, ed.. International Tables for Crystallography, Volume A: Space Group Symmetry. A. Berlin, New York: Springer-Verlag. Doi:10.1107/97809553602060000100. ISBN 978-0-7923-6590-7
In crystallography, the terms crystal system, crystal family, lattice system each refer to one of several classes of space groups, point groups, or crystals. Informally, two crystals are in the same crystal system if they have similar symmetries, although there are many exceptions to this. Crystal systems, crystal families and lattice systems are similar but different, there is widespread confusion between them: in particular the trigonal crystal system is confused with the rhombohedral lattice system, the term "crystal system" is sometimes used to mean "lattice system" or "crystal family". Space groups and crystals are divided into seven crystal systems according to their point groups, into seven lattice systems according to their Bravais lattices. Five of the crystal systems are 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, in order to eliminate this confusion.
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, orthorhombic, rhombohedral and cubic. 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 lattice system, in which case both the crystal and lattice systems have the same name. However, five point groups are assigned to two lattice systems and hexagonal, because both exhibit threefold rotational symmetry; these point groups are assigned to the trigonal crystal system. In total there are seven crystal systems: triclinic, orthorhombic, trigonal and cubic. A crystal family is determined by lattices and point groups, it is formed by combining crystal systems which have space groups assigned to a common lattice system. In three dimensions, the crystal families and systems are identical, except the hexagonal and trigonal crystal systems, which are combined into one hexagonal crystal family.
In total there are six crystal families: triclinic, orthorhombic, tetragonal and cubic. Spaces with less than three dimensions have the same 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 and hexagonal; the relation between three-dimensional crystal families, crystal systems and lattice systems is shown in the following table: Note: there is no "trigonal" lattice system. To avoid confusion of terminology, the term "trigonal lattice" is not used; the 7 crystal systems consist of 32 crystal classes as shown in the following table: The point symmetry of a structure can be further described as follows. Consider the points that make up the structure, reflect them all through a single point, so that becomes; this is the'inverted structure'. If the original structure and inverted structure are identical the structure is centrosymmetric. Otherwise it is non-centrosymmetric.
Still in the non-centrosymmetric case, the inverted structure can in some cases be rotated to align with the original structure. This is a non-centrosymmetric achiral structure. If the inverted structure cannot be rotated to align with the original structure the structure is chiral or enantiomorphic and its symmetry group is enantiomorphic. A direction is called polar if its two directional senses are physically different. A symmetry direction of a crystal, polar is called a polar axis. Groups containing a polar axis are called polar. A polar crystal possesses a unique polar axis; some geometrical or physical property is different at the two ends of this axis: for example, there might develop a dielectric polarization as in pyroelectric crystals. A polar axis can occur only in non-centrosymmetric structures. There cannot be a mirror plane or twofold axis perpendicular to the polar axis, because they would make the two directions of the axis equivalent; the crystal structures of chiral biological molecules can only occur in the 65 enantiomorphic space groups.
The distribution of the 14 Bravais lattices into lattice systems and crystal families is given in the following table. In geometry and crystallography, a Bravais lattice is a category of translative symmetry groups in three directions; such symmetry groups consist of translations by vectors of the form R = n1a1 + n2a2 + n3a3,where n1, n2, n3 are integers and a1, a2, a3 are three non-coplanar vectors, called primitive vectors. These lattices are classified by the space group of the lattice itself, viewed as a collection of points, they represent the maximum symmetry. All crystalline materials must, by definition, fit into one of these arrangements. For convenience a Bravais lattice is depicted by a unit cell, a factor 1, 2, 3 or 4 larger than the primitive cell. Depending on the symmetry of a crystal or other pattern, the fundamental domain is again smaller, up to a factor 48; the Bravais lattices were studied by Moritz Ludwig Frankenheim in 1842, who found that there we
Iron is a chemical element with symbol Fe and atomic number 26. It is a metal, that belongs to group 8 of the periodic table, it is by mass the most common element on Earth, forming much of Earth's inner core. It is the fourth most common element in the Earth's crust. Pure iron is rare on the Earth's crust being limited to meteorites. Iron ores are quite abundant, but extracting usable metal from them requires kilns or furnaces capable of reaching 1500 °C or higher, about 500 °C higher than what is enough to smelt copper. Humans started to dominate that process in Eurasia only about 2000 BCE, iron began to displace copper alloys for tools and weapons, in some regions, only around 1200 BCE; that event is considered the transition from the Bronze Age to the Iron Age. Iron alloys, such as steel and special steels are now by far the most common industrial metals, because of their mechanical properties and their low cost. Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts with oxygen and water to give brown to black hydrated iron oxides known as rust.
Unlike the oxides of some other metals, that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing fresh surfaces for corrosion. The body of an adult human contains about 3 to 5 grams of elemental iron in hemoglobin and myoglobin; these two proteins play essential roles in vertebrate metabolism oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals. Chemically, the most common oxidation states of iron are +2 and +3. Iron shares many properties of other transition metals, including the other group 8 elements and osmium. Iron forms compounds in a wide range of oxidation states, −2 to +7. Iron forms many coordination compounds. At least four allotropes of iron are known, conventionally denoted α, γ, δ, ε; the first three forms are observed at ordinary pressures.
As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a body-centered cubic crystal structure. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a face-centered cubic crystal structure, or austenite. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope; the physical properties of iron at high pressures and temperatures have been studied extensively, because of their relevance to theories about the cores of the Earth and other planets. Above 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into another hexagonal close-packed structure, known as ε-iron; the higher-temperature γ-phase changes into ε-iron, but does so at higher pressure. Some controversial experimental evidence exists for a stable β phase at pressures above 50 GPa and temperatures of at least 1500 K, it is supposed to have a double hcp structure. The inner core of the Earth is presumed to consist of an iron-nickel alloy with ε structure.
The melting and boiling points of iron, along with its enthalpy of atomization, are lower than those of the earlier 3d elements from scandium to chromium, showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus. This same trend appears for ruthenium but not osmium; the melting point of iron is experimentally well defined for pressures less than 50 GPa. For greater pressures, published data still varies by tens of gigapascals and over a thousand kelvin. Below its Curie point of 770 °C, α-iron changes from paramagnetic to ferromagnetic: the spins of the two unpaired electrons in each atom align with the spins of its neighbors, creating an overall magnetic field; this happens because the orbitals of those two electrons do not point toward neighboring atoms in the lattice, therefore are not involved in metallic bonding. In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into magnetic domains, about 10 micrometres across, such that the atoms in each domain have parallel spins, but different domains have other orientations.
Thus a macroscopic piece of iron will have a nearly zero overall magnetic field. Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field; this effect is exploited in devices that needs to channel magnetic fields, such as electrical transformers, magnetic recording heads, electric motors. Impurities, lattice defects, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists after the external field is removed -- thus turning the iron object into a magnet. Similar behavior is exhibited by some iron compounds, such as the fer
Johan Gadolin was a Finnish chemist and mineralogist. Gadolin discovered a "new earth" containing the first rare-earth compound yttrium, determined to be a chemical element, he is considered the founder of Finnish chemistry research, as the second holder of the Chair of Chemistry at the Royal Academy of Turku. Gadolin was ennobled for his achievements and awarded the Order of Saint Vladimir and the Order of Saint Anna. Johan Gadolin was born in Finland. Johan was the son of professor of physics and theology at Åbo. Johan began to study mathematics at the Royal Academy of Turku, he changed his major to chemistry, studying with Pehr Adrian Gadd, the first chair of chemistry at Åbo. In 1779 Gadolin moved to Uppsala University. In 1781, he published his dissertation Dissertatio chemica de analysi ferri, under the direction of Torbern Bergman. Bergman founded an important research school, many of his students, including Gadolin, Johan Gottlieb Gahn, Carl Wilhelm Scheele, became close friends. Gadolin was fluent in Latin, Russian, German and French in addition to his native Swedish.
He was a candidate for the chair of chemistry at Uppsala in 1784, but Johann Afzelius was selected instead. Gadolin became an extraordinary professor at Åbo in 1785. Beginning in 1786, he made a chemical "grand tour" of Europe, visiting universities and mines in various countries, he worked with Lorenz Crell, editor of the journal Chemische Annalen in Germany, with Adair Crawford and Richard Kirwan in Ireland. Gadolin was elected a member of the Royal Swedish Academy of Sciences in 1790. Gadolin became the ordinary professor of chemistry at the Royal Academy of Turku in 1797, after the death of Pehr Adrian Gadd, he retained the position until his retirement in 1822. He was one of the first chemists, he allowed the students to use his private laboratory. Gadolin made contributions in a variety of areas. Although he never visited France, he became a proponent of Antoine Lavoisier's theory of combustion. Gadolin's Inledning till Chemien was the first chemistry textbook in the Nordic countries that questioned the theory of phlogiston and discussed the role of oxygen in combustion in a modern way.
Gadolin studied the relationship of heat to chemical changes, in particular, the ability of different substances to absorb heat and the absorption of heat during state changes. This thermochemical work required precise measurements. Gadolin published important papers on specific heat by 1784, on the latent heat of steam in 1791, he demonstrated that the heat of ice was equal to the heat of snow, published a standard set of heat tables. "The best series of experiments on the distribution of heat among different bodies was performed before the year 1784 by Professor Gadolin of Åbo, rejecting the notion of Capacity, introduced the unexceptionable expression, Specific Heat. One of the most beautiful consequences derived from this theory, was the determination of the absolute zero or lowest point in the scale of Heat." Gadolin became famous for his description of yttrium. In 1792 Gadolin received a sample of black, heavy mineral found in a quarry in a Swedish village Ytterby near Stockholm by Carl Axel Arrhenius.
By careful experiments, Gadolin determined that 38% of the sample was a unknown "earth", an oxide, named yttria. Yttria, or yttrium oxide, was the first known rare-earth metal compound — at that time, it was not yet regarded as an element in the modern sense, his work was published in 1794. The mineral that Gadolin examined was named gadolinite in 1800; the element gadolinium and its oxide gadolinia were named after Gadolin by its discoverers. In an earlier paper in 1788 Gadolin showed that the same element can show several oxidation states, in his case Sn and Sn'by combining itself with larger or smaller amounts of the calcinating substance', he vividly described the disproportionation reaction: 2 Sn ⇌ Sn + Sn. Having established the composition of Prussian blue, Gadolin suggested a method for precipitating ferrous iron as ferro ferricyanide, preceding the work of Gay-Lussac by forty years. Reports of many of Gadolin’s chemical investigations appeared in German in Crell’s Chemische Annalen für die Freunde der Naturlehre, Haushaltungskeit und Manufacturen.
In 1825 he published Systema fossilium analysibus chemicis examinatorum secundum partium constitutivarum rationes ordinatorium, a system of mineral classification based upon chemical principles. The introduction outlines Gadolin's theories, the text presents mineral species in a systematic ordering. One of Gadolin's latest studies was the chemical analysis of the Chinese alloy pak tong in 1810 and 1827. Known as alpacca or German silver, it was a less expensive silver substitute containing copper, zinc and tin. Gadolin is famous for publishing one of the earliest examples of counter-current condensers. In 1791 he improved a condenser design of his father's by using the "counter-current principle". By requiring water coolant to flow uphill, the effectiveness of the condenser was increased; this principle was used by Justus Liebig, in what is today referred to as a Liebig condenser. Gadolin is registered under number 245 in the Finnish House of Nobility, he was made awarded the Order of Saint Vladimir and the Order of Saint Anna.
His heraldic device was: Argent, on a bend Azure with two mullets Or between a rose Gules
Euxenite or euxenite- is a brownish black mineral with a metallic luster. It contains calcium, tantalum, titanium and uranium and thorium, with some other metals; the chemical formula is: 2O6. It is partially amorphous due to radiation damage. Euxenite forms a continuous series with the titanium rich polycrase- having the following formula: 2O6 It was first described in 1870 and named for from the Greek, hospitable or friendly to strangers, in allusion to the many rare elements that it contains, it occurs in detrital black sands. It is found in many locations worldwide, notably its type locality in Jølster, Norway. Other locations include the Ural Mountains of Russia. Euxenite is used as an ore of the rare earth elements it contains. Rare large crystals have been used in jewelry
Conchoidal fracture describes the way that brittle materials break or fracture when they do not follow any natural planes of separation. Mindat.org defines conchoidal fracture as follows "a fracture with smooth, curved surfaces slightly concave, showing concentric undulations resembling the lines of growth of a shell". Materials that break in this way include quartz, flint, quartzite and other fine-grained or amorphous materials with a composition of pure silica, such as obsidian and window glass, as well as a few metals, such as solid gallium. Conchoidal fractures can occur in other materials under favorable circumstances; this material property was used in the Stone Age to make sharp tools, minerals that fractured in this fashion were traded as a desirable raw material. Conchoidal fractures result in a curved breakage surface that resembles the rippling, gradual curves of a mussel shell. A swelling appears at the point of impact called the bulb of percussion. Shock waves emanating outwards from this point leave their mark on the stone as ripples.
Other conchoidal features include small fissures emanating from the bulb of percussion. They are defined in contrast to the faceted fractures seen in single crystals such as semiconductor wafers and gemstones, the high-energy ductile fracture surfaces desirable in most structural applications. Several subdefinitions exist, for instance on the Webmineral website: Brittle - conchoidal - brittle fracture producing small, conchoidal fragments Brittle - subconchoidal - brittle fracture with subconchoidal fragments Conchoidal - irregular - irregular fracture producing small, conchoidal fragments Conchoidal - uneven - uneven fracture producing small, conchoidal fragments Subconchoidal - fractures developed in brittle materials characterized by semi-curving surfaces In lithic stone tools, conchoidal fractures form the basis of flint knapping, since the shape of the broken surface is controlled only by the stresses applied, not by some preferred orientation of the material; this property makes such fractures useful in engineering, since they provide a permanent record of the stress state at the time of failure.
As conchoidal fractures can be produced only by mechanical impact, rather than frost cracking for example, they can be a useful method of differentiating prehistoric stone tools from natural stones. Fracture The dictionary definition of conchoid at Wiktionary