The streak of a mineral is the color of the powder produced when it is dragged across an un-weathered surface. Unlike the apparent color of a mineral, which for most minerals can vary the trail of finely ground powder has a more consistent characteristic color, is thus an important diagnostic tool in mineral identification. If no streak seems to be made, the mineral's streak is said to be colorless. Streak is important as a diagnostic for opaque and colored materials, 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 because of trace impurities or a disturbed macroscopic crystal structure. Small amounts of an impurity that absorbs a particular wavelength can radically change the wavelengths of light that are reflected by the specimen, thus change the apparent color. However, when the specimen is dragged to produce a streak, it is broken into randomly oriented microscopic crystals, small impurities do not affect the absorption of light.
The surface across which the mineral is dragged is called a "streak plate", is made of unglazed porcelain tile. In the absence of a streak plate, the unglazed underside of a porcelain bowl or vase or the back of a glazed tile will work. Sometimes a streak is more or 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, a streak can only be made of minerals softer than the streak plate, around 7 on the Mohs scale of mineral hardness. For harder minerals, the color of the powder can be determined by filing or crushing with a hammer a small sample, usually rubbed on a streak plate. Most minerals that are harder have an unhelpful white streak; some minerals leave a streak similar to their natural color, such as lazurite. Other minerals leave surprising colors, such as fluorite, which always has a white streak, although it can appear in purple, yellow, or green crystals. Hematite, black in appearance, leaves a red streak which accounts for its name, which comes from the Greek word "haima", meaning "blood."
Galena, which can be similar in appearance to hematite, is distinguished by its gray streak. Bishop, A. C.. R.. R.. Cambridge Guide to Minerals and Fossils. Cambridge: Cambridge University Press. Pp. 12–13. Holden, Martin; the Encyclopedia of Gemstones and Minerals. New York: Facts on File. p. 251. ISBN 1-56799-949-2. Schumann, Walter. Minerals of the World. New York: Sterling Publishing. Pp. 18–16. ISBN 0-00-219909-2. Physical Characteristics of Minerals, at Introduction to Mineralogy by Andrea Bangert What is Streak? from the Mineral Gallery
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
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 quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are birefringent, as are plastics under mechanical stress. Birefringence is responsible for the phenomenon of double refraction whereby a ray of light, when incident upon a birefringent material, is split by polarization into two rays taking different paths; 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. However it was not until the 19th century that Augustin-Jean Fresnel described the phenomenon in terms of polarization, understanding light as a wave with field components in transverse polarizations. A mathematical description of wave propagation in a birefringent medium is presented below.
Following is a qualitative explanation of the phenomenon. The simplest type of birefringence is described as uniaxial, meaning that there is a single direction governing the optical anisotropy whereas all directions perpendicular to it are optically equivalent, thus rotating the material around this axis does not change its optical behavior. This special direction is known as the optic axis of the material. Light propagating parallel to the optic 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, this is called an ordinary ray. However, for ray directions not parallel to the optic axis, the polarization direction perpendicular to the ordinary ray's polarization will be in the direction of the optic axis, this is called an extraordinary ray. I.e. when unpolarized light enters an uniaxial birefringent material it is split into two beams travelling different directions.
The ordinary ray will always experience a refractive index of no, whereas the refractive index of the extraordinary ray will be in between no and ne, depending on the ray direction as described by the index ellipsoid. The magnitude of the difference is quantified by the birefringence: Δ n = n e − n o; the propagation of the ordinary ray is described by no as if there were no birefringence involved. However the extraordinary ray, as its name suggests, propagates unlike any wave in a homogenous optical material, its refraction at a surface can be understood using the effective refractive index. However it is in fact an inhomogeneous wave whose power flow is not in the direction of the wave vector; this causes an additional shift in that beam 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 extraordinary ray, to rotate around that of the ordinary ray, which remains fixed.
When the light propagates either along or orthogonal to the optic axis, such a lateral shift does not occur. 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. A crystal with its optic axis in this orientation, parallel to the optical surface, may be used to create a waveplate, in which there is no distortion of the image but an intentional modification of the state of polarization of the incident wave. For instance, a quarter-wave plate is used to create circular polarization from a linearly polarized source; the case of so-called biaxial crystals is more complex. 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, 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. Although there is no axis of symmetry, there are two optical axes or binormals which are defined as directions along which light may propagate without birefringence, i.e. directions along which the wavelength is independent of polarization. For this reason, birefringent materials with three distinct refractive indices are called biaxial. Additionally, there are two distinct axes known as optical ray axes or biradials along which the group velocity of the light is independent of polarization; when an arbitrary beam of light strikes the surface of a b
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, physical properties of minerals and mineralized artifacts. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization. Early writing on mineralogy on gemstones, comes from ancient Babylonia, the ancient Greco-Roman world and medieval China, Sanskrit texts from ancient India and the ancient Islamic World. Books on the subject included the Naturalis Historia of Pliny the Elder, which not only described many different minerals but explained many of their properties, Kitab al Jawahir by Persian scientist Al-Biruni; the German Renaissance specialist Georgius Agricola wrote works such as De re metallica and De Natura Fossilium which began the scientific approach to the subject. Systematic scientific studies of minerals and rocks developed in post-Renaissance Europe; the modern study of mineralogy was founded on the principles of crystallography and to the microscopic study of rock sections with the invention of the microscope in the 17th century.
Nicholas Steno first observed the law of constancy of interfacial angles in quartz crystals in 1669. This was generalized and established experimentally by Jean-Baptiste L. Romé de l'Islee in 1783. René Just Haüy, the "father of modern crystallography", showed that crystals are periodic and established that the orientations of crystal faces can be expressed in terms of rational numbers, as encoded in the Miller indices. In 1814, Jöns Jacob Berzelius introduced a classification of minerals based on their chemistry rather than their crystal structure. William Nicol developed the Nicol prism, which polarizes light, in 1827–1828 while studying fossilized wood. James D. Dana published his first edition of A System of Mineralogy in 1837, in a edition introduced a chemical classification, still the standard. X-ray diffraction was demonstrated by Max von Laue in 1912, developed into a tool for analyzing the crystal structure of minerals by the father/son team of William Henry Bragg and William Lawrence Bragg.
More driven by advances in experimental technique and available computational power, the latter of which has enabled accurate atomic-scale simulations of the behaviour of crystals, the science has branched out to consider more general problems in the fields of inorganic chemistry and solid-state physics. It, retains a focus on the crystal structures encountered in rock-forming minerals. In particular, the field has made great advances in the understanding of the relationship between the atomic-scale structure of minerals and their function. To this end, in their focus on the connection between atomic-scale phenomena and macroscopic properties, the mineral sciences display more of an overlap with materials science than any other discipline. An initial step in identifying a mineral is to examine its physical properties, many of which can be measured on a hand sample; these can be classified into density. Hardness is determined by comparison with other minerals. In the Mohs scale, a standard set of minerals are numbered in order of increasing hardness from 1 to 10.
A harder mineral will scratch a softer, so an unknown mineral can be placed in this scale by which minerals it scratches and which scratch it. A few minerals such as calcite and kyanite have a hardness that depends on direction. Hardness can be measured on an absolute scale using a sclerometer. Tenacity refers to the way a mineral behaves when it is broken, bent or torn. A mineral can be brittle, sectile, flexible or elastic. An important influence on tenacity is the type of chemical bond. Of the other measures of mechanical cohesion, cleavage is the tendency to break along certain crystallographic planes, it is described by the orientation of the plane in crystallographic nomenclature. Parting is the tendency to break along planes of weakness due to twinning or exsolution. Where these two kinds of break do not occur, fracture is a less orderly form that may be conchoidal, splintery, hackly, or uneven. If the mineral is well crystallized, it will have a distinctive crystal habit that reflects the crystal structure or internal arrangement of atoms.
It is affected by crystal defects and twinning. Many crystals are polymorphic, having more than
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency. Media having this common property may be termed dispersive media. Sometimes the term chromatic dispersion is used for specificity. Although the term is used in the field of optics to describe light and other electromagnetic waves, dispersion in the same sense can apply to any sort of wave motion such as acoustic dispersion in the case of sound and seismic waves, in gravity waves, for telecommunication signals along transmission lines or optical fiber. In optics, one important and familiar consequence of dispersion is the change in the angle of refraction of different colors of light, as seen in the spectrum produced by a dispersive prism and in chromatic aberration of lenses. Design of compound achromatic lenses, in which chromatic aberration is cancelled, uses a quantification of a glass's dispersion given by its Abbe number V, where lower Abbe numbers correspond to greater dispersion over the visible spectrum.
In some applications such as telecommunications, the absolute phase of a wave is not important but only the propagation of wave packets or "pulses". The most familiar example of dispersion is a rainbow, in which dispersion causes the spatial separation of a white light into components of different wavelengths. However, dispersion has an effect in many other circumstances: for example, group velocity dispersion causes pulses to spread in optical fibers, degrading signals over long distances. Most chromatic dispersion refers to bulk material dispersion, that is, the change in refractive index with optical frequency. However, in a waveguide there is the phenomenon of waveguide dispersion, in which case a wave's phase velocity in a structure depends on its frequency due to the structure's geometry. More "waveguide" dispersion can occur for waves propagating through any inhomogeneous structure, whether or not the waves are confined to some region. In a waveguide, both types of dispersion will be present, although they are not additive.
For example, in fiber optics the material and waveguide dispersion can cancel each other out to produce a zero-dispersion wavelength, important for fast fiber-optic communication. Material dispersion can be a desirable or undesirable effect in optical applications; the dispersion of light by glass prisms is used to construct spectrometers and spectroradiometers. Holographic gratings are used, as they allow more accurate discrimination of wavelengths. However, in lenses, dispersion causes chromatic aberration, an undesired effect that may degrade images in microscopes and photographic objectives; the phase velocity, v, of a wave in a given uniform medium is given by v = c n where c is the speed of light in a vacuum and n is the refractive index of the medium. In general, the refractive index is some function of the frequency f of the light, thus n = n, or alternatively, with respect to the wave's wavelength n = n; the wavelength dependence of a material's refractive index is quantified by its Abbe number or its coefficients in an empirical formula such as the Cauchy or Sellmeier equations.
Because of the Kramers–Kronig relations, the wavelength dependence of the real part of the refractive index is related to the material absorption, described by the imaginary part of the refractive index. In particular, for non-magnetic materials, the susceptibility χ that appears in the Kramers–Kronig relations is the electric susceptibility χe = n2 − 1; the most seen consequence of dispersion in optics is the separation of white light into a color spectrum by a prism. From Snell's law it can be seen that the angle of refraction of light in a prism depends on the refractive index of the prism material. Since that refractive index varies with wavelength, it follows that the angle that the light is refracted by will vary with wavelength, causing an angular separation of the colors known as angular dispersion. For visible light, refraction indices n of most transparent materials decrease with increasing wavelength λ: 1 < n < n < n, or alternatively: d n d λ < 0. In this case, the medium is said to have normal dispersion.
Whereas, if the index increases with increasing wavelength, the medium is said to have anomalous dispersion. At the interface of such a material with air or vacuum, Snell's law predicts that light incident at an angle θ to the normal will be refracted at an angle arcsin. Thus, blue light, with a higher refractive index, will be bent more than red light, resulting in the well-known rainbow pattern. Another consequence of dispersion manifests itself as a temporal effect; the formula v = c/n calculates the
The serpentine subgroup are greenish, brownish, or spotted minerals found in serpentinite rocks. They are used as a source of magnesium and asbestos, as a decorative stone; the name is thought to come from the greenish color being that of a serpent. The serpentine group describes a group of common rock-forming hydrous magnesium iron phyllosilicate minerals, resulting from the metamorphism of the minerals that are contained in ultramafic rocks, they may contain minor amounts of other elements including chromium, cobalt or nickel. In mineralogy and gemology, serpentine may refer to any of 20 varieties belonging to the serpentine group. Owing to admixture, these varieties are not always easy to individualize, distinctions are not made. There are three important mineral polymorphs of serpentine: antigorite and lizardite; the serpentine group of minerals are polymorphous, meaning that they have the same chemical formulae, but the atoms are arranged into different structures, or crystal lattices. Chrysotile, which has a fiberous habit, is one polymorph of serpentine and is one of the more important asbestos minerals.
Other polymorphs in the serpentine group may have a platy habit. Antigorite and lizardite are the polymorphs with platy habit. Many types of serpentine have been used for jewellery and hardstone carving, sometimes under the name false jade or Teton jade, their olive green colour and smooth or scaly appearance is the basis of the name from the Latin serpentinus, meaning "serpent rock," according to Best. They have their origins in metamorphic alterations of pyroxene. Serpentines may pseudomorphously replace other magnesium silicates. Alterations may be incomplete. Where they form a significant part of the land surface, the soil is unusually high in clay. Antigorite is the polymorph of serpentine that most forms during metamorphism of wet ultramafic rocks and is stable at the highest temperatures—to over 600 °C at depths of 60 km or so. In contrast and chrysotile form near the Earth's surface and break down at low temperatures well below 400 °C, it has been suggested that chrysotile is never stable relative to either of the other two serpentine polymorphs.
Samples of the oceanic crust and uppermost mantle from ocean basins document that ultramafic rocks there contain abundant serpentine. Antigorite contains water in about 13 percent by weight. Hence, antigorite may play an important role in the transport of water into the earth in subduction zones and in the subsequent release of water to create magmas in island arcs, some of the water may be carried to yet greater depths. Soils derived from serpentine are toxic to many plants, because of high levels of nickel and cobalt; the flora is very distinctive, with specialised, slow-growing species. Areas of serpentine-derived soil will show as strips of shrubland and open, scattered small trees within otherwise forested areas. Most serpentines are opaque to translucent, soft and susceptible to acids. All are massive in habit, never being found as single crystals. Lustre may be greasy or silky. Colours range from white to grey, yellow to green, brown to black, are splotchy or veined. Many are intergrown such as calcite and dolomite.
Occurrence is worldwide. Serpentines find use in industry for a number of purposes, such as railway ballasts, building materials, the asbestiform types find use as thermal and electrical insulation; the asbestos content can be released to the air when serpentine is excavated and if it is used as a road surface, forming a long term health hazard by breathing. Asbestos from serpentine can appear at low levels in water supplies through normal weathering processes, but there is as yet no identified health hazard associated with use or ingestion. In its natural state, some forms of serpentine react with carbon dioxide and re-release oxygen into the atmosphere; the more attractive and durable varieties are termed "noble" or "precious" serpentine and are used extensively as gems and in ornamental carvings. The town of Bhera in the historic Punjab province of the Indian subcontinent was known for centuries for finishing a pure form of green serpentine obtained from quarries in Afghanistan into lapidary work, ornamental sword hilts, dagger handles.
This high-grade serpentine ore was known as sang-i-yashm or to the English, false jade, was used for generations by Indian craftsmen for lapidary work. It is carved, taking a good polish, is said to have a pleasingly greasy feel. Less valuable serpentine ores of varying hardness and clarity are sometimes dyed to imitate jade. Misleading synonyms for this material include "Suzhou jade", "Styrian jade", "New jade". New Caledonian serpentine is rich in nickel; the Māori of New Zealand once carved beautiful objects from local serpentine, which they c
Axel Fredrik Cronstedt
Baron Axel Fredrik Cronstedt was a Swedish mineralogist and chemist who discovered nickel in 1751 as a mining expert with the Bureau of Mines. He found the mineral, which Cronstedt described as kupfernickel, in the cobalt mines of Los, Hälsingland, Sweden; this name arises because the ore has a similar appearance to copper and a mischievous sprite was supposed by miners to be the cause of their failure to extract copper from it. Cronstedt named it nickel in 1754, he was a pupil of the discoverer of cobalt. Cronstedt is one of the founders of modern mineralogy and is described as the founder by John Griffin in his 1827 A Practical Treatise on the Use of the Blowpipe, he remains to this day to be an outstanding idol for young swedes. Cronstedt discovered the mineral scheelite in 1751, he named the mineral tungsten. Carl Wilhelm Scheele suggested that a new metal could be extracted from the mineral. In English, this metal is now known as the element tungsten. In 1753, Cronstedt was elected a member of the Royal Swedish Academy of Sciences.
In 1756, Cronstedt coined the term zeolite after heating the mineral stilbite with a blowpipe flame. Gusenius, E M. "Beginnings of greatness in Swedish Chemistry. II. Axel Fredrick Cronstedt". Trans. Kans. Acad. Sci. Kansas Academy of Science. 72: 476–85. Doi:10.2307/3627648. ISSN 0022-8443. JSTOR 3627648. PMID 4918973. Cheetham, A. K.. Solid State Chemistry. Clarendon Press. ISBN 0-19-855165-7. A Practical Treatise on the Use of the Blowpipe by John Griffin, 1827, from Google Book Search