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
Tourmaline is a crystalline boron silicate mineral compounded with elements such as aluminium, magnesium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gemstone comes in a wide variety of colors. According to the Madras Tamil Lexicon the name comes from the Sinhalese word "thoramalli" or "tōra- molli", applied to a group of gemstones found in Sri Lanka. According to the same source, the Tamil "tuvara-malli" and "toramalli" are derived from the Sinhalese root word; this etymology is given in other standard dictionaries including the Oxford English Dictionary. Brightly colored Sri Lankan gem tourmalines were brought to Europe in great quantities by the Dutch East India Company to satisfy a demand for curiosities and gems. At the time, it was not realised that schorl and tourmaline were the same mineral, as it was only about 1703 that it was discovered that some colored gems were not zircons. Tourmaline was sometimes called the "Ceylonese Magnet" because it could attract and repel hot ashes due to its pyroelectric properties.
Tourmalines were used by chemists in the 19th century to polarize light by shining rays onto a cut and polished surface of the gem. Encountered species and varieties: Schorl species: Brownish black to black—schorl,Dravite species: from the Drave district of Carinthia Dark yellow to brownish black—dravite,Elbaite species: named after the island of Elba, Italy Red or pinkish-red—rubellite variety, Light blue to bluish green—Brazilian indicolite variety, Green—verdelite or Brazilian emerald variety, Colorless—achroite variety; the most common species of tourmaline is the sodium iron endmember of the group. It may account for 95% or more of all tourmaline in nature; the early history of the mineral schorl shows that the name "schorl" was in use prior to 1400 because a village known today as Zschorlau was named "Schorl", the village had a nearby tin mine where, in addition to cassiterite, black tourmaline was found. The first description of schorl with the name "schürl" and its occurrence was written by Johannes Mathesius in 1562 under the title "Sarepta oder Bergpostill".
Up to about 1600, additional names used in the German language were "Schurel", "Schörle", "Schurl". Beginning in the 18th century, the name Schörl was used in the German-speaking area. In English, the names shorl and shirl were used in the 18th century. In the 19th century the names common schorl, schörl, schorl and iron tourmaline were the English words used for this mineral. Dravite called brown tourmaline, is the sodium magnesium rich tourmaline endmember. Uvite, in comparison, is a calcium magnesium tourmaline. Dravite forms multiple series, including schorl and elbaite; the name dravite was used for the first time by Gustav Tschermak, Professor of Mineralogy and Petrography at the University of Vienna, in his book Lehrbuch der Mineralogie for magnesium-rich tourmaline from village Dobrova near Unterdrauburg in the Drava river area, Austro-Hungarian Empire. Today this tourmaline locality at Dobrova, is a part of the Republic of Slovenia. Tschermak gave this tourmaline the name dravite, for the Drava river area, the district along the Drava River in Austria and Slovenia.
The chemical composition, given by Tschermak in 1884 for this dravite corresponds to the formula NaMg36B3Si6O27, in good agreement with the endmember formula of dravite as known today. Dravite varieties include the vanadium dravite. A lithium-tourmaline elbaite was one of three pegmatitic minerals from Utö, Sweden, in which the new alkali element lithium was determined in 1818 by Johan August Arfwedson for the first time. Elba Island, was one of the first localities where colored and colorless Li-tourmalines were extensively chemically analysed. In 1850 Karl Friedrich August Rammelsberg described fluorine in tourmaline for the first time. In 1870 he proved. In 1889 Scharitzer proposed the substitution of by F in red Li-tourmaline from Sušice, Czech Republic. In 1914 Vladimir Vernadsky proposed the name Elbait for lithium-, sodium-, aluminum-rich tourmaline from Elba Island, with the simplified formula HAl6B2Si4O21. Most the type material for elbaite was found at Fonte del Prete, San Piero in Campo, Campo nell'Elba, Elba Island, Province of Livorno, Italy.
In 1933 Winchell published an updated formula for elbaite, H8Na2Li3Al3B6Al12Si12O62, used to date written as NaAl633. The first crystal structure determination of a Li-rich tourmaline was published in 1972 by Donnay and Barton, performed on a pink elbaite from San Diego County, United States; the tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. Its composition varies because of isomorphous replacement, its general formula can be written as XY3Z63V3W,where: X = Ca, Na, K, ▢ = vacancy Y = Li, Mg, Fe2+, Mn2+, Zn, Al, Cr3+, V3+, Fe3+, Ti4+, vacancy Z = Mg, Al, Fe3+, Cr3+, V3+ T = Si, Al, B B = B, vacancy V = OH, O W = OH, F, OA revised nomenclature for the tourmaline group was published in 2011. Tourmaline is a six-member ring cyclosilicate having a trigonal crystal system, it occurs
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 Snell's Law. Translucency is a superset of transparency: it allows light to pass through, but does not follow Snell's 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. Transparent materials appear clear, with the overall appearance of one color, or any combination leading up to a brilliant spectrum of every color; the opposite property of translucency is opacity. When light encounters a material, it can interact with it in several different ways; these interactions depend on the nature of the material. Photons interact with an object by some combination of reflection and transmission; some materials, such as plate glass and clean water, transmit much of the light that falls on them and reflect little of it.
Many liquids and aqueous solutions are transparent. Absence of structural defects and molecular structure of most liquids are 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 white light frequencies, they absorb certain portions of the visible spectrum while reflecting others. The frequencies of the spectrum which are not absorbed are either reflected or transmitted for our physical observation; this is. The attenuation of light of all frequencies and wavelengths is due to the combined mechanisms of absorption and scattering. Transparency can provide perfect camouflage for animals able to achieve it; this is easier in turbid seawater than in good illumination. Many marine animals such as jellyfish are transparent. With regard to the absorption of light, primary material considerations include: At the electronic level, absorption in the ultraviolet and visible portions of the spectrum depends on whether the electron orbitals are spaced such that they can absorb a quantum of light of a specific frequency, does not violate selection rules.
For example, in most glasses, electrons have no available energy levels above them in range of that associated with visible light, or if they do, they violate selection rules, meaning there is no appreciable absorption in pure glasses, making them ideal transparent materials for windows in buildings. 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, on selection rules. Nitrogen and oxygen are not greenhouse gases because there is no absorption, but because there is no molecular dipole moment. With regard to the scattering of light, the most critical factor is the length 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 grain boundaries, crystallographic defects and microscopic pores. Organic materials: scattering centers include fiber and cell structures and boundaries. Diffuse reflection - Generally, when light strikes the surface of a solid material, it bounces off in all directions due to multiple reflections by the microscopic irregularities inside the material, by its surface, if it is rough. Diffuse reflection is characterized by omni-directional reflection angles. Most of the objects visible to the naked eye are identified via diffuse reflection. Another term used for this type of reflection is "light scattering". Light scattering from the surfaces of objects is our primary mechanism of physical observation. Light scattering in liquids and solids depends on the wavelength of the light being scattered. Limits to spatial scales of visibility therefore arise, depending on the frequency of the light wave and the physical dimension of the scattering center.
Visible light has a wavelength scale on the order of a half a micrometer. Scattering centers as small. Optical transparency in polycrystalline materials is limited by the amount of light, scattered by their microstructural features. Light scattering depends on the wavelength of the light. Limits to spatial scales of visibility therefore arise, depending on the frequency of the light wave and the physical dimension of the scattering center. For example, since visible light has a wavelength scale on the order of a micrometer, scattering centers will have dimensions on a similar spatial scale. Primary scattering centers in polycrystalline materi
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
Beryl is a mineral composed of beryllium aluminium cyclosilicate with the chemical formula Be3Al2Si6O18. Well-known varieties of beryl include aquamarine. Occurring, hexagonal crystals of beryl can be up to several meters in size, but terminated crystals are rare. Pure beryl is colorless, but it is tinted by impurities. Beryl is an ore source of beryllium; the name "beryl" is derived from Greek βήρυλλος beryllos which referred to a "precious blue-green color-of-sea-water stone". The term was adopted for the mineral beryl more exclusively; when the first eyeglasses were constructed in 13th century Italy, the lenses were made of beryl as glass could not be made clear enough. Glasses were named Brillen in German. Beryl of various colors is found most in granitic pegmatites, but occurs in mica schists in the Ural Mountains, limestone in Colombia. Beryl is associated with tin and tungsten ore bodies. Beryl is found in Europe in Norway, Germany, Sweden and Russia, as well as Brazil, Madagascar, Pakistan, South Africa, the United States, Zambia.
US beryl locations are in California, Connecticut, Idaho, New Hampshire, North Carolina, South Dakota and Utah. New England's pegmatites have produced some of the largest beryls found, including one massive crystal from the Bumpus Quarry in Albany, Maine with dimensions 5.5 by 1.2 m with a mass of around 18 metric tons. As of 1999, the world's largest known occurring crystal of any mineral is a crystal of beryl from Malakialina, Madagascar, 18 m long and 3.5 m in diameter, weighing 380,000 kg. Beryl belongs to the hexagonal crystal system. Beryl forms hexagonal columns but can occur in massive habits; as a cyclosilicate beryl incorporates rings of silicate tetrahedra of Si 6 O 18 that are arranged in columns along the C axis and as parallel layers perpendicular to the C axis, forming channels along the C axis. These channels permit a variety of ions, neutral atoms, molecules to be incorporated into the crystal thus disrupting the overall charge of the crystal permitting further substitutions in Aluminium and Beryllium sites in the crystal structure.
These impurities give rise to the variety of colors of beryl. Increasing alkali content within the silicate ring channels causes increases to the refractive indices and birefringence. Aquamarine is a cyan variety of beryl, it occurs at most localities. The gem-gravel placer deposits of Sri Lanka contain aquamarine. Green-yellow beryl, such as that occurring in Brazil, is sometimes called chrysolite aquamarine; the deep blue version of aquamarine is called maxixe. Maxixe is found in the country of Madagascar, its color fades to white when exposed to sunlight or is subjected to heat treatment, though the color returns with irradiation. The pale blue color of aquamarine is attributed to Fe2+. Fe3+ ions produce golden-yellow color, when both Fe2+ and Fe3+ are present, the color is a darker blue as in maxixe. Decoloration of maxixe by light or heat thus may be due to the charge transfer between Fe3+ and Fe2+. Dark-blue maxixe color can be produced in green, pink or yellow beryl by irradiating it with high-energy particles.
In the United States, aquamarines can be found at the summit of Mt. Antero in the Sawatch Range in central Colorado. In Wyoming, aquamarine has been discovered near Powder River Pass. Another location within the United States is the Sawtooth Range near Stanley, although the minerals are within a wilderness area which prevents collecting. In Brazil, there are mines in the states of Minas Gerais, Espírito Santo, Bahia, minorly in Rio Grande do Norte; the mines of Colombia, Madagascar, Malawi and Kenya produce aquamarine. The largest aquamarine of gemstone quality mined was found in Marambaia, Minas Gerais, Brazil, in 1910, it weighed over 110 kg, its dimensions were 48.5 cm long and 42 cm in diameter. The largest cut aquamarine gem is the Dom Pedro aquamarine, now housed in the Smithsonian Institution's National Museum of Natural History; the ancient Romans believed that aquamarine would protect against any dangers while travelling at sea, that it provided energy and cured laziness. Emerald is green beryl, sometimes vanadium.
Most emeralds are included, so their brittleness is classified as poor. The modern English word "emerald" comes via Middle English Emeraude, imported from modern French via Old French Ésmeraude and Medieval Latin Esmaraldus, from Latin smaragdus, from Greek σμάραγδος smaragdos meaning ‘green gem’, from Hebrew ברקת bareket, meaning ‘lightning flash’, referring to ‘emerald’, relating to Akkadian baraqtu, meaning ‘emerald’, relating to the Sanskrit word मरकत marakata, meaning ‘green’; the Semitic word אזמרגד izmargad, meaning ` emerald', is a back-loan. Emeralds in antiquity were mined by the Egyptians and in what is now Austria, as well as Swat in contemporary Pakistan. A ra
Phosphate minerals are those minerals that contain the tetrahedrally coordinated phosphate anion along with the substituting arsenate and vanadate. Chlorine and hydroxide anions that fit into the crystal structure; the phosphate class of minerals is a large and diverse group, only a few species are common. Phosphate rock is rock with high concentration of phosphate minerals, most of the apatite group, it is the major resource mined to produce phosphate fertilizers for the agriculture sector. Phosphate is used in animal feed supplements, food preservatives, anti-corrosion agents, fungicides, water treatment and metallurgy; the largest use of minerals mined for their phosphate content is the production of fertilizer. Phosphate minerals are used for control of rust and prevention of corrosion on ferrous materials applied with electrochemical conversion coatings. Phosphate minerals include: Triphylite LiPO4 Monazite PO4,rare earth metals Hinsdalite PbAl36 Pyromorphite Pb53Cl Vanadinite Pb53Cl Erythrite Co32·8H2O Amblygonite LiAlPO4F lazulite Al222 Wavellite Al323·5H2O Turquoise CuAl648·5H2O Autunite Ca22·10-12H2O Carnotite K222·3H2O Phosphophyllite Zn22•4H2O Struvite MgPO4·6H2O Xenotime-Y Y Apatite group Ca53 hydroxylapatite Ca53OH fluorapatite Ca53F chlorapatite Ca53Cl bromapatite Mitridatite group: Arseniosiderite-mitridatite series Arseniosiderite-robertsite series IMA-CNMNC proposes a new hierarchical scheme.
This list uses it to modify the classification of Nickel–Strunz. Abbreviations: "*" – discredited. "?" – questionable/doubtful. "REE" – Rare-earth element "PGE" – Platinum-group element 03. C Aluminofluorides, 06 Borates, 08 Vanadates, 09 Silicates: Neso: insular Soro: grouping Cyclo: ring Ino: chain Phyllo: sheet Tekto: three-dimensional framework Nickel–Strunz code scheme: NN. XY.##x NN: Nickel–Strunz mineral class number X: Nickel–Strunz mineral division letter Y: Nickel–Strunz mineral family letter ##x: Nickel–Strunz mineral/group number, x add-on letter 08. A Phosphates, etc. without additional anions, without H2O 08. AA With small cations: 05 Berlinite, 05 Rodolicoite. AB With medium-sized cations: 05 Farringtonite. AC With medium-sized and large cations: 10 IMA2008-054, 10 Alluaudite, 10 Hagendorfite, 10 Ferroalluaudite, 10 Maghagendorfite, 10 Varulite, 10 Ferrohagendorfite*. AD With only large cations: 05 Nahpoite, 10 Monetite, 15 Archerite, 15 Biphosphammite. 08. B Phosphates, etc. with Additional Anions, without H2O 08.
BA With small and medium-sized cations: 05 Vayrynenite. BB With only medium-sized cations,:RO4 £1:1: 05 Amblygonite, 05 Natromontebrasite?, 05 Montebrasite?, 05 Tavorite. BC With only medium-sized cations,:RO4 > 1:1 and < 2:1: 10 Plimerite, 10 Frondelite, 10 Rockbridgeite 08. BD With only medium-sized cations,:RO4 = 2:1: 05 Pseudomalachite, 05 Reichenbachite, 10 Gatehouseite, 25 Ludjibaite 08. BE With only medium-sized cations,:RO4 > 2:1: 05 Augelite, 10 Grattarolaite, 15 Cornetite, 30 Raadeite, 85 Waterhouseite 08. BF With medium-sized and large cations,:RO4 < 0.5:1: 05 Arrojadite, 05 Arrojadite-, 05 Arrojadite-, 05 Arrojadite-, 05 Arrojadite-, 05 Arrojadite-, 05 Arrojadite-, 05 Arrojadite-, 05 Fluorarrojadite-, 05 Fluorarrojadite-, 05 Fluorarrojadite-, 05 Ferri-arrojadite-, 05 Dickinsonite, 05 Dickinsonite-, 05 Dickinsonite-, 05 Dickinsonite-, 05 Dickinsonite-. BG With medium-sized and large cations,:RO4 = 0.5:1: 05 Bearthite, 05 Goedkenite, 05 Tsumebite. BH With medium-sized and large cations,:RO4 = 1:1: 05 Thadeuite.
Feldspars are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight. Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are present in many types of metamorphic rock. Rock formed entirely of calcic plagioclase feldspar is known as anorthosite. Feldspars are found in many types of sedimentary rocks; the name feldspar derives from the German Feldspat, a compound of the words Feld, "field", Spat meaning "a rock that does not contain ore". The change from Spat to -spar was influenced by the English word spar, meaning a non-opaque mineral with good cleavage. Feldspathic refers to materials; the alternate spelling, has fallen out of use. This group of minerals consists of tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers: potassium feldspar endmember KAlSi3O8, albite endmember NaAlSi3O8, anorthite endmember CaAl2Si2O8. Solid solutions between K-feldspar and albite are called "alkali feldspar".
Solid solutions between albite and anorthite are called "plagioclase", or more properly "plagioclase feldspar". Only limited solid solution occurs between K-feldspar and anorthite, in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the Earth. Albite is considered both alkali feldspar. Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminum, or silicon; the first of these include: orthoclase KAlSi3O8, sanidine AlSi3O8, microcline KAlSi3O8, anorthoclase AlSi3O8. Potassium and sodium feldspars are not miscible in the melt at low temperatures, therefore intermediate compositions of the alkali feldspars occur only in higher temperature environments. Sanidine is stable at the highest temperatures, microcline at the lowest. Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition; the perthitic textures in the alkali feldspars of many granites can be seen with the naked eye.
Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope. Barium feldspars are considered alkali feldspars. Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure; the barium feldspars are monoclinic and include the following: celsian BaAl2Si2O8, hyalophane 4O8. The plagioclase feldspars are triclinic; the plagioclase series follows: albite NaAlSi3O8, oligoclase AlSi2O8, andesine NaAlSi3O8—CaAl2Si2O8, labradorite AlSi2O8, bytownite AlSi2O8, anorthite CaAl2Si2O8. Intermediate compositions of plagioclase feldspar may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, the resulting two-feldspar intergrowths are too fine-grained to be visible with optical microscopes; the immiscibility gaps in the plagioclase solid solutions are complex compared to the gap in the alkali feldspars. The play of colours visible in some feldspar of labradorite composition is due to fine-grained exsolution lamellae.
The specific gravity in the plagioclase series increases from albite to anorthite. Chemical weathering of feldspars results in the formation of clay minerals such as illite and kaolinite. About 20 million tonnes of feldspar were produced in 2010 by three countries: Italy and China. Feldspar is a common raw material used in glassmaking, to some extent as a filler and extender in paint and rubber. In glassmaking, alumina from feldspar improves product hardness and resistance to chemical corrosion. In ceramics, the alkalis in feldspar act as a flux. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the other components of the system together. In the US, about 66% of feldspar is consumed in glassmaking, including glass containers and glass fiber. Ceramics and other uses, such as fillers, accounted for the remainder. In earth sciences and archaeology, feldspars are used for K-Ar dating, argon-argon dating, luminescence dating. In October 2012, the Mars Curiosity rover analyzed a rock that turned out to have a high feldspar content.
List of minerals – A list of minerals for which there are articles on Wikipedia List of countries by feldspar production This article incorporates public domain material from the United States Geological Survey document: "Feldspar and nepheline syenite". Bonewitz, Ronald Louis. Rock and Gem. New York: DK Publishing. ISBN 978-0-7566-3342-4. Media related to Feldspar at Wikimedia Commons