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07/02/2009, 01:31 AM
What does mean Andalusite¿
Andalusite
Occurrence
It was first reported from Andalucia, Spain in 1789.
Andalusite is a common regional metamorphic mineral which forms under low pressure and moderate to high temperatures. Called Lapis Crucifer in ancient texts. The minerals kyanite and sillimanite are polymorphs of andalusite, each occurring under different temperature-pressure regimes and are therefore rarely found together in the same rock. Because of this the three minerals are a useful tool to help identify the pressure-temperature paths of the host rock in which they are found.
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.
The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section.
A clear variety first found in Andalusia, Spain can be cut into an interesting gemstone.[2] 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.
Category: mineral
Chemical formula : Al2SiO5[1]
Identification:
Color: transparent to opaque brownish or yellowish green to orangy brown;[1] may be pure green,[1] brown,[1] pink,[1] violet (rare),[1] and red
Crystal system : orthorhombic[1]
Twinning : lamellar
Cleavage : distinct in one direction[1]
Fracture : uneven to conchoidal[1]
Mohs Scale hardness 7 - 7.5[1]
Luster : vitreous[1]
Polish luster: vitreous[1]
Refractive index : 1.634 - 1.643 (+/-.005)[1]
Optical Properties: double refractive, biaxial negative; chiastolite has anomalous aggregate reaction.[1] Can also display chatoyancy.
Birefringence : .007 - .013[1]
Dispersion : .016[1]
Pleochroism : Strong. Brownish to yellowish green and brownish orange to brownish red
Ultraviolet fluorescence : inert in long wave, inert to moderate green to yellowish green in short wave[1]
Specific gravity : 3.17 (+/- .04)
Andalusite
Dark, rod-like andalusite crystals in a light green cordierite schist matrix (writing pen shows size).
From Wikipedia, the free encyclopedia
http://upload.wikimedia.org/wikipedia/commons/b/b8/AndalusiteSchistUSGS.jpg
http://en.wikipedia.org/wiki/File:AndalusiteSchistUSGS.jpg
andalusite
(Al2SiO5), aluminum silicatemineral that occurs in relatively small amounts in various metamorphic rocks, particularly in altered sediments. It is found in commercial quantities in the Inyo Mountains, Mono county, Calif., in the United States; in Kazakstan; and in South Africa. Such deposits are mined as a raw material for refractories and porcelain used in spark plugs and other products. For detailed physical properties, see silicate mineral (table 2).
Andalusite of gem quality occurs as greenish or reddish pebbles in Minas Gerais, Brazil, and in Sri Lanka. The variety chiastolite (also called cross-stone, or macle), characteristic of clay slates near a granite contact, forms elongated prismatic crystals enclosing symmetrically arranged wedges of carbonaceous material. In crosssection, it shows a black cross on a grayish ground; polished cross sections of the mineral are sometimes worn as charms.
The silicates make up about 95 percent of the Earth's crust and upper mantle, occurring as the majorconstituents of most igneous rocks and in appreciable quantities in sedimentary and metamorphic varieties as well. They also are important constituents of lunar samples, meteorites, and most asteroids. In addition, planetary probes have detected their occurrence on the surfaces of Mercury, Venus, and Mars. Of the approximately 600 known silicate minerals, only the feldspars, amphiboles, pyroxenes, micas, olivines, feldspathoids, and zeolites are significant in rock formation.
The basic structural unit of all silicate minerals is the silicon tetrahedron in which one silicon atom is surrounded by and bonded to (i.e., coordinated with) four oxygen atoms, each at the corner of a regular tetrahedron. These SiO4 tetrahedral units can share oxygen atoms and be linked in a variety of ways, which results in different structures. The topology of these structures forms the basis for silicate classification. For example, sorosilicates are silicate minerals consisting of double tetrahedral groups in which one oxygen atom is shared by two tetrahedrons. Inosilicates show a single-chain structure wherein each tetrahedron shares twooxygen atoms. Phyllosilicates have a sheet structure in which each tetrahedron shares one oxygen atom with each of three other tetrahedrons. Tectosilicates show a three-dimensional network of tetrahedrons, with each tetrahedral unit sharing all of its oxygen atoms.
Details of the linkage of tetrahedrons became known early in the 20th century when X-ray diffraction made the determination of crystal structure possible. Prior to this, the classification of silicates was based on chemical and physical similarities, which often proved to be ambiguous. Although many properties of a silicate mineral group are determined by tetrahedral linkage, an equally important factor is the type and location of other atoms in the structure.
Silicate minerals can be thought of as three-dimensional arrays of oxygen atoms that contain void spaces (i.e., crystallographic sites) where various cations can enter. Besides the tetrahedral (4-fold coordination) sites, 6-fold, 8-fold, and 12-fold sites are common. A correlation exists between the size of a cation (a positively charged ion) and the type of site it can occupy: the larger the cation, the greater the coordination, because large cations have more surface area with which the oxygen atoms can make contact. Tetrahedral sites are generally occupied by silicon and aluminum; 6-fold sites by aluminum, iron, titanium, magnesium, lithium, manganese, andsodium; 8-fold sites by sodium, calcium, and potassium; and 12-foldsites by potassium. Elements of similar ionic size often substitute for one another. An aluminum ion, for example, is only slightly larger than a silicon ion, allowing substitution for silicon in both tetrahedral and 6-fold sites.
Ref:
http://en.wikipedia.org/wiki/Andalusite
Pictures:
1-Andalusite from Tirol, Austria
Courtesy of the Field Museum of Natural History, Chicago; photograph, John H. Gerard
2-silicate mineral
any of a large group of silicon-oxygencompounds that are widely distributed throughout much of the solar system.
Andalusite
Occurrence
It was first reported from Andalucia, Spain in 1789.
Andalusite is a common regional metamorphic mineral which forms under low pressure and moderate to high temperatures. Called Lapis Crucifer in ancient texts. The minerals kyanite and sillimanite are polymorphs of andalusite, each occurring under different temperature-pressure regimes and are therefore rarely found together in the same rock. Because of this the three minerals are a useful tool to help identify the pressure-temperature paths of the host rock in which they are found.
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.
The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section.
A clear variety first found in Andalusia, Spain can be cut into an interesting gemstone.[2] 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.
Category: mineral
Chemical formula : Al2SiO5[1]
Identification:
Color: transparent to opaque brownish or yellowish green to orangy brown;[1] may be pure green,[1] brown,[1] pink,[1] violet (rare),[1] and red
Crystal system : orthorhombic[1]
Twinning : lamellar
Cleavage : distinct in one direction[1]
Fracture : uneven to conchoidal[1]
Mohs Scale hardness 7 - 7.5[1]
Luster : vitreous[1]
Polish luster: vitreous[1]
Refractive index : 1.634 - 1.643 (+/-.005)[1]
Optical Properties: double refractive, biaxial negative; chiastolite has anomalous aggregate reaction.[1] Can also display chatoyancy.
Birefringence : .007 - .013[1]
Dispersion : .016[1]
Pleochroism : Strong. Brownish to yellowish green and brownish orange to brownish red
Ultraviolet fluorescence : inert in long wave, inert to moderate green to yellowish green in short wave[1]
Specific gravity : 3.17 (+/- .04)
Andalusite
Dark, rod-like andalusite crystals in a light green cordierite schist matrix (writing pen shows size).
From Wikipedia, the free encyclopedia
http://upload.wikimedia.org/wikipedia/commons/b/b8/AndalusiteSchistUSGS.jpg
http://en.wikipedia.org/wiki/File:AndalusiteSchistUSGS.jpg
andalusite
(Al2SiO5), aluminum silicatemineral that occurs in relatively small amounts in various metamorphic rocks, particularly in altered sediments. It is found in commercial quantities in the Inyo Mountains, Mono county, Calif., in the United States; in Kazakstan; and in South Africa. Such deposits are mined as a raw material for refractories and porcelain used in spark plugs and other products. For detailed physical properties, see silicate mineral (table 2).
Andalusite of gem quality occurs as greenish or reddish pebbles in Minas Gerais, Brazil, and in Sri Lanka. The variety chiastolite (also called cross-stone, or macle), characteristic of clay slates near a granite contact, forms elongated prismatic crystals enclosing symmetrically arranged wedges of carbonaceous material. In crosssection, it shows a black cross on a grayish ground; polished cross sections of the mineral are sometimes worn as charms.
The silicates make up about 95 percent of the Earth's crust and upper mantle, occurring as the majorconstituents of most igneous rocks and in appreciable quantities in sedimentary and metamorphic varieties as well. They also are important constituents of lunar samples, meteorites, and most asteroids. In addition, planetary probes have detected their occurrence on the surfaces of Mercury, Venus, and Mars. Of the approximately 600 known silicate minerals, only the feldspars, amphiboles, pyroxenes, micas, olivines, feldspathoids, and zeolites are significant in rock formation.
The basic structural unit of all silicate minerals is the silicon tetrahedron in which one silicon atom is surrounded by and bonded to (i.e., coordinated with) four oxygen atoms, each at the corner of a regular tetrahedron. These SiO4 tetrahedral units can share oxygen atoms and be linked in a variety of ways, which results in different structures. The topology of these structures forms the basis for silicate classification. For example, sorosilicates are silicate minerals consisting of double tetrahedral groups in which one oxygen atom is shared by two tetrahedrons. Inosilicates show a single-chain structure wherein each tetrahedron shares twooxygen atoms. Phyllosilicates have a sheet structure in which each tetrahedron shares one oxygen atom with each of three other tetrahedrons. Tectosilicates show a three-dimensional network of tetrahedrons, with each tetrahedral unit sharing all of its oxygen atoms.
Details of the linkage of tetrahedrons became known early in the 20th century when X-ray diffraction made the determination of crystal structure possible. Prior to this, the classification of silicates was based on chemical and physical similarities, which often proved to be ambiguous. Although many properties of a silicate mineral group are determined by tetrahedral linkage, an equally important factor is the type and location of other atoms in the structure.
Silicate minerals can be thought of as three-dimensional arrays of oxygen atoms that contain void spaces (i.e., crystallographic sites) where various cations can enter. Besides the tetrahedral (4-fold coordination) sites, 6-fold, 8-fold, and 12-fold sites are common. A correlation exists between the size of a cation (a positively charged ion) and the type of site it can occupy: the larger the cation, the greater the coordination, because large cations have more surface area with which the oxygen atoms can make contact. Tetrahedral sites are generally occupied by silicon and aluminum; 6-fold sites by aluminum, iron, titanium, magnesium, lithium, manganese, andsodium; 8-fold sites by sodium, calcium, and potassium; and 12-foldsites by potassium. Elements of similar ionic size often substitute for one another. An aluminum ion, for example, is only slightly larger than a silicon ion, allowing substitution for silicon in both tetrahedral and 6-fold sites.
Ref:
http://en.wikipedia.org/wiki/Andalusite
Pictures:
1-Andalusite from Tirol, Austria
Courtesy of the Field Museum of Natural History, Chicago; photograph, John H. Gerard
2-silicate mineral
any of a large group of silicon-oxygencompounds that are widely distributed throughout much of the solar system.