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Andalusite
From Wikipedia, the free encyclopedia
| Andalusite | |
|---|---|
 Dark, rod-like andalusite crystals in a light green cordierite schist matrix (writing pen shows size).
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| General | |
| 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 is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.
The variety chiastolite commonly contains dark inclusions of carbon or clay which form an X or a cross in 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.
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Andalusite is a common regional metamorphic mineral which forms under high pressure and/or temperature. 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.
It was first reported from Andalucia, Spain in 1789.
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