Andalusite

Category  :  Mineral
Hardness  :  7 - 7.5
Gravity  :  3.17 (+/- .04)
Reflection  :  1.64 / 1.63
Color  :  transparent to opaque brownish or yellowish green to orangy brown; may be pure green, brown, pink, violet (rare), and red
Chemical formula  :  Al2SiO5
Crystal system  :  orthorhombic
Major varieties  :  Andalusite

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. 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.

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.

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