Andalusite

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Andalusite
Dark, rod-like andalusite crystals in a light green cordierite schist matrix (writing pen shows size).
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.

1 Occurrence
2 Related minerals
3 See also
4 References
5 Notes

[edit] Occurrence

Andalusite is a common regional metamorphic mineral which forms under low pressure and moderate to high temperatures. 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.