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]
Specific gravity	3.17 (+/- .04)
Polish luster	vitreous[1]
Optical properties	double refractive, biaxial negative; chiastolite has anomalous aggregate reaction.[1] Can also display chatoyancy.
Refractive index	1.634 - 1.643 (+/-.005)[1]
Birefringence	.007 - .013[1]
Pleochroism	Strong. Brownish to yellowish green and brownish orange to brownish red
Dispersion	.016[1]
Ultraviolet fluorescence	inert in long wave, inert to moderate green to yellowish green in short wave[1]

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.

It is associated with mica schist which increases alkali content in ultimate product and so it has not been exploited economically so far.

Contents 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. 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.

It was first reported from Andalucia, Spain in 1789.

[edit] Related minerals

• Kyanite
• Sillimanite
• Chiastolite

[edit] See also

Wikimedia Commons has media related to: Andalusite

• List of minerals

[edit] References

• Mineral galleries
• Webmineral
• Mindat w/ locations
• http://www.gemstone.org/gem-by-gem/english/andalusite.html

[edit] Notes

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