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Andalusite



Andalusite

Dark, rod-like andalusite crystals in a light green cordierite schist matrix (writing pen shows size).
General
Categorymineral
Chemical formulaAl2SiO5[1]
Identification
Colortransparent 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 systemorthorhombic[1]
Twinninglamellar
Cleavagedistinct in one direction[1]
Fractureuneven to conchoidal[1]
Mohs Scale hardness7 - 7.5[1]
Lustervitreous[1]
Polish lustervitreous[1]
Refractive index1.634 - 1.643 (+/-.005)[1]
Optical Propertiesdouble refractive, biaxial negative; chiastolite has anomalous aggregate reaction.[1] Can also display chatoyancy.
Birefringence.007 - .013[1]
Dispersion.016[1]
PleochroismStrong. Brownish to yellowish green and brownish orange to brownish red
Ultraviolet fluorescenceinert in long wave, inert to moderate green to yellowish green in short wave[1]
Specific gravity3.17 (+/- .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.

Additional recommended knowledge

Contents

Occurrence

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.

Related minerals

See also

References

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

Notes

  1. ^ a b c d e f g h i j k l m n o p q Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
  2. ^ International Colored Gem Association: Anadalusite
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Andalusite". A list of authors is available in Wikipedia.
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