Andalusite

Common aluminosilicate mineral.
Andalusite is the low pressure mid temperature polymorph (trimorph) of kyanite and sillimanite. May convert to sillimanite at higher temperature and pressure.
Andalusite provides clues as to the temperature and pressure involved in the formation of it´s host rock and thus is an index mineral of aluminosilicates

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Chiastolite is an andalusite variety in which black graphite inclusions form a cross-shaped pattern.
chiastolith chiastolitechiastolith chiastolite

Origin of name: andalusite was named by French natural scientist Jean-Claude Delamétherie (1743-1817), who erroneously thought the origin to be in Andalusia, Spain.
However, it is assumed that the specimens, which Delamétherie analyzed, came from El Cardoso, near the town of Guadalajara in Castilla-La Mancha.
Thus the type locality should be El Cardoso, even though there are several deposits in Andalusia.
Quelle: Mindat.org

Synonyms and trade names: none
Chiastolite is sometimes falsely called cross-stone, a name which refers to staurolite

Can be confused with: other gemstones of similar colour like dravite (tourmaline), smoky quartz, zircon, epidote, kornerupine, enstatite
However, due to it`s strong pleochroism and by means of measuring optical data like refractive index and absorption spectrum, identification usually poses no problem for the gemmologist

Localities: common worldwide. Mindat lists 1221 deposits (as of January 2015) on all continents, even Antarctica. In Austria there are more than 30 known deposits.
Gem quality andalusite is quite rare. Important localities are Brazil (Espirito Santo and Minas Gerais) and Sri Lanka.
Good chiastolite is found in Australia, USA, Spain, Myanmar (Burma) and China

Handling: andalusite shows good cleavage. Sensitive to heat, brines and acids. Keep away from borax, boric acid and galvanic baths. Do not clean ultrasonically.

Worth knowing: andalusite owes it´s attractiveness to the strong pleochroism.

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Gemmological Properties of Andalusite

Formula:
Al2(SiO4)O
Crystal system:
orthorhombic
Mohs hardness:
6.5 - 7.5
Specific gravity:
3.13 - 3.21, average 3.17
Refractive index:
1.629 - 1.650
Max. Birefringence:
0.010
Dispersion:
0.016
Pleochroism:
strong, yellow, olive, brownish-red to red
Luminiscence:
weak
Lustre:
vitreous
Cleavage:
good
Fracture:
brittle, uneven to sub-conchoidal
Colour:
brown, red-brown, green