Andalusite

Andalusite
Andalusite, Tyrol Austria.
General
Category	Nesosilicates
Formula (repeating unit)	Al2SiO5
Strunz classification	09.AF.10
Crystal symmetry	2/m 2/m 2/m - Dipyramidal
Unit cell	a = 7.7980 Å b = 7.9031 Å c = 5.5566 Å; Z = 4
Identification
Color	Pink, violet, yellow, green, white, gray; in thin section, colorless to pink or green
Crystal habit	As euhedral crystals or columnar aggregates having nearly square cross sections; fibrous compact to massive
Crystal system	orthorhombic
Twinning	On {101}, rare
Cleavage	Good on {110}, poor on {100}
Fracture	uneven to subconchoidal
Mohs scale hardness	6.5 - 7.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to nearly opaque with inclusions
Specific gravity	3.17 (+/- .04)
Optical properties	double refractive, biaxial negative; chiastolite has anomalous aggregate reaction.[1]
Refractive index	nα = 1.629 - 1.640 nβ = 1.633 - 1.644 nγ = 1.638 - 1.650
Birefringence	δ = 0.009 - 0.010
Pleochroism	Weak
2V angle	71 - 86°
Dispersion	r < v strong
Ultraviolet fluorescence	non-fluorescent
References	[1][2][3][4]

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 a gemstone.^[5] 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.

Occurrence

Andalusite is a common regional metamorphic mineral which forms under low pressure and low 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. An example rock includes hornfels.
It was first described and named after the type locality in the Ronda Massif, Málaga, Andalusia, Spain in 1789.^[3][4]