Andalusite

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 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. Refractive index n_α = 1.629 - 1.640 n_β = 1.633 - 1.644 n_γ = 1.638 - 1.650 Birefringence δ = 0.009 - 0.010 Pleochroism strongly trichroic 2V angle 71 - 86° Dispersion r < v strong Ultraviolet fluorescence non-fluorescent References

Andalusite is trimorphic with kyanite and sillimanite, being the lower pressure mid temperature polymorph. At higher temperatures and pressures, andalusite may convert to sillimanite. Thus, as with its other polymorphs, andalusite is an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock.

Kyanite

Andalusite

Sillimanite

Phase diagram of Al₂SiO₅
(nesosilicates).

Varieties

The variety chiastolite commonly contains dark inclusions of carbon or clay which form a cruciform pattern when shown in cross-section. This stone was known at least from the sixteenth century, being taken to many European countries, as a souvenir, by pilgrims returning from Santiago de Compostela.

A clear variety found in Brazil and Sri-Lanka can be cut into a gemstone. 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.

Occurrence

Andalusite is a common 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. It is particularly associated with mica schist.

Uses

Andalusite is used as a refractory in furnaces, kilns and other industrial processes. South Africa possesses by far the largest portion of the world's known andalusite deposits.

This article uses material from the Wikipedia article Metasyntactic variable, which is released under the Creative Commons Attribution-ShareAlike 3.0 Unported License.