Andalusite Mineral Andalusite Mineral Andalusite_(Chiastolite)
Andalusite is a rock forming minerals and is an aluminium nesosilicate minerals with the chemical formula Al2SiO5.At higher temperatures and pressures, andalusite may convert to sillimanite.andalusite is an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock.
Physical Properties of Andalusite
Category Nesosilicates
Formula Al2SiO5
Crystal system Orthorhombic
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 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 nα = 1.629 – 1.640 nβ = 1.633 – 1.644 nγ = 1.638 – 1.650
Birefringence δ = 0.009 – 0.010
Pleochroism strongly trichroic
2V angle 71 – 86°
Dispersion r < v strong
Ultraviolet fluorescence
Occurrence of Andalusite
It is a common metamorphic mineral which forms under low pressure and low to high temperatures. The minerals kyaniteand 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.
What is Chiastolite ?
Chiastolite is a variety of andalusite that contains black particles of graphite arranged in geometric patterns. The graphite is pushed aside by crystal growth within a rock that is being metamorphosed. As growth occurs, the particles become concentrated at crystal interfaces. The result can be a cross-shaped pattern within the mineral – similar to the “cross-stone” shown in the photo here. People have known about these cross stones for centuries and have valued them for their perceived religious or spiritual meaning. Attractive specimens are often cut and polished for use as amulets, charms, and novelty gems.
Uses of Andalusite
The rare transparent variety of Andalusite is used as a minor gemstone. Chiastolite cross-sections are often cut and polishedto bring out the distinctive cross-shaped pattern, and Chiastolite crystals may also be cut along their cross-section and used in jewelry as a Christian symbol.