Andalusite and Chiastolite


Staff member
Jun 3, 2020
Andalusite is an Aluminosilicate index mineral which provides the conditions of formation of its host rock. It is a common metamorphic mineral and trimorphic with Kyanite and Sillimanite. They occur under different temperature-pressure regimes and are therefore rarely found together in the same rock. Andalusite forms under low pressure and low to high temperatures. Sillimanite is the high-temperature mineral, and kyanite forms at high pressures and lower temperatures. Andalusite forms during the regional metamorphism of shale and is found in both schist and gneiss. It also forms during the contact metamorphism of shale.


Chiastolite is a variety of andalusite with black particles of graphite arranged in geometric patterns. A cross-shaped pattern is formed in the mineral due to these inclusions. Chiastolite has been valued by people for their perceived religious or spiritual meaning and collected as souvenirs.

Andalusite can withstand high temperatures without alteration. This property makes it useful in high-temperature ceramics and refractories. The white porcelain of some spark plugs is made using andalusite. The world's largest portion of andalusite deposits are in South Africa. The attractive specimens are used as gems.

Physical Properties
Crystal SystemOrthorhombic
ColorReddish brown, olive green, white to gray
Mohs Scale Hardness6.5 to 7.5
DiaphaneityTransparent to nearly opaque
Specific Gravity3.17
Chemical CompositionAl2SiO5
Diagnostic PropertiesCrystal form, associated minerals, strongly pleochroic, symmetrical inclusions

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