Structure : Orthorhombic
Hardness : 7.5
Specific Gravity : 3.16
Refractive index : 1.63 - 1.6
Doubly Refracting (bi-refraction) : 0.01
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 an interesting 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 regional metamorphic mineral which forms under low pressure and moderate to high temperatures. Called Lapis Crucifer in ancient texts. 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 was first reported from Andalucia, Spain in 1789.
Chemical formula Al2SiO5
Identification Color transparent to opaque brownish or yellowish green to orangy brown; may be pure green, brown, pink, violet (rare), and red
Crystal system orthorhombic
Cleavage distinct in one direction
Fracture uneven to conchoidal
Mohs Scale hardness 7 - 7.5
Luster vitreous Polish luster vitreous
Refractive index 1.634 - 1.643 (+/-.005)
Optical Properties double refractive, biaxial negative; chiastolite has anomalous aggregate reaction. Can also display chatoyancy.
Birefringence .007 - .013
Pleochroism Strong. Brownish to yellowish green and brownish orange to brownish red
Ultraviolet fluorescence inert in long wave, inert to moderate green to yellowish green in short wave
Specific gravity 3.17 (+/- .04)