Andalusite
Image: Robert M. Lavinsky (CC BY-SA 3.0)
Mineral NameAndalusite
Chemical FormulaAl2SiO5
Mohs Hardness6.5
Crystal Systemorthorhombic
Lustervitreous
Streak Colorwhite
Cleavagegood on {110}
Fractureuneven
Specific Gravity3.15
Colorswhite, gray, pink, red, brown, yellow, green, black
Transparencytransparent to opaque
Type LocalityTyrol, Austria
Mineral GroupSilicates
Usesrefractory material, gemstone, collector
Associated Mineralsquartz, muscovite, biotite, cordierite, kyanite, sillimanite, staurolite, garnet
Raritycommon
Description
This specimen is a chiastolite variety of andalusite, characterized by its brownish-grey color and a distinctive black cross pattern formed by carbonaceous inclusions within the prismatic crystal.
Geological Context
Andalusite is a common metamorphic mineral, typically forming in medium-grade regional metamorphism of pelitic rocks (shales and mudstones). It is one of three polymorphs of Al₂SiO₅, stable at lower temperatures and pressures.
