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
Colorsgreenish-brown, brown, gray
Transparencytranslucent
Type LocalityLüsens valley (Lisens valley; Lüsens alp; Lisens alp), Sellrain valley, North Tyrol, Tyrol, Austria
Mineral GroupSilicates
Usesrefractory material, gemstone, indicator mineral
Associated Mineralsquartz, muscovite, biotite, cordierite
Rarityuncommon
Description
This specimen displays several blocky, greenish-brown andalusite crystals, some with a rectangular cross-section, embedded in a grayish, fine-grained matrix. The crystals exhibit a sub-vitreous to dull luster.
Geological Context
Andalusite is a metamorphic mineral typically found in pelitic rocks that have undergone regional or contact metamorphism under conditions of relatively low pressure and moderate to high temperature. It is one of the aluminum silicate polymorphs, indicating specific pressure-temperature conditions of formation.
