Staurolite
Image: Robert M. Lavinsky (CC BY-SA 3.0)
Mineral NameStaurolite
Chemical Formula(Fe,Mg)2Al9(Si,Al)4O22(OH)2
Mohs Hardness7
Crystal Systemmonoclinic
Lustervitreous
Streak Colorwhite to grayish
Cleavagedistinct on {010}, poor on {110}
Fracturesubconchoidal
Specific Gravity3.78
Colorsreddish-brown, dark brown, yellowish-brown, blackish-brown
Transparencytranslucent to opaque
Type LocalityRubelita, Minas Gerais, Southeast Region, Brazil
Mineral GroupSilicates
Usescollector, jewelry, good luck charm
Associated Mineralsgarnet, kyanite, sillimanite, andalusite, muscovite, biotite, quartz
Rarityuncommon
Description
This specimen features a dark reddish-brown, opaque staurolite crystal, prominently displaying its characteristic 90-degree cruciform twinning, which forms a distinct cross shape. The crystal has a somewhat rough, dull to resinous luster.
Geological Context
Staurolite is a nesosilicate mineral formed during regional metamorphism of argillaceous rocks, typically found in schists and gneisses. Its presence indicates medium- to high-grade metamorphic conditions, making it a valuable index mineral for geologists.
