Anorthosite
Image: James St. John (CC BY 2.0)
Mineral NameAnorthosite
Chemical FormulaNaAlSi3O8 to CaAl2Si2O8 (Plagioclase Feldspar)
Mohs Hardness6
Streak ColorWhite
CleavageGood in two directions at nearly 90 degrees (from plagioclase)
FractureUneven to conchoidal
Specific Gravity2.7
Colorslight grey, whitish, greenish-grey
TransparencyTranslucent to opaque
Type LocalitySplit Rock Lighthouse, Minnesota, USA
Mineral GroupIgneous Rock (Plutonic); dominated by feldspar group minerals
Usesdimension stone, aggregate, building material
Associated Mineralsplagioclase feldspar, pyroxene, olivine, magnetite, ilmenite
Raritycommon
Description
A coarsely-crystalline, light grey to whitish intrusive igneous rock, characterized by interlocking plagioclase feldspar crystals, exhibiting a rough and somewhat weathered surface.
Geological Context
Anorthosite forms from the slow cooling of plagioclase-rich magma deep within the Earth's crust. This sample is a Precambrian inclusion within a Mesoproterozoic diabase intrusion, indicating ancient magmatic activity.
