Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight.[2][3] Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock.[4] Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite.[5] Feldspars are also found in many types of sedimentary rocks.