Feldspars are a group of rock-forming tectosilicate minerals that make up about 40% of the Earth's continental crust.

Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock.Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rocks. (source)