Feldspars (KAlSi3O8 NaAlSi3O8 CaAl2Si2O8) are a group of
rock-forming tectosilicate minerals that make up about *1% of the
Earth\'s continental crust by weight.[2]
Feldspars crystallize from magma as veins in both intrusive and
extrusive igneous rocks and are also present in many types of
metamorphic rock.[3] Rock formed almost entirely of calcic
plagioclase feldspar (see below) is known as anorthosite.[4]
Feldspars are also found in many types of sedimentary rocks.
