» This article's about a mineral. For the Malcolm in the Middle character, see List of characters in Malcolm in the Middle#Recurring characters. Feldspars (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust. Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock. Rock formed entirely of plagioclase feldspar (see below)'s k… (More on Feldspar)