» This article is about a mineral. For the Malcolm in the Middle character see list of 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.Feldspar. What is Feldspar? Industrial Minerals Association. Retrieved on July 18, 2007. Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock."Metamorphic Rocks."… (More on Feldspar)