· Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rockforming tectosilie minerals that make up as much as 60% of the Earth's crust. Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock.