Feldspar

Feldspar is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium-sodium) feldspars. Feldspars make up about 60% of the Earth’s crust and 41% of the Earth’s continental crust by weight.

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 known as anorthosite. Feldspars also found in many types of sedimentary rocks.

The name derives from the German Feldspat, a compound of the words Feld, “field”, and Spat, “a rock that does not contain ore”. The change from Spat to -spar influenced by the English word “spar”, a synonym for “mineral”. Feldspathic refers to materials that contain feldspar. The alternate spelling, felspar, has largely fallen out of use.