Feldspar is the name of a group of rock-forming tectosilicate minerals that make up as much as 60% of the Earth's crust.
Feldspar forms crystals from magma in both intrusive and extrusive rocks, and they can also happen as compact minerals, as veins, and are also present in many types of metamorphic rock. Rock formed entirely of plagioclase feldspar is known as anorthosite. Feldspar is also found in many types of sedimentary rock.
Feldspar is typically reddish or pale pink in color, and has a hardness of 6 on the Mohs scale, making it roughly as hard to scratch as glass.
There are two groups of feldspar, both made out of silica and aluminum:
- Orthoclase or "alkali" feldspars: a group of minerals that all have the same formula (KAlSi3O8 with sodium sometimes replacing the potassium in some pieces), but are formed in different ways because of how different the times and places where they formed were.
- Plagioclase feldspars: A series of minerals that are made up of a series of two chemicals (Albite, NaAlSi3O8, and anorthite CaAl2Si2O8), with a given piece having its place on the series determined by the relative amounts of the two chemicals.