Feldspar is the mineral group that includes orthoclase, microcline, and plagioclase.
Feldspars are aluminum silicate minerals containing potassium, sodium, calcium, or combinations of these elements. They are divided into two principal compositional families: the potassium feldspars, including orthoclase and microcline, and the plagioclase feldspars, which form a solid-solution series between sodium-rich albite and calcium-rich anorthite.
Feldspar is the most abundant mineral group in Earth’s crust. It occurs in many igneous rocks, including granite, basalt, and gabbro, and it is also common in metamorphic and sedimentary rocks. Its crystals commonly show two cleavage directions meeting at close to right angles.
Feldspar can weather to clay minerals, helping produce soils and sediment. Quartz is also abundant in continental crust, but quartz is chemically distinct: it is silicon dioxide and contains no aluminum, potassium, sodium, or calcium.