1. n. [Geology]

[alkali feldspar (K,Na)AlSi3O8]
[plagioclase feldspar NaAlSi3O8 - CaAl2Si2O8] A group of rock-forming silicate minerals that are essential constituents of igneous rocks and are common in sandstones. Feldspar can weather to form clay minerals. Feldspar can occur in all three major rock types and forms approximately 60% of the crust of the Earth.

See: felsic,  granite,  illite,  kaolinite,  metamorphic,  porosity,  sandstone,  sediment,  sedimentary