feldspargroup

The feldspar group is a large group of rock-forming tectosilicate minerals that make up a substantial portion of Earth's crust. They are framework silicates in which aluminum substitutes for silicon in a three-dimensional silica network, balanced by alkali or alkaline-earth cations. The general composition of feldspars ranges from potassium and sodium-rich end-members to calcium-rich plagioclase, with a wide solid-solution series between them.

The group is divided into two primary subgroups: alkali feldspars and plagioclase feldspars. Alkali feldspars include orthoclase (KAlSi3O8), microcline, and sanidine, which are typically potassium-rich. Plagioclase feldspars form a continuous solid solution between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8) and include varieties such as oligoclase, andesine, labradorite, and bytownite. Diagnostic features include two cleavage directions at about 90 degrees, and distinctive twinning patterns, such as the cross-hatched twinning of microcline and the polysynthetic twinning of plagioclase.

Feldspars are among the most abundant minerals in igneous rocks, especially felsic rocks like granites and rhyolites, and they also occur in metamorphic and some sedimentary rocks. They crystallize from magmas and weather to clay minerals through chemical alteration. Composition of plagioclase records magmatic differentiation and can be used to infer tectonic and thermal histories; potassium feldspar is often used in radiometric dating (K–Ar and Ar–Ar systems).

Economically, feldspars are important industrial minerals. They are widely used as fluxes in glassmaking and ceramic bodies, as fillers in paints and plastics, and as sources of sodium and potassium oxides.