feldspars

Feldspars are a large and important group of rock-forming minerals within the tectosilicate (framework silicate) family. Their defining feature is a framework of silicon-oxygen tetrahedra in which aluminum can substitute for silicon, balanced by alkali or alkaline-earth cations in interstitial sites. The general chemical formula is (K, Na, Ca)AlSi3O8, with the specific composition determining the subgroup and member.

The feldspar group is divided into two main subgroups: potassium feldspars (K-feldspars) and plagioclase feldspars. K-feldspars include orthoclase, microcline, and sanidine. Plagioclase feldspars form a continuous solid solution series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), with intermediate members such as oligoclase and andesine; admixtures between sodium and calcium yield distinctive plagioclase varieties such as labradorite and bytownite. Microcline is the low-temperature, highly ordered form of K-feldspar, while orthoclase and sanidine are other crystallographic forms.

In rocks, feldspars are among the most abundant minerals, commonly found in igneous rocks (granites, rhyolites, diorites), as well as in metamorphic and some sedimentary rocks. They weather to clay minerals (such as kaolinite and illite) and dissolved ions, releasing potassium in the process. Physical properties include a Mohs hardness near 6 and two good cleavage directions at or near 90 degrees; plagioclase commonly shows striations on cleavage surfaces, while K-feldspars may display characteristic twinning in microcline.

Feldspars have significant economic use as industrial minerals, especially in glassmaking and ceramic industries, where they act as fluxes and fillers.