Potassium Feldspar

Potassium feldspar, or K-feldspar, is the group of feldspars built around the composition KAlSi3O8. It includes three main polymorphs that differ in structure and formation temperature: sanidine (high-temperature, volcanic), orthoclase (intermediate), and microcline (low-temperature, fully ordered).

It is one of the most abundant minerals in the crust and a defining ingredient of granite, syenite, and many gneisses. Its often distinctive salmon-pink to cream color makes it one of the easier rock-forming minerals to spot in granite.

Varieties include the blue-green gem amazonite (a microcline), the moonstone-forming adularia, and perthitic intergrowths with albite.

Potassium feldspar crystallizes from silica- and potassium-rich magmas, making it a hallmark of felsic igneous rocks such as granite, granodiorite, syenite, and rhyolite. The specific polymorph reflects cooling rate: rapidly cooled volcanic rocks contain sanidine, while slowly cooled plutonic and metamorphic rocks develop orthoclase and, with time, microcline.

It is also a major component of pegmatites, where it can form enormous crystals, and it occurs in some metamorphic and sedimentary (arkose) rocks. Adularia forms at low temperatures in hydrothermal veins. K-feldspar localities are global, with notable pegmatite and amazonite sources in Colorado, Brazil, Russia, and Madagascar.

Look for salmon-pink, cream, or white crystals with a hardness of 6, vitreous luster, white streak, and two cleavages meeting at about 90 degrees. Unlike plagioclase, potassium feldspar typically lacks fine twin striations, though microcline can show a cross-hatched (tartan) pattern under magnification.

It is distinguished from quartz by its cleavage (quartz has none) and lower hardness, and from plagioclase by the absence of albite striations and its frequent pink color. Simple Carlsbad twins are common in orthoclase. Amazonite's blue-green color is diagnostic for that microcline variety.

Potassium feldspar is an industrial workhorse: it is a primary raw material in ceramics, porcelain, glass, glazes, and enamel, and is used as a mild abrasive and filler. Pegmatite K-feldspar has been mined in large tonnages for these uses.

Gem and ornamental varieties include amazonite, moonstone (adularia), and ornamental pink feldspar. In the metaphysical trade, amazonite and moonstone carry their own lore of communication and intuition. K-feldspar is also widely used in geology for potassium-argon and argon-argon radiometric dating.