Microclina

Microclina, commonly referred to as microcline in English, is a triclinic potassium feldspar mineral with the chemical formula KAlSi3O8. It is one of the main potassium feldspar polymorphs, along with orthoclase and sanidine, and represents the low‑temperature form of the K‑feldspar solid solution.

Formation and occurrence

Microclina forms during the final stages of crystallization in felsic igneous rocks and is especially common in granitic pegmatites and metamorphosed granitoids. It often occurs as well-formed crystals within granites and related rocks. A characteristic feature is perthitic texture, produced by exsolution of albite lamellae within the K‑feldspar crystal, giving a mottled or acid‑etched appearance that can reveal a granular or shimmering interior.

Properties and identifying features

Microclina is typically colorless to white, though it can display pink, cream, or brown hues. It has a hardness of about 6 on the Mohs scale and exhibits two cleavages at approximately 90 degrees, like other feldspars. The luster is vitreous, and the mineral is usually transparent to translucent in fine crystals. The green gemstone variety amazonite is a well-known variety of microcline colored by trace elements such as copper and lead.

Uses and significance

As a major constituent of granitic rocks, microclina contributes to the potassium content used in ceramics and glass production. In petrography, its presence and perthitic texture help identify granitoid rocks and provide clues to their thermal histories. In addition to industrial uses, certain colored varieties like amazonite have long been valued as gemstones.