orthoklaas

Orthoklaas is the Dutch name for the mineral orthoclase, a potassium feldspar with the chemical composition KAlSi3O8. It is a major constituent of many felsic igneous rocks and belongs to the alkali feldspar group. The name derives from Greek orthos, meaning straight, and klasis, meaning fracture, referring to its cleavage characteristics.

Crystallography and appearance: Orthoclase crystallizes in the monoclinic system and commonly forms tabular or prismatic crystals. It has a Mohs hardness of about 6 and two cleavages at nearly 90 degrees. The luster is vitreous, and colors range from colorless to white, with pink to peach hues produced by impurities. In rocks, it often occurs in perthitic intergrowths with microcline.

Geologic occurrence: It forms during crystallization of silica-rich magmas and is abundant in granites, granodiorites, rhyolites, and pegmatites. It is also found in some metamorphic rocks as a late-stage product of felsic systems.

Variants and relations: Orthoclase is one of the potassium feldspars; at lower temperatures it can exsolve into microcline, and at higher temperatures it is related to sanidine as a high-temperature polymorph. In granitoids, orthoclase and microcline commonly occur together as perthite, reflecting cooling history and exsolution textures.

Uses and significance: Feldspars, including orthoclase, are important industrial minerals used in ceramics and glass manufacture as fluxes and functional fillers. Pure mineral samples are mainly of interest to collectors and researchers, while the bulk material is typically used in industrial applications.