Potassium Feldspar (Orthoclase)

Physical Properties

Hardness: 6 (Mohs); Color: Salmon-pink to reddish-orange; Luster: Vitreous to pearly on cleavage planes; Crystal Structure: Monoclinic; Cleavage: Two planes at 90 degrees; SG: 2.55-2.63

Formation & Geological History

Formed primarily in felsic igneous rocks like granite and rhyolite, as well as in pegmatites and metamorphic rocks. It crystallizes from cooling magma or during high-grade metamorphism.

Uses & Applications

Used in the manufacture of glass and ceramics (as a flux), as a mild abrasive in scouring powders, and occasionally as a gemstone (moonstone) or architectural stone.

Geological Facts

Orthoclase is one of the index minerals of the Mohs scale of mineral hardness (set at 6). It is the mineral that often gives pink granite its distinctive color.

Field Identification & Locations

Identify by the characteristic salmon-pink color and perfect cleavage at 90-degree angles. It is commonly found in mountain ranges with large granitic batholiths.

Identified on: 4/29/2026

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