mineral

Orthoclase Feldspar

Potassium Feldspar (KAlSi3O8)

Hardness: 6 (Mohs scale); Color: Salmon pink, flesh-colored, or white; Luster: Vitreous to pearly; Crystal System: Monoclinic; Cleavage: Two planes at 90 degrees; Specific Gravity: 2.5-2.6

Hardness: 6 (Mohs scale)
Color: Salmon pink, flesh-colored, or white
Luster: Vitreous to pearly

Identified April 10, 2026More mineral →

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Physical properties

Hardness: 6 (Mohs scale); Color: Salmon pink, flesh-colored, or white; Luster: Vitreous to pearly; Crystal System: Monoclinic; Cleavage: Two planes at 90 degrees; Specific Gravity: 2.5-2.6

Formation & geological history

Formed as a primary mineral in igneous rocks like granite and syenite through the cooling of magma, as well as in metamorphic rocks like gneiss. It is a fundamental component of the Earth's crust.

Uses & applications

Used extensively in the manufacture of glass and ceramics (as a flux to lower melting points), as a filler in paints and plastics, and occasionally as a gemstone (moonstone or sunstone varieties).

Geological facts

Orthoclase is one of the index minerals for a hardness of 6 on the Mohs scale. The name comes from the Greek 'orthos' (straight) and 'klasis' (fracture), referring to its two cleavage planes at right angles to each other.

Field identification & locations

Identify in the field by its characteristic pinkish-orange color, 90-degree cleavage, and the absence of striations (which distinguishes it from plagioclase feldspar). It is found globally within granitic batholiths.