mineral

Orthoclase Feldspar

Orthoclase (KAlSi3O8)

Hardness: 6 (Mohs scale); Color: White, colorless, flesh-pink, grayish, or yellowish-white; Luster: Vitreous to pearly on cleavage faces; Crystal Structure: Monoclinic; Cleavage: Two directions at nearly 90 degrees; Specific Gravity: 2.55-2.63

Hardness: 6 (Mohs scale)
Color: White, colorless, flesh-pink, grayish, or yellowish-white
Luster: Vitreous to pearly on cleavage faces

Identified March 22, 2026More mineral →

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Physical properties

Formation & geological history

Formed through the cooling of molten rock (magma) or through hydrothermal activity. It is a major constituent of granitic igneous rocks and is found in pegmatites and metamorphic rocks primarily from the Precambrian to Cenozoic eras.

Uses & applications

Major raw material for making glass and ceramics (porcelain); used as a mild abrasive in scouring powders; gem-quality varieties like moonstone are used in jewelry.

Geological facts

Orthoclase is one of the ten index minerals in the Mohs scale of mineral hardness. The name is derived from the Greek for 'straight fracture,' referring to its two cleavage planes at right angles to each other.

Field identification & locations

Identify in the field by its blocky shape, 6.0 hardness (will scratch glass but not quartz), and its characteristic right-angle cleavage. Common in granite-rich regions like the Appalachians or the Rockies. For collectors, look for pearly luster on flat surfaces.