mineral

Orthoclase Feldspar

Orthoclase (KAlSi3O8)

Hardness: 6 on Mohs scale; Color: Pink, flesh-colored, or yellowish-tan; Luster: Vitreous to pearly; Crystal Structure: Monoclinic; Cleavage: Two planes at right angles (90 degrees); Specific Gravity: 2.55-2.63

Hardness: 6 on Mohs scale
Color: Pink, flesh-colored, or yellowish-tan
Luster: Vitreous to pearly

Identified April 29, 2026More mineral →

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Physical properties

Formation & geological history

Formed as a primary constituent in igneous rocks like granite and syenite through the cooling of magma. It can also be found in high-grade metamorphic rocks and as grains in sedimentary rocks.

Uses & applications

Used in the manufacture of glass and ceramics (porcelain); sometimes used as a mild abrasive in scouring pads. High-quality varieties like moonstone are used in jewelry.

Geological facts

Orthoclase is one of the ten index minerals on 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.

Field identification & locations

Identify in the field by its distinct 90-degree cleavage, flesh-like color, and pearly luster. Found worldwide in granitic batholiths and pegmatites. Collectors look for well-defined crystal faces.