mineral

Orthoclase Feldspar

KAlSi3O8 (Potassium aluminum silicate)

AI-generated identification · may be incorrect

Hardness: 6 on Mohs scale; Color: Tan, peach, or off-white; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Two directions at 90 degrees; Specific gravity: 2.55-2.63.

Hardness: 6 on Mohs scale
Color: Tan, peach, or off-white
Luster: Vitreous to pearly

Identified July 2, 2026More mineral →

Explore Orthoclase Feldspar in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 6 on Mohs scale; Color: Tan, peach, or off-white; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Two directions at 90 degrees; Specific gravity: 2.55-2.63.

Formation & geological history

Forms primarily in igneous rocks like granite and syenite through the cooling of magma, and in metamorphic rocks like gneiss. Found in environments spanning from the Precambrian to recent eras.

Uses & applications

Used in the manufacture of glass and ceramics (porcelain), as a filler in paints and plastics, and occasionally as a gemstone (moonstone variety).

Geological facts

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

Field identification & locations

Identify by its blocky appearance and prominent 90-degree cleavage planes. Commonly found in granitic pegmatites worldwide, such as in the Appalachian Mountains or the Alps.