mineral

Orthoclase Feldspar

KAlSi3O8 (Potassium Aluminum Silicate)

Hardness: 6.0 on Mohs scale; Color: Pink, flesh-red, tan, or yellowish-white; Luster: Vitreous to pearly; Crystal System: Monoclinic; Cleavage: Two directions meeting at 90 degrees; Specific Gravity: 2.55-2.63.

Hardness: 6
Color: Pink, flesh-red, tan, or yellowish-white
Luster: Vitreous to pearly

Identified July 2, 2026More mineral →

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Physical properties

Formation & geological history

Forms primarily in igneous rocks such as granite and syenite through the cooling of silica-rich magma; also found in metamorphic rocks like gneiss and in sedimentary arkose sandstones.

Uses & applications

Major ingredient in the manufacture of glass and ceramics (porcelain, floor tiles, and sanitary ware). Rare clear varieties are used as gemstones (Moonstone).

Geological facts

Orthoclase is the defining mineral for level 6 on the Mohs scale of mineral hardness. The name comes from the Greek for 'straight fracture,' referring to its two cleavage planes at right angles to each other.

Field identification & locations

Identifiable by its blocky shape, pearly luster on cleavage faces, and pinkish-tan color. Look for parallel lines (exsolution lamellae) on surface faces. Extremely common in continental crust worldwide.

Other mineral specimens

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Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Schist

Metamorphic

Epidote

Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)

metamorphic

Gypsum (variety Selenite or Alabaster)

Calcium sulfate dihydrate (CaSO4·2H2O)

Mineral

Nephrite Jade

Nephrite

Mineral/Rock

Granite

Granite (Phaneritic intrusive igneous rock)

igneous