mineral

Orthoclase Feldspar

Potassium aluminum silicate (KAlSi3O8)

Hardness: 6 on Mohs scale; Color: Pink, salmon-orange, or flesh-colored; Luster: Vitreous to pearly; Crystal Structure: Monoclinic; Cleavage: Perfect at or near 90 degrees; Specific Gravity: 2.55-2.63.

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

Identified March 14, 2026More mineral →

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Physical properties

Formation & geological history

Primary constituent of igneous rocks like granite and syenite. It forms through the cooling of magma or in high-grade metamorphic environments. It can also be found in sedimentary arkose sandstones.

Uses & applications

Used in the manufacture of glass and ceramics (as a flux to lower melting points), as a mild abrasive in scouring powders, and occasionally as a gemstone (Moonstone is a variety of orthoclase).

Geological facts

Orthoclase is one of the ten defining minerals of 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 appearance, flat cleavage faces that reflect light, and its distinctive pinkish-orange 'flesh' color. Commonly found in mountain ranges with intrusive igneous bedrock like the Rockies or Appalachians.

Other mineral specimens

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Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Epidote

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

metamorphic

Nephrite Jade

Nephrite

Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock

Green Apatite on Albite

Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]

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Epidote

Epidote - Ca2(Al2,Fe3+)3(SiO4)3(OH)

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