mineral

Orthoclase Feldspar (Twinning Specimen)

Orthoclase (KAlSi3O8)

Hardness: 6 (Mohs scale), Color: Flesh-pink to tan/brownish, Luster: Vitreous to pearly, Crystal structure: Monoclinic, Cleavage: Two directions at 90 degrees, Specific gravity: 2.55–2.63

Identified March 16, 2026More mineral →

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Physical properties

Hardness: 6 (Mohs scale), Color: Flesh-pink to tan/brownish, Luster: Vitreous to pearly, Crystal structure: Monoclinic, Cleavage: Two directions at 90 degrees, Specific gravity: 2.55–2.63

Formation & geological history

Formed primarily in granitic igneous rocks and pegmatites through the cooling of silica-rich magma; also found in metamorphic rocks. Commonly formed during the late stages of magmatic crystallization.

Uses & applications

Used in the manufacture of glass and ceramics (as a flux), in abrasive cleansers, and occasionally as gemstones (Moonstone variety) or architectural stone.

Geological facts

Orthoclase is a defining mineral for level 6 on the Mohs scale of mineral hardness. This specific specimen exhibits 'Carlsbad twinning,' where two crystals grow together in a symmetrical fashion, common in volcanic and plutonic rocks.

Field identification & locations

Identify in the field by its blocky shape, 90-degree cleavage planes, and lack of striations (which distinguish it from Plagioclase). Commonly found in granite-rich regions like the Appalachian Mountains or the Rockies.

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