mineral

Orthoclase Feldspar

Orthoclase (KAlSi3O8)

Hardness: 6 on Mohs scale; Color: typically salmon-pink to flesh-red; 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: typically salmon-pink to flesh-red
Luster: Vitreous to pearly

Identified March 29, 2026More mineral →

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Physical properties

Hardness: 6 on Mohs scale; Color: typically salmon-pink to flesh-red; Luster: Vitreous to pearly; Crystal Structure: Monoclinic; Cleavage: Perfect at or near 90 degrees; Specific Gravity: 2.55–2.63

Formation & geological history

Formed as a primary mineral in igneous rocks such as granite and pegmatites through the slow cooling of magma. It can also form in metamorphic environments through the alteration of other minerals.

Uses & applications

Used in the manufacture of glass and ceramics (as a flux), in commercial scouring powders, and occasionally as a gemstone (Moonstone is a variety of orthoclase).

Geological facts

Orthoclase is one of the ten defining 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 to each other.

Field identification & locations

Identify in the field by its distinct pinkish-orange color, blocky cleavage, and hardness (it can scratch glass but is scratched by quartz). Commonly found in granite-rich regions like the Appalachian Mountains or the Ural Mountains.