mineral

Orthoclase Feldspar

Orthoclase (KAlSi3O8)

Hardness: 6 on Mohs scale; Color: typically flesh-pink to reddish-brown (as seen here); Luster: Vitreous to pearly; Crystal Structure: Monoclinic; Cleavage: Two planes at right angles (90 degrees); Specific gravity: 2.55–2.63.

Hardness: 6 on Mohs scale
Color: typically flesh-pink to reddish-brown (as seen here)
Luster: Vitreous to pearly

Identified March 6, 2026More mineral →

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Physical properties

Formation & geological history

Formed primarily in plutonic igneous rocks like granite and syenite, as well as in metamorphic rocks like gneiss. This specimen appears to be a weathered crystal fragment from a pegmatite or granitic source.

Uses & applications

Used extensively in the manufacture of glass and ceramics (porcelain), as a filler in paints and plastics, and occasionally as a gemstone (Moonstone) when featuring adularescence.

Geological facts

Orthoclase is one of the most abundant minerals in the Earth's crust. Its name comes from the Greek 'orthos' meaning straight and 'klasis' meaning fracture, referring to Its two cleavages at right angles.

Field identification & locations

Identify in the field by its distinct pinkish-tan color and blocky cleavage planes. It cannot be scratched by a steel knife (H<5.5) but can scratch glass. Common in mountainous regions with granitic outcrops.