Orthoclase Feldspar (Potassium Feldspar)

Physical Properties

Hardness: 6.0 on Mohs scale; Color: Pinkish-orange, salmon, or tan; Luster: Vitreous to pearly on cleavage surfaces; Crystal Structure: Monoclinic; Cleavage: Two directions at nearly 90 degrees; Specific Gravity: 2.55-2.63

Formation & Geological History

Formed through the cooling of silica-rich igneous magmas (granites, pegmatites) and within metamorphic rocks like gneiss. This specimen appears water-worn, likely from a fluvial (river) or glacial environment.

Uses & Applications

Used in the manufacture of glass, ceramics (enamel and porcelain), and as an abrasive in scouring powder. Transparent/vibrant varieties are used as gemstones (e.g., Moonstone).

Geological Facts

Orthoclase is one of the ten index minerals in the Mohs scale of mineral hardness. It is the defining mineral for a hardness of 6. It is a primary constituent of the Earth's continental crust.

Field Identification & Locations

Identify in the field by its distinct salmon-pink color, blocky cleavage planes that reflect light, and its inability to be scratched by a steel knife (but can be scratched by quartz). Commonly found in granite outcrops and creek beds.

Identified on: 4/27/2026

Mode: Standard