Feldspar (Orthoclase/Potassium Feldspar)

Physical Properties

Hardness: 6 (Mohs); Color: Pinkish-orange, peach, to tan; 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 intrusive and extrusive igneous rocks (like granite and rhyolite) through the cooling of magma. Also forms in metamorphic rocks such as gneiss. It is a major component of the Earth's crust.

Uses & Applications

Used in the manufacture of glass, ceramics (glazes and porcelain), and as a filler in paints and plastics. Finer specimens are used in jewelry as moonstone or sunstone.

Geological Facts

Feldspar accounts for approximately 60% of the Earth's crust. The name is derived from the German 'Feldspath', meaning 'field spar' because of its common appearance in rural fields.

Field Identification & Locations

Identify in the field by its distinct salmon-pink color, blocky rectangular cleavage, and hardness (will scratch glass but be scratched by quartz). Commonly found in granite outcrops and gravel beds worldwide.

Identified on: 5/18/2026

Mode: Standard