mineral

Feldspar (specifically Orthoclase)

Potassium Aluminum Silicate (KAlSi3O8)

Hardness: 6.0 on Mohs scale; Color: Pink, reddish-brown, tan, or white; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Two planes at nearly right angles; Specific gravity: 2.55-2.63

Hardness: 6
Color: Pink, reddish-brown, tan, or white
Luster: Vitreous to pearly

Identified March 17, 2026More mineral →

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Physical properties

Formation & geological history

Formed through the cooling of silica-rich igneous magma or through metamorphism of existing rocks; commonly found in granite and pegmatite formations. Age varies greatly based on the host rock, from Ma to several Ga.

Uses & applications

Used in the manufacturing of glass and ceramics (as a flux); also used as a filler in paints, plastics, and rubber. Some high-quality varieties like moonstone or sunstone are used in jewelry.

Geological facts

Feldspar is the most abundant mineral group in the Earth's crust, making up about 60% of its composition. The name comes from the German words 'Feld' (field) and 'Spath' (flake).

Field identification & locations

Identify in the field by its blocky shape, two directions of cleavage at 90 degrees, and its hardness (it will scratch glass but not quartz). Commonly found in granite-rich mountain ranges and quarries. Use a hand lens to look for internal 'exsolution lamellae' streaks.

Other mineral specimens

See all

Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Epidote

Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)

metamorphic

Nephrite Jade

Nephrite

Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock

Green Apatite on Albite

Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]

mineral

Epidote

Epidote - Ca2(Al2,Fe3+)3(SiO4)3(OH)

mineral