Orthoclase Feldspar
Orthoclase (Potassium Feldspar) · Mineral Specimen / Rock-Forming Mineral
Crystal System
Monoclinic; typically occurs in prismatic or tabular habits, or as massive granular aggregates.
Mineral Group
Silicates; Tectosilicate (Feldspar Group)
Chemical Formula
KAlSi3O8
Formation Process
Igneous; forms from the cooling of silica-rich magma in plutonic environments or volcanic rocks.
About This Crystal
This specimen appears as a massive, opaque to translucent fragment with a fleshy pink to creamy tan color. It exhibits a vitreous to pearly luster on its cleavage faces.
Physical Characteristics
Color: Pink, tan, or white; Streak: White; Cleavage: Perfect in two directions at nearly 90 degrees; Fracture: Conchoidal to uneven; Specific Gravity: 2.55-2.63; Fluorescence: Weak red or green under UV (sometimes inert).
Optical Properties
Refractive Index: 1.518-1.526; Birefringence: 0.005-0.008; Optically biaxial; sometimes exhibits adularescence (moonstone effect) in high-quality varieties.
Hardness & Durability
Origin Region
Found worldwide; notable sources include Madagascar, Russia, Sri Lanka, and the USA. Commonly occurs in granites and pegmatites.
Hardness & Durability
Mohs Hardness: 6 (the index mineral for 6); Durability: Fair, but prone to splitting along cleavage planes if struck.
Care & Maintenance
Clean with warm soapy water and a soft brush; avoid ultrasonic cleaners due to cleavage planes. Store separately from harder stones like quartz or topaz.
Rarity & Value
Common rock-forming mineral; as a massive specimen, it has low commercial value but high educational value. Gem-quality varieties (Moonstone) are more valuable.
Special Characteristics
Exhibits two perfect cleavage planes at 90 degrees, a characteristic that defines the name 'orthoclase' (Greek for 'straight fracture').
Lore & History
Historically used in ceramic glazes and glassmaking. In traditional lore, it is associated with encouraging cooperation and finding creative solutions to problems.