Orthoclase
Species
Common · KAlSi₃O₈
The common potassium feldspar of granites, granite pegmatites and syenites, colourless, white, pale yellow or flesh-red. It is the reference mineral for 6 on the Mohs scale. Its twins are named after the places they were first described: Carlsbad, Baveno and Manebach. Clear yellow gem crystals come from Madagascar.
Identity
- Formula
- KAlSi₃O₈
- Chemical class
- Silicate
- Part of
- Moonstone
- Mineral group
- Albite, Labradorite, Moonstone, Sunstone
- Also known as
- Orthoclase feldspar, Potassium feldspar
Crystallography
- Crystal system
- Monoclinic
- Habits
- Prismatic, Tabular, Massive, Granular
- Fracture
- Conchoidal
Physical properties
- Mohs hardness
- 6
- Density
- 2.56 g/cm³
- Luster
- Vitreous, Pearly
- Colors
- colorless, white, gray, pale yellow, pink, green
- Streak
- white
- Transparency
- Transparent, Translucent
Geology
- Origins
- Igneous, Metamorphic, Hydrothermal
- Host rocks
- Granite, Volcanic rock, Schist and gneiss
- Associated minerals
- Albite, Mica, biotite, Hornblende, Tourmaline, Beryl
- Found in
- Granite, Pegmatite deposit
- Geography
- Switzerland (St. Gotthard), Italy (Baveno, Elba), Czech Republic (Karlovy Vary), Russia (Urals), Madagascar (Itrongay), USA
Classification
- Rarity
- Common
- Gem quality
- Yes
Sources
- M. Pasero and others, IMA Commission on New Minerals, Nomenclature and Classification, The New IMA List of Minerals, updated September 2026. International Mineralogical Association (CNMNC), 2026.
- J. W. Anthony, R. A. Bideaux, K. W. Bladh, M. C. Nichols, Orthoclase. Mineral Data Publishing, Handbook of Mineralogy, 2001.
- Orthoclase. Mindat.org (Hudson Institute of Mineralogy).
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