Mineral Guide · Mineral
Orthoclase is a potassium feldspar with monoclinic symmetry and an important role in granites and pegmatites.
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Orthoclase is a potassium feldspar with monoclinic symmetry and an important role in granites and pegmatites.
The reference formula is KAlSi3O8. Orthoclase is classified with the silicates and its crystal system is monoclinic. Natural specimens can vary in composition, habit and degree of crystallinity, so the locality and observed form remain important.
Typical colours include white, cream, pink, yellow and colourless. Lustre, transparency, inclusions and surface condition should be described for the individual specimen rather than inferred from the name alone.
Orthoclase forms in or is associated with granitic igneous rocks, pegmatites and some metamorphic rocks. Notable collecting regions include Madagascar, Brazil, Austria, Norway, Russia and the United States. A precise locality is more useful than a broad country label when it is documented.
Orthoclase is useful in a reference collection because its appearance connects mineral chemistry with geological setting. Store specimens dry, protected from impact and clearly labelled with the confirmed material, locality and any treatment history.
Reviewed by NotOnlyRocks Editorial Team
Last reviewed: 31 August 2026
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