Mineral · Feldspar group
Orthoclase
K(AlSi3O8)
Orthoclase is the monoclinic potassium feldspar, the pink to white mineral of granites that defines hardness 6; Carlsbad-twinned crystals from Colorado and Madagascar, Alpine adularia (with its moonstone sheen) and yellow gem orthoclase from Itrongay, Madagascar, are the collector forms. Two cleavages at 90° and blocky habit identify feldspars. Microcline is the triclinic twin.
| Formula | K(AlSi3O8) |
|---|---|
| IMA status | grandfathered |
| Crystal system | Monoclinic |
| Mohs hardness | 6 |
| Group | Feldspar group |
| Type locality | None |
| Streak | White |
| Specific gravity | 2.56 |
| Reference list | IMA List of Minerals (RRUFF) — verify formula and status against the current list |
Elsewhere
Cite this article
Stratum Study. "Orthoclase" (Mineral). Stratum Study pack 2026.09, as of 2026-09-19. https://stratum.black/study/minerals/orthoclase. Accessed 2026-09-20.
@misc{stratum-orthoclase,
title = {Orthoclase},
author = {{Stratum Study}},
howpublished = {\url{https://stratum.black/study/minerals/orthoclase}},
year = {2026},
note = {Mineral article, Stratum Study pack 2026.09, as of 2026-09-19; sources: stratum, wikidata, commons. Accessed 2026-09-20.}
} Citations carry the pack version and the article's as-of date so they resolve to the same text later. The facts themselves belong to the sources listed below; cite those for the science.