Orthoclase (KAlSi₃O₈) is the monoclinic potassium feldspar, isochemical with microcline but stable at higher temperatures. Pink Orthoclase is a defining color in Capitol Reef and Pikes Peak granites. Adularia (transparent variety from Alpine clefts) and moonstone (with adularescent schiller) are prized collector forms.
Key Facts
- Mohs hardness 6–6.5.
- Monoclinic; isochemical with microcline (triclinic) — Carlsbad twinning is diagnostic.
- Adularia variety: gem-clear from Swiss Alpine clefts.
- Moonstone variety: schiller from K-Na feldspar exsolution.
- Major rock-forming mineral in granites and syenites.
Notable Localities
Carrara (Italy) and St. Gotthard (Switzerland) yield adularia. Madagascar and Sri Lanka supply moonstone.
Found at these Localities
- Italian Volcanic Province (Vesuvius / Lipari / Etna) (意大利火山矿物产地)
- Swiss Alps (Alpine-Cleft Province) (瑞士阿尔卑斯高山裂隙矿物产地)
