Sanidine (KAlSi₃O₈) is the high-temperature monoclinic K-feldspar polymorph that quenches into volcanic and rapidly-cooled metamorphic rocks. It typically forms glassy, transparent, tabular phenocrysts in trachyte, rhyolite, and obsidian — including the famous Drachenfels (Germany) Sanidine in trachyte and Eifel volcanic ejecta crystals.
Key Facts
- Mohs hardness 6.
- Monoclinic; high-temperature disordered K-feldspar polymorph.
- Carlsbad and Baveno twinning common.
- Quenched volcanic rocks preserve the disordered structure (vs. orthoclase’s partial Si-Al ordering).
- Diagnostic for volcanic and contact-metamorphic origin in thin section.
Notable Localities
Drachenfels (Germany) trachyte and Eifel volcanic field (Germany) are classic sources. Mt. Vesuvius (Italy) and Madagascar volcanic ejecta also yield specimens.
