Sanidine

Feldspar Group. The high-temperature, fully disordered (in its Al and Si distribution), form of K-feldspar; other potassium-dominant feldspars are microcline (fully ordered) and orthoclase (partly ordered). It is metastable at room temperature, and can revert to orthoclase over long geological periods. It forms an (alkali feldspar) series at high temperature with high albite. Petrologically, intermediates in this series from 10% up to 36% K-feldspar were termed anorthoclase, and anything from 36 - 100% K-feldspar was termed sanidine, even where Na>K. With current classifications, the name anorthoclase is discontinued and the intermediates should be called either albite (if Na>K) or sanidine (if K>Na). It is common in many potassic volcanic rocks, some high level intrusive rocks and other high-temperature rocks like buchites. The Fe analogue, UM1993-23-SiO:AlFeK, is now IMA-approved as ferrisanidine.