17 Photos
Sanidine
Chemical Formula	(K,Na)(Si,Al)4O8
Species	Silicates
Crystal System	Monoclinic
Mohs Scale	6
Specific Gravity	2.52
Color	Colorless to white
Streak	White
Luster	Vitreous
Refractive Index	n = 1.518 - 1.525 n = 1.523 - 1.530 n = 1.525 - 1.531
Diaphaneity	Transparent, Translucent
Cleavage	Perfect Perfect, distinct
Fracture	Irregular/Uneven, Conchoidal
Crystal Habit：Tabular crystals, may be acicular
Geological Setting：Acidic volcanic rocks

Sanidine is the high temperature form of potassium feldspar (K,Na)(Si,Al)₄O₈. Sanidine most typically occurs in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower temperatures. At yet lower temperatures, microcline, a triclinic polymorph of potassium feldspar, is stable.

Due to the high temperature and rapid quenching, sanidine can contain more sodium in its structure than the two polymorphs that equilibrated at lower temperatures. Sanidine and high albite constitute a solid solution series with intermediate compositions termed anorthoclase. Exsolution of an albite phase does occur; resulting cryptoperthite can best be observed in electron microprobe images.