Sanidine

Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi₃O₈).[1] Sanidine is found most typically 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.

Sanidine
Sanidine - Puy de Sancy, Monts-Dore massif, Puy-de-Dôme, Auvergne, France. (5x4.5cm)
General
Category	Feldspar
Formula (repeating unit)	K(AlSi3O8)
Strunz classification	9.FA.30
Dana classification	76.01.01.02
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Identification
Color	Colorless to white
Crystal habit	Tabular crystals, may be acicular
Twinning	Carlsbad twinning common
Cleavage	{001} perfect, {010} good
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	6
Luster	Vitreous, pearly on cleavage
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.52
Optical properties	Biaxial (-)
Refractive index	nα = 1.518 - 1.525 nβ = 1.523 - 1.530 nγ = 1.525 - 1.531
Birefringence	δ = 0.007
2V angle	Measured: 18° - 42° (low); 15° - 63° (high)
References	[1][2][3]

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.