Sanidine

Sanidine
Sanidine from Puy de Sancy, Monts-Dore massif, Puy-de-Dôme, France. Size 5 cm × 4.5 cm (2.0 in × 1.8 in)
General
Category	Feldspar
Formula (repeating unit)	K(AlSi3O8)
IMA symbol	Sa[1]
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	[2][3][4]

Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi₃O₈).^[2] 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.

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.