Sanidine
Form of potassium feldspar / From Wikipedia, the free encyclopedia
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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.
Sanidine | |
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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] |
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.