Sanidine
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| 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 | Tectosilicate minerals |
| Group | Feldspar group |
| Series | Alkali feldspar series |
| Formula | K(AlSi3O8) |
| IMA symbol | Sa[1] |
| IMA status | Grandfathered (before 1959) |
| Strunz classification | 9.FA.30 |
| Dana classification | 76.01.01.02 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) |
| Space group | B2/m (no. 12) |
| 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.
In addition to its presence in the groundmass of felsic rocks, sanidine is a common phenocryst in rhyolites and, to a lesser extent, rhyodacites.[5] Trachyte consists largely of fine-grained sanidine.[6]
Fallout ash beds in sedimentary rock of the western United States have been classified in part by whether sanidine phenocrysts are present and, if present, whether they are sodium-enriched. W-type rhyolite ash beds contain sodium-poor sanidine; G-type rhyolite ash beds contain sodium-rich sanidine; and dacite fallout ash beds frequently lack sanidine. Because of their high potassium content, sanidine phenocrysts are also very useful for radiometric dating of rhyolite ash beds by the K–Ar dating method.[7]
Composition
Although the ideal composition of sanidine is 64.76 wt% SiO2, 18.32 wt% Al2O3, and 16.72 wt% K2O, natural sanidine incorporates significant sodium, calcium, and iron(III). Calcium and sodium substitute for potassium (with concurrent substitution of additional aluminum for silicon, in the case of calcium) while ferric iron substitutes for aluminum. A typical natural composition is:[8]
| Component | Weight % |
|---|---|
| SiO2 | 64.03 |
| Al2O3 | 19.92 |
| Fe2O3 | 0.62 |
| CaO | 0.45 |
| Na2O | 4.57 |
| K2O | 10.05 |
At elevated temperature, a complete solid solution exists between sanidine and albite. Rapid cooling of the sanidine freezes the composition, though most sanidine is cryptoperthitic, showing separate layers of low-sodium sanidine and albite at a sub-micron scale that can be detected only by X-ray crystallography or electron microscope methods.[9]