Sanidine
A variety of Minerals
What is Sanidine?
Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8). 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.
Optical Properties
- Refractive Index
- 1.518-1.536
- Birefringence
- 0.005-0.008
- Pleochroism
- Weak
- Optical Character
- Biaxial negative
Market Value Factors
Pricing varies for every rock and mineral, so use these universal factors to gauge Sanidine before comparing listings or appraisals.
Size & Weight
Larger, intact specimens usually command higher prices.
Rarity & Demand
Scarce material or popular varieties sell at a premium.
Condition & Finish
Chips, repairs, and heavy wear lower value; clean prep helps.
Treatment & Provenance
Untreated specimens with documented locality are prized.
Sanidine Localities Map
See where Sanidine is found with a localities map, collecting zones, and geology context. Generate a sample map preview below.
Key Characteristics
Formation of Sanidine
In addition to its presence in the groundmass of felsic rocks, sanidine is a common phenocryst in rhyolites and, to a lesser extent, rhyodacites. Trachyte consists largely of fine-grained sanidine. 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 method.
Composition of Sanidine
Although the ideal composition of sanidine is 64.76 wt% SiO2, 18.32 wt% AlsO3, and 16.72 wt% K2O, natural sanidine incorporates significant sodium, calcium, and ferric iron. 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: 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.
Quick Facts
Physical Properties
- Color
- Colourless, white, grey, yellowish white, reddish white
- Hardness (Mohs)
- 6
- Density
- 2.56 g/cm³
- Streak
- White
Chemical Properties
- Chemical Formula
- K(AlSi3O8)
- Elements
- Al, K, O, Si
Also Known As
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Sanidine FAQs
How do I identify Sanidine?
Sanidine can be identified by its hardness of 6 on the Mohs scale, Colourless color. Look for these key characteristics when examining specimens.
What color is Sanidine?
Sanidine typically appears in Colourless, white, grey, yellowish white, reddish white. Color can vary depending on impurities and formation conditions.
How hard is Sanidine?
Sanidine has a hardness of 6 on the Mohs scale. This gives it moderate hardness.
What is the formation of sanidine of Sanidine?
In addition to its presence in the groundmass of felsic rocks, sanidine is a common phenocryst in rhyolites and, to a lesser extent, rhyodacites. Trachyte consists largely of fine-grained sanidine. 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 method.
What is the composition of sanidine of Sanidine?
Although the ideal composition of sanidine is 64.76 wt% SiO2, 18.32 wt% AlsO3, and 16.72 wt% K2O, natural sanidine incorporates significant sodium, calcium, and ferric iron. 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: 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.
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