A member of the solid-solution series of potassium and sodium feldspars, sanidine is the high-temperature form of potassium feldspar, forming at 1,065°F (575°C) or above. Crystals are usually colorless or white, glassy, and transparent, but they may also be gray, cream, or occur in other pale tints. They are generally short prismatic or tabular, with a square cross section. Twinning is common. Crystals have been known to reach 20 in (50 cm) in length. Sanidine is also found as granular or cleavable masses. A widespread mineral, sanidine occurs in feldsparand quartz-rich volcanic rocks, such as rhyolite, phonolite, and trachyte. It is also found in eclogites, contact metamorphic rocks, and metamorphic rocks formed at low pressure and high temperature. Sanidine forms spherular masses of needlelike crystals in obsidian, giving rise to what is called snowflake obsidian. Significant occurrences of sanidine are at the Alban Hills near Rome, Italy; Mont St.-Hilaire, Canada; and Eifel, Germany
Name: From the Greek for tablet or board, in allusion to the mineral’s common habit.
Polymorphism & Series: High sanidine forms a series with high albite.
Contents
Chemical Properties of Sanidine
| Chemical Classification | Tectosilicate |
| Chemical Composition | K(AlSi3O8) |
Physical Properties of Sanidine
| Color | Colorless to white |
| Streak | White |
| Luster | Vitreous, pearly on cleavage |
| Cleavage | {001} perfect, {010} good |
| Diaphaneity | Transparent to translucent |
| Mohs Hardness | 6 |
| Specific Gravity | 2.52 |
| Crystal System | Monoclinic |
| Tenacity | Brittle |
| Parting | {100} |
| Fracture | Irregular/Uneven, Conchoidal |
| Density | 2.56 – 2.62 g/cm3 (Measured) 2.56 g/cm3 (Calculated) |
Optical Properties of Sanidine
| Type | Anisotropic |
| Twinning | Carlsbad – common Baveno, Manebach – rarer |
| Optic Sign | Biaxial (-) |
| Birefringence | δ = 0.007 |
| Relief | Low |
Occurrence
Most common in felsic volcanic and hypabyssal rocks as rhyolites, phonolites, trachytes; as spherulites in volcanic glass. Also from ultrapotassic ma¯c, high-temperature contact metamorphic (sanidinite facies), and hydrothermally altered rocks. From eclogite nodules in kimberlite.
Uses Area
It used as gemstone
Association: Quartz, sodic plagioclase, muscovite, biotite, \hornblende,” magnetite
Distribution
Not uncommon, but rare in crystals of any size.
- In Germany, from Drachenfels, Siebengebirge, Rhine; and at Hohenfels, Mendig, Mayen, and elsewhere around the Laacher See, Eifel district.
- In France, at Mt. Dore, Auvergne, and Puy Gros du Laney, Puy-de-Dome.
- From Vesuvius and Monte Somma, Campania, and Monte Cimine, Lazio, Italy.
- At Daichi, Wakayama Prefecture, Japan.
- From Kanchin-do, Meisem-gun, northeast Korea.
- In the USA, at Tooele, Tooele Co., Utah; Cottonwood Canyon, Peloncillo Mountains, Cochise Co., Arizona; as large crystals in Rabb Canyon and near the crest of the Black Range, Grant Co., New Mexico. From Bernic Lake, Manitoba, and Mont Saint-Hilaire, Quebec, Canada.
- In the Sierra de San Francisco, Durango, Mexico.
References
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
- Mindat.org. (2019). Sanidine: Mineral information, data and localities.. [online] Available at: https://www.mindat.org
