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Sanidine Crystal in Granite Matrix

The Mineral sanidine

Sanidine is polymorphous with Orthoclase and Microcline. These three minerals form the Potassium Feldspar group.They are almost identical in physical properties, and sometimes it is impossible to distinguish one another without x-ray analysis. The only difference between them is their crystal structure. Microcline crystallizes in the triclinic system, and Orthoclase and Sanidine crystallize in the monoclinic system. Sanidine forms at high temperatures and has a disordered monoclinic symmetry, whereas Orthoclase forms at low temperatures and cools slowly, forming more ordered monoclinic crystals.

In some mineral reference guides, Microcline and Sanidine are wrongly categorized as variety of Orthoclase. Since it is so difficult to distinguish between Orthoclase, Sanidine, and Microcline, they may be simply called "Potassium Feldspar".

Chemical Formula KAlSi3O8
Composition Potassium aluminum silicate. Forms a series with Albite (NaAlSi3O8), in which the intermediary member is Anorthoclase.
Variable Formula (K,Na)AlSi3O8
Color Colorless, white, cream, yellow, gray, light brown
Streak White
Hardness 6
Crystal System Monoclinic
Crystal Forms
and Aggregates
Most commonly as tabular crystals, but occasionally in prismatic form. Crystals frequently form penetration twins and repeated twins. Also occurs massive and grainy.

Click here for more detailed information on the crystal structure of the Feldspars.
Transparency Transparent to translucent
Specific Gravity 2.5 - 2.6
Luster Vitreous to pearly
Cleavage 1,1 - basal ; 2,1 - prismatic The cleavage angle is about 90º
Fracture Conchoidal to uneven
Tenacity Brittle
Complex Tests Soluble in hydrofluoric acid
In Group Silicates; Tectosilicates; Feldspar Group
Striking Features Crystal habits, cleavage, hardness, and environments.
Environment In recent alkaline volcanic environments rich in potassium.
Rock Type Igneous
Popularity (1-4) 2
Prevalence (1-3) 2
Demand (1-3) 2


 -  White or colorless, transparent to translucent variety of Orthoclase (or Sanidine).
Orthoclase, Microcline

A transparent, yellow variety of Sanidine (previously thought to have been Orthoclase) is used as a gemstone. It is still called Orthoclase on the gemstone market. See the gemstone Orthoclase for additional information.

Good crystals of Sanidine are not nearly as common as the other potassium feldspars. European localities include Roc de Courlande, Chastreix, France; Drachenfels, Konigswinter, Germany; Capo d'Enfola, Elba, Italy; and Kakourou, Aridhaia, Greece. In the U.S., Sanidine comes from the Ragged Mountain, Gunnison Co., Colorado; the Black Range, Grant Co., New Mexico; the Sandia Mts., Bernalillo Co., New Mexico, Bisbee, Cochise Co., Arizona; and the Thomas Range, Juab Co., Utah. In Canada, it occurs with Aegirine in Mont Saint Hilaire, Quebec.

Quartz, Muscovite, Biotite, Plagioclase feldspars, Aegirine, Sodalite

Orthoclase - Indistinguishable without complex methods. Sanidine, however, is never opaque, whereas Orthoclase may be.
Microcline - Indistinguishable without complex methods. However, Microcline can be a deep green color that Sanidine cannot achieve.
Plagioclase Feldspars - May exhibit striations on twinned crystal surfaces, otherwise difficult to distinguish.
Spodumene - Has a splintery fracture.
Calcite - Much lower hardness.

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