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

Color

Colorless, white, cream, yellow, gray, light brown

Crystal System

Monoclinic

Properties

Streak White

Hardness 6

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

Crystal Habits

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.

Additional Information

Composition Potassium aluminum silicate. Forms a series with Albite (NaAlSi3O8), in which the intermediary member is Anorthoclase.

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

Other Names

Rhyacolite

Varieties

Adularia

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

Uses

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.

Noteworthy Localities

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.

Common Mineral Associations

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

Distingushing Similar Minerals

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.

sanidine Photos

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