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".

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.

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.