alkali feldspar mineral, a high-temperature form of potassium aluminosilicate (KAlSi₃O₈) that sometimes occurs in surface rocks. Sanidine forms colourless or white, glassy, transparent crystals in acidic volcanic rocks.

Because sanidine that occurs in the Earth's crust has cooled quickly (its structure is stable only above 700° C 【1,300° F】), it has trapped a disordered distribution of the aluminum and silicon atoms in its aluminosilicate crystal framework. Among the various forms of potassium aluminosilicate, there is a complete variation in ordering from the fully random arrangement in high-sanidine (a very-high-temperature form), through sanidine and then orthoclase, to the fully ordered arrangement in microcline.

The name anorthoclase is applied to the intermediate members of the high-albite–sanidine series that contain less than 40 percent KAlSi₃O₈. Anorthoclase occurs, often as crystals, only in sodium-rich volcanic rocks, such as the andesitic lavas of Pantelleria, southwest of Sicily. Other occurrences include Kilimanjaro, Tanzania; Mt. Kenya, Kenya; Yellowstone National Park, Wyoming; and the German Rhineland.