The feldspar orthoclase

	Chemical Formula	KAlSi₃O₈
	Composition	Potassium aluminum silicate
	Color	White, yellow, colorless, pink, orange, light blue, light green, brown, gray
	Streak	White
	Hardness	6
	Crystal Forms and Aggregates	(Monoclinic) Occurs in well shaped prismatic and tabular crystals. Crystals often form penetration and repeated twins, and are frequently striated. Also occurs massive, grainy, and as rounded, waterworn stones.
	Transparency	Transparent to opaque
	Specific Gravity	2.6
	Luster	Vitreous to pearly
	Cleavage	2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal The cleavage angle is about 90º
	Fracture	Conchoidal to uneven
	Tenacity	Brittle
	Varieties	Adularia - White or colorless, transparent to translucent variety of Orthoclase and Sanidine. Moonstone - Adularia displaying adularescence. Noble Orthoclase - Transparent, yellow variety of Orthoclase. Microcline and Sanidine are sometimes incorrectly classified as a variety of Orthoclase.
	Polymorphs	Microcline, Sanidine
	In Group	Silicates ; Tectosilicates ; Feldspar group ; Potassium feldspars
	All About	Orthoclase is one of the most common minerals, and occurs in numerous mineral environments. It is polymorphous with Microcline and Sanidine. These three minerals form the Potassium Feldspar group. They are almost identical in physical properties, and it is sometimes 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. [For more information on the crystal structure of the feldspars, see "All about" in the mineral Anorthite.] In many 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". Moonstone is a form of Adularia (or several other feldspars) that displays adularescence, exhibiting a color sheen on its surface. This phenomenon gives Moonstone important status and recognition on the gem market.
	Uses	Orthoclase is industrially important in the manufacture of glass and ceramics. Crystals and twins are studied by scientists, as they provide information on the formation of minerals and environmental factors. Well shaped crystals and twins are highly sought after by mineral collectors, and the variety Moonstone is an important gem. A transparent yellow variety from Madagascar is faceted as a gem for collectors.
	Striking Features	Crystal habits, cleavage, and hardness
	Popularity (1-4)	2
	Prevalence (1-3)	1
	Demand (1-3)	2
	Distinguishing Similar Minerals	Sanidine - Indistinguishable without complex methods. Sanidine, however, is never opaque. Microcline - Indistinguishable without complex methods. However, Microcline can have a deep green color that Orthoclase cannot achieve. Plagioclase feldspars - exhibits striations on twinned crystal surfaces Spodumene - has a splintery cleavage Calcite - much softer (3)
	Commonly Occurs With	Quartz, Muscovite, Plagioclase feldspars, Biotite, Tourmaline, Garnet
	Noteworthy Localities	Orthoclase is one of the most common minerals, and is found worldwide. Only a select few localities are mentioned: Transparent yellow crystals have come from a pegmatite in Madagascar, and a similar material is found in the gem gravels of Myanmar (Burma). Much Adularia is found in the Alps in Switzerland, especially at Adular, the locality this variety was named after. "Carlsbad" twins are famous from Carlsbad, Czechoslovakia, and fine "Baveno" twins have come from Baveno, Italy. Large, well-formed crystals were found in Disentis, Switzerland. Light blue crystals occur in the Lake Baikal area in Russia, and large masses occur on Mt. Kilimanjaro, Tanzania. In the U.S., enormous crystals were found on Sandia Mountain, Bernalillo Co, New Mexico. Famous collector specimens have come from Robinson, Colorado, and Goodsprings, Clark Co., Nevada. Orthoclase has also come from the French Creek mine, St. Peters, Chester Co., Pennsylvania. Much Adularia Moonstone has come from New Mexico.
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