The feldspar sanidine

	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 Forms and Aggregates	(Monoclinic) Most commonly as tabular crystals, but occasionally in prismatic form. Crystals frequently form penetration and repeated twins, and they are often striated. Also occurs massive and grainy.
	Transparency	Transparent to translucent
	Specific Gravity	2.5 - 2.6
	Luster	Vitreous
	Cleavage	1,1 - basal ; 2,1 - prismatic The cleavage angle is about 90º
	Fracture	Conchoidal to uneven
	Tenacity	Brittle
	Varieties	Adularia - White or colorless, transparent to translucent variety of Orthoclase or Sanidine. Moonstone - Adularia displaying adularescence.
	Polymorphs	Orthoclase, Microcline
	In Group	Silicates ; Tectosilicates ; Feldspar group ; Potassium feldspars
	All About	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. [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	Sanidine 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.
	Striking Features	Crystal habits, cleavage, and hardness
	Complex Tests	Soluble in hydrofluoric acid
	Popularity (1-4)	2
	Prevalence (1-3)	2
	Demand (1-3)	2
	Distinguishing 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 - exhibit striations on twinned crystal surfaces Spodumene - has a splintery cleavage Calcite - much softer (3)
	Commonly Occurs With	Quartz, Muscovite, Plagioclase feldspars
	Noteworthy Localities	Some famous localities are the Island of Elba, Italy; the Caucasus Mountains, Russia; Ragged Mountain, Gunnison Co., Colorado; the Black Range, Grant Co., New Mexico; and Bisbee, Cochise Co., Arizona.
	Picture Icon Links	Sorry. No Images currently available.
	Picture Links	Sorry. No Images currently available.