The plagioclase feldspar anorthite

	Chemical Formula	CaAl₂Si₂O₈
	Composition	Calcium aluminum silicate. May contain some sodium replacing the calcium, but that amount must be less than 10 percent.
	Variable Formula	(Ca,Na)Al₂Si₂O₈
	Color	Colorless, white, cream, gray, pink, pale yellow, pale green
	Streak	White
	Hardness	6 - 6½
	Crystal Forms and Aggregates	(Triclinic) Crystals are prismatic or tabular, and are usually twinned. Also occurs grainy and massive.
	Transparency	Transparent to translucent
	Specific Gravity	2.74 - 2.76
	Luster	Vitreous. Pearly on cleavage surfaces
	Cleavage	2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal The cleavage angle is about 90º
	Fracture	Conchoidal to uneven
	Tenacity	Brittle
	Polymorphs	Svyatoslavite - Rare, orthorhombic polymorph of Anorthite Dmisteinbergite - Rare, hexagonal polymorph of Anorthite
	In Group	Silicates ; Tectosilicates ; Feldspar group ; Plagioclase Feldspars
	All About	Anorthite belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Albite is one end member, containing sodium and no calcium. The other end member, Anorthite, contains calcium and no sodium. The intermediary members are Oligoclase, Andesine, Labradorite, and Bytownite.
	Uses	Anorthite is a rare Feldspar and its only real use is as specimens for collectors.
	Striking Features	Color, crystal habits, cleavage, and hardness
	Complex Tests	Soluble in hydrochloric acid
	Popularity (1-4)	2
	Prevalence (1-3)	3
	Demand (1-3)	2
	Distinguishing Similar Minerals	Potassium feldspars - don't exhibit striations on twinned crystal surfaces, whereas the Plagioclase feldspars do Other Plagioclase Feldspars - usually cannot be determined by practical means Rhodonite - Although crystals are generally more elongated, it is very difficult to distinguish pink Anorthite from Rhodonite. However, Rhodonite usually has characteristic black veins running through it. Spodumene - has a splintery cleavage Calcite - much softer (3)
	Commonly Occurs With	Quartz, Muscovite, Biotite, Hornblende, Augite
	Noteworthy Localities	Localities include Mount Vesuvius, Italy; the Aleutian Islands of Alaska; Lake Co., California; Nevada Co., California; and Franklin, Sussex Co., New Jersey.
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