The feldspar anorthoclase

	Chemical Formula	(Na,K)AlSi₃O₈
	Composition	Sodium potassium silicate, with a varying ratio of sodium to potassium. If sodium is not present, the mineral is Sanidine, and if potassium is not present, the mineral is Albite.
	Color	Colorless, white, cream, yellow, pink, brown, reddish, greenish, gray
	Streak	White
	Hardness	6 - 6½
	Crystal Forms and Aggregates	(Monoclinic or triclinic [see "all about"]) Occurs in well formed crystals, usually prismatic. Crystals also occur in dense aggregates, grainy, and massive. Crystals frequently twin.
	Transparency	Transparent to opaque
	Specific Gravity	2.5 - 2.6
	Luster	Vitreous
	Cleavage	2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal The cleavage angle is about 90º
	Fracture	Conchoidal to uneven
	Tenacity	Brittle
	Other Names	Anorthose
	In Group	Silicates ; Tectosilicates ; Feldspar group If Anorthoclase contains a considerable amount of potassium, it is also classified as in the Potassium feldspar group.
	All About	Technical Feldspar crystallization information: The following text attempts to resolve a confusing aspect of Feldspar minerals. Anorthoclase is the intermediary member of the Albite-Sanidine series. It is interesting to note that this mineral series contains two end members crystallizing in different systems. Albite crystallizes in the triclinic system, and Sanidine in the monoclinic system. The difference between the triclinic system and the monoclinic system is only one angle. Monoclinic minerals have two asymmetrical angles and one symmetrical angle, whereas triclinic minerals have three asymmetrical angles and no symmetrical angles. Albite, which is triclinic, has its third angle slightly asymmetrical, and it is therefore classified as a triclinic mineral, but the angle of asymmetry (i.e. off of 90º) is so minute it can hardly be detected, and it is therefore very similar to orthorhombic minerals. (The same is true for all Feldspar minerals that crystallize in the triclinic system -- their third angle of symmetry is asymmetrical by only a tiny amount. This is the reason all Feldspar crystals, even those crystallizing in different systems, are almost identical in appearance. This is also the reason why Orthoclase and Microcline are usually regarded as a single mineral, since the only difference between them is one minute asymmetrical angle.) Sanidine can have up to ten percent of its potassium replaced by sodium. If more than ten percent sodium is present, the mineral becomes Anorthoclase. The same is true for the other end of the series. If Albite contains more than ten percent potassium, it is no longer Albite, but Anorthoclase. When Anorthoclase forms, its atoms form in a specific arrangement. This arrangement of the atoms forms the crystal structure of the mineral. Depending on the environment, it will either form a monoclinic or triclinic crystal. If it forms a monoclinic crystal, it leans toward Sanidine in regard to its crystal structure, and if it forms in the triclinic system, it leans toward Albite. For this reason, Anorthoclase crystals can either be triclinic or monoclinic. The above determination of the classification of the particular Feldspar can obviously only be determined with x-ray analysis. However, certain localities produce particular known examples of Sanidine, Anorthoclase, and Albite, and therefore the mineral type can be determined without complex analysis.
	Striking Features	Crystal habits, cleavage, and hardness
	Popularity (1-4)	2
	Prevalence (1-3)	1
	Demand (1-3)	2
	Distinguishing Similar Minerals	Albite, Sanidine, Orthoclase, and Microcline - Indistinguishable without x-ray analysis
	Commonly Occurs With	Quartz, Muscovite, Biotite
	Noteworthy Localities	The most famous Anorthoclase localities are Mount Kilimanjaro, Tanzania; Pantelleria Island, Italy; Tjolling, Norway; and Obsidian Cliff in Yellowstone National Park, Wyoming.
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