Alkali Fesldspar - KAlSi3O8

The alkali feldspars are primarily important as constituents of rocks; they are very widespread and abundant in alkali and acidic igneous rocks (particularly syenites, granites, and granodiorites), in pegmatites, and in gneisses. The alkali feldspars may be regarded as mixtures of sodium aluminosilicate (NaAlSi3O8) and potassium aluminosilicate (KAlSi3O8). Both the sodium and potassium aluminosilicates have several distinct forms, each form with a different structure. The form stable at high temperatures is sanidine (a sodium aluminosilicate), which has a random distribution of aluminum and silicon atoms in its crystal structure.

Low-temperature forms include orthoclase, microcline, and adularia; these have an ordered arrangement of such atoms.
If specimens of the high-temperature varieties are rapidly cooled, the random distribution is preserved. In the Earth’s crust the alkali feldspars display a range of ordering from the fully random distribution of sanidine and orthoclase to the fully ordered distribution of microcline.

Sanidine
Sanidine generally occurs with an equant habit (almost square) and shows perfect {001} and {010} cleavages, which readily distinguish it from quartz. Rarely does sanidine show twinning, but when it does, it is usually simple twinning.

Anorthoclase
Anorthoclase is a Na - rich feldspar with approximately equal amounts of the Anorthite (Ca) and orthoclase (K) components. Generally anorthoclase occurs in Na - rich volcanic rocks. Like the other alkali feldspars, it has perfect {001} and {010} cleavages. Anorthoclase sometimes shows twinning, but generally not the multiple twinning seen in the plagioclase feldspars, but a crosshatched twinning similar to that seen in microcline, but on a very fine scale.

Optical Properties:
• Relief: low
• Color: colorless
• Interference colors: 1° gray to 1° white interference colors
• Form: Sanidine crystals are commonly euhedral in phenocrysts, either essentially square in section or elongated parallel to a axis.
• twinning: Only Carlsbad twinning is common in Sanidine, bisecting phenocrysts or microlites parallel to either length. Anorthoclase is distinguished by combined albite and pericline twinning, which form a diffuse grid, or quadrille pattern, like in Microcline.

Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen


Sanidine
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Sanidine crystals in a Trachyte. PPL image , 2x (Field of view = 7mm)
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Sanidine crystal in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystal in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystal in a Trachyte. PPL image , 2x (Field of view = 7mm)
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Sanidine and Aegirine crystals in a Trachyte. PPL image , 2x (Field of view = 7mm)
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Sanidine crystal in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystal in a Trachyte. PPL image , 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. PPL image , 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. PPL image , 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystal in a Trachyte. XPL image , 2x (Field of view = 7mm)
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Sanidine crystal in a Trachyte. XPL image , 10x (Field of view = 2mm)


Anorthoclase
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Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 10x (Field of view = 2mm)
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Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 10x (Field of view = 2mm)
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Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 10x (Field of view = 2mm)
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Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 2x (Field of view = 7mm)
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Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 2x (Field of view = 7mm)
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Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 2x (Field of view = 7mm)
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Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 2x (Field of view = 7mm)
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Anorthoclase (with pericline twinningin) a Trachyte. XPL image , 2x (Field of view = 7mm)