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.

Orthoclase
Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite. Typically, the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember (NaAlSi3O8), of plagioclase. While slowly cooling within the earth, sodium-rich albite lamellae form by exsolution, enriching the remaining orthoclase with potassium. The resulting intergrowth of the two feldspars is called perthite. The higher-temperature polymorph of orthoclase is sanidine.

Microcline
Microcline is the lowest temperature form of alkali feldspar. Upon cooling, orthoclase must rearrange its structure from monoclinic to triclinic. When this happens, twinning usually results. The twinning characteristic of microcline is a combination of albite twinning and pericline twinning. This results in a cross-hatched pattern (often called tartan twinning) that is the most distinguishing characteristic of microcline.

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.

Adularia
A more ordered low-temperature variety of Orthoclase or partially disordered Microcline. Generally found in alpine-type parageneses.

Bibliography



• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc


Orthose
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Orthose with carlsbad twinning. XPL image, 2x (Field of view = 7mm)
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Orthose with carlsbad twinning. XPL image, 2x (Field of view = 7mm)
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Orthose with carlsbad twinning and perthite unmixing. XPL image, 2x (Field of view = 7mm)
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Orthose altered by sericite. PPL image, 2x (Field of view = 7mm)
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Orthose with carlsbad twinning. XPL image, 2x (Field of view = 7mm)
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Perthite unmixing in orthose . XPL image, 10x (Field of view = 2mm)
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Perthite unmixing in orthose . XPL image, 10x (Field of view = 2mm)
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Perthite unmixing in orthose . PPL image, 10x (Field of view = 2mm)
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Perthite unmixing in orthose . XPL image, 10x (Field of view = 2mm)
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?erthite unmixing in orthose . XPL image, 10x (Field of view = 2mm)
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Perthite unmixing in orthose . PPL image, 10x (Field of view = 2mm)
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Perthite unmixing in orthose . XPL image, 10x (Field of view = 2mm)


Microcline
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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Cross-hatched twinning in Microcline. XPL image, 10x (Field of view = 2mm)
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Cross-hatched twinning in Microcline. XPL image, 10x (Field of view = 2mm)
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Cross-hatched twinning in Microcline. XPL image, 10x (Field of view = 2mm)
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Cross-hatched twinning in Microcline. XPL image, 10x (Field of view = 2mm)
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Cross-hatched twinning in Microcline. XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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Cross-hatched twinning in Microcline. XPL image, 10x (Field of view = 2mm)
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Cross-hatched twinning in Microcline. XPL image, 10x (Field of view = 2mm)
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Cross-hatched twinning in Microcline. XPL image, 10x (Field of view = 2mm)
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Cross-hatched twinning in Microcline. XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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perthite unmixing in Microcline . XPL image, 10x (Field of view = 2mm)
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Cross-hatched twinning in Microcline. XPL image, 10x (Field of view = 2mm)
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Cross-hatched twinning in Microcline. XPL image, 10x (Field of view = 2mm)


Adularia
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Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Adularia in a hydrothermal vein from Rio marina, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)