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(POTASSIC and ALKALI FELDSPARS)
Ka[AlSi3O8]
Crystal system: Triclinic system
Refractive index: 1.514-1.539
Birefringence: 0.007-0.010
Cleavage:1 perfect, 1 good intersecting not quite at right-angles
Twinning: albite/pericline "tartan" twinning
Colour in PPL: colourless
Pleochroism: -
Relief: low
Distinction: any feldspar with "tartan" twinning is considered to be microcline, perthitic intergrowth with Na-feldspar
Occurrence: common in granitic rocks
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Microcline, cross polarized light - Granite petrofabric - Belize
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