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(POTASSIC and ALKALI FELDSPARS) Ka[AlSi3O8]
Crystal system: Monoclinic
Refractive index: 1.518-1.539
Birefringence: 0.006-0.010
Cleavage: 1 perfect, 1 good intersecting at right-angles
Twinning: Carlsbad interpenetrant or contact, both show straight extinction
Colour in PPL: colourless
Pleochroism: none
Relief: low
Distinction: absence of multiple twinning , perthitic intergrowth with Na-feldspar, lower RI than quartz
Occurrence: common in granitic rocks
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Orthoclase - Plain polarised light - Ma'arrat al -Nu'man - Syria
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