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Na[AlSi3O8] - Ca[Al2Si2O8]2
Crystal system: Triclinic
Refractive index: 1.527-1.590
Birefringence: 0.10-0.13
Cleavage: 1 perfect, 1 good intersecting at angle
Twinning: lamellar twinning
Colour in PPL: colourless
Pleochroism: -
Relief: low
Distinction: lamellar twinning distinctive, untwinned plagioclase is practically impossible to distinguish from untwinned potassic feldspars
Occurrence: most abundant mineral in basic and intermediate igneous rocks, widespread in metamorphic rocks
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Plagioclase - Cross polarised light - Faza petrofabric, Yemen - field of view 0.5mm
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