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CaCO3 (calcium carbonate)
Crystal system: Trigonal
Refractive index: 1.486-1.740
Birefringence: 0.172-0.190
Cleavage: {1011} perfect
Twinning: lamellar very common
Colour in PPL: colourless
Pleochroism: none
Relief: low to moderate
Distinction: extreme birefringence, cleavage, variable relief distinct from other minerals; from other carbonates by twin being parallel to long diagonal of cleavage rhomb and effervescence in dilute HCl in hand-specimen
Occurrence: common in carbonate sedimenatry rocks (limestones) and their metamorphosed equivalents (marble), and some igneous rocks
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Micrite - Cross polarised light - Fustat - Egypt
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