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Dolomite
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CaMg(CO3)2 (calcium-magnesium carbonate)
Crystal system: Trigonal
Refractive index: 1.500-1.703
Birefringence: 0.179-0.185
Cleavage: {1011} perfect
Twinning: lamellar very common
Colour in PPL: colourless
Pleochroism: -
Relief: low to moderate
Distinction: extreme birefringence, cleavage, variable relief distinct from other minerals; from calcite by twin being parallel to short diagonal of cleavage rhomb
Occurrence: typically secondary sedimentary mineral, in dolostones
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