Marble. Polarized light micrograph of a thin section of marble. Marble is a calcareous metamorphic rock, derived from fairly pure limestone (calcium carbonate). It is composed of crystals of calcite. The calcite crystals (stripes) in this sample show sutured edges and lamellar twinning and are arranged parallel to the diagonals of this rhombohedral cleavage. Magnification: x90 when printed at 10 centimetres wide.
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