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| Photo credit: Rob Lavinsky/iRocks.com |
Chocolate calcite
Resembling an artful edible decoration from some top Swiss chocolate, Incredibly deep brown crystals that are thoroughly infused with microscopic hematite grains, giving them a deep ruddy red color. This is a classic style for Tsumeb. Measuring 9 x 9 x 4.3 cm, large speciments of tis nature are very rare.
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate. The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite".
Calcite is a common constituent of sedimentary rocks, limestone in particular, much of which is formed from the shells of dead marine organisms. Approximately 10% of sedimentary rock is limestone.
Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300°C, and vaterite is even less stable.
