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Calcite (CaCO3) Bizarre nailhead calcite stalactite from Hilton Mine, Cumbria.The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.Calcite crystals are trigonal-rhombohedral, though actual calcite rhombohedra are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedra, tabular forms, prisms, or various scalenohedra.
Calcite Crystal Cluster Chocolate Calcite by FenderMinerals