13" Green-Black Calcite Crystal Cluster - Sweetwater Mine

These green-black calcite aggregations (also known as "black calcite" and "dragon scale calcite") come from the Sweetwater Mine in Reynolds County, Missouri. They are collected at about 1,400 ft elevation from stope J-21 (copper side). Their green-black coloration can be attributed to chalcopyrite inclusions suspended in layers throughout the crystals, resulting in fascinating phantoms. Both microscopic and larger chalcopyrite crystals can be found beneath the exterior-most layers of calcite.

Calcite (CaCO3) is a carbonate mineral, and the most stable polymorph of calcium carbonate. The other polymorphs are the minerals aragonite and vaterite. 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 and prisms. Calcite exhibits several twinning types as well, adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form.