2.5" Nailhead Spar Calcite after Dogtooth Calcite - China
This is a gorgeous cluster of hematite included nailhead spar calcite crystals that formed over dogtooth spar calcite. This specimen was collected from Daye City, China. Under shortwave UV, the underlying dogtooth calcite crystal fluoresces a vibrant orange color at the point of breakage ("underside" of the crystal).
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 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.