1.8" Lustrous Olmiite on Calcite - N'Chwaning Mine, South Africa
This is an association of lustrous olmiite crystals that formed over calcite crystals. The entire specimen measures 1.8" wide and was collected from the N'Chwaning Mine in the Kalahari Manganese Fields of Kuruman, Africa. Under shortwave UV illumination, these crystals fluoresce a red-pink color.
Olmiite is a calcium and manganese based mineral with the chemical formula CaMn[SiO3(OH)](OH). Olmiite crystals typically exhibit a salmon color, with little to no transparency depending on the specimens' Mn content. It is often found in association with poldervaartite in few known locations in South Africa, most commonly throughout the Kalahari Manganese Fields. Like poldervaartite, Olmiite commonly displays a dull-red fluorescence when placed under shortwave UV-light.
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.