Rhodochrosite

Rhodochrosite is a manganese carbonate mineral with chemical composition MnCO3. In its pure form (rare), it is typically a rose-red colour, but it can also be shades of pink to pale brown. It streaks white, and its Mohs hardness varies between 3.5 and 4.5. Its specific gravity is between 3.45 and 3.6. It crystallizes in the trigonal system, and cleaves with rhombohedral carbonate cleavage in three directions. The crystal structure of rhodochrosite is a rhombohedral system, which is a subset of the trigonal system. The carbonate ions (CO3) are arranged in a triangular planar configuration, and the manganese ions (Mn) are surrounded by six oxygen ions in an octahedral arrangement. The MnO6 octahedra and CO3 triangles are linked together to form a three-dimensional structure. Crystal twinning often is present. It is often confused with the manganese silicate, rhodonite, but is distinctly softer. Rhodochrosite is formed by the oxidation of manganese ore, and is found in South Africa, China, and the Americas. It is officially listed as one of the National symbols of Argentina. Rhodochrosite forms a complete solid solution series with iron carbonate (siderite). Calcium (as well as magnesium and zinc, to a limited extent) frequently substitutes for manganese in the structure, leading to lighter shades of red and pink, depending on the degree of substitution. This is the reason for the pink color of rhodochrosite. Rhodochrosite occurs as a hydrothermal vein mineral along with other manganese minerals in low temperature ore deposits as in the silver mines of Romania where it was first found. Banded rhodochrosite is mined in Capillitas, Argentina. It was first described in 1813 in reference to a sample from Cavnic, Maramureş, present-day Romania. The name is derived from the combination of Greek words ροδόν (rodon, meaning rose) and χρωσις (chrosis, meaning coloring). Rhodochrosite is mainly used as an ore of manganese, which is a key component of low-cost stainless steel formulations and certain aluminium alloys.