|Rhodochrosite Cluster from South-Africa|
Beautiful pink rhodochrosite crystal cluster from the famous N'Chwaning Mines, Kuruman, Kalahari manganese fields, Northern Cape Province, South Africa.
Rhodochrosite is a manganese carbonate mineral with chemical composition MnCO3. In its (rare) pure form, it is typically a rose-red color, but impure specimens can be shades of pink to pale brown. It streaks white, and its Mohs hardness varies between 3.5 and 4. Its specific gravity is between 3.5 and 3.7.
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. It is for this reason that the most common color encountered is pink.
To the west of Kuruman (Northern Cape Province) below the arid shrub savanah and the thick layer of red Kalahari Desert sand one finds the largest land-based sedimentary manganese deposits of the world.
The manganese deposits are confined to the Hotazel Formation of the Griqualand West Supergroup of the middle Proterzoic age. The base of the Hotazel Formation consists of a bright-red banded iron-formation bed (varieng from massive to fine-grained specularite and/or euhedral magnetite cystals) overlying volcanic glass breccias and lavas of the Ongeluk Formation.
About 1300 million years ago a widespread hydrothermal event occured in the north-western portion of the Kalahari manganese field which reached temperatures up to 450°C in the Wessels, N'Chwaning and Black Rock Mines. This event decarbonated and desilicated portions of the Hotazel Formation to the north-west and thus upgraded the manganese content of the ore.
Furthermore the hydrothermic event is of great significance for from the collectors point of view as a wide range of rare as well as unusual mineral combinations where produced
Photo: Joe Budd Photography