Fig. 1. Massive rhodochrosite stalactites exposed in the Capillitas Mine, Argentina. Photo courtesy of J. A. Saadi and The Giant Crystal Project.
The carbonates are a common and important group of minerals found in rocks of the earth’s crust. All carbonates are distinguished by having a CO3 ion in their chemical composition. Many carbonates are quite colorful such as the shades of calcite (calcium carbonate) and the blues of azurite (copper carbonate). Most mineral collections contain numerous specimens of carbonates but three related minerals are of special interest: iron carbonate (siderite), calcium carbonate (calcite), and manganese carbonate (rhodochrosite). The reason behind this interest is their solid solution relationship. For example, in the popular collectable mineral rhodochrosite (MnCO3), calcium, as well as iron, may substitute/replace the manganese; therefore, the mineral may exhibit many different shades of pink and red and the exact chemical formula varies with the differing amounts of manganese, iron, and calcium. It is my understanding that “pure” rhodochrosite is rather rare.