Sunday, October 9, 2011


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.

The most “famous” rhodochrosite” crystals are the specimens collected from the Sweet Home Mine near Alma, Colorado.  These rose-red colored crystals are prized by collectors the world over and the Denver Museum of Nature and Science has a fantastic display.  However, also of interest to many rockhounds are the “sliced and polished stalactites” from the Capillitas Mine in Argentina. These non-crystal specimens are very recognizable and occur as the principle gangue mineral in this lead-zinc sulfide mine (Fig. 1).  The minerals are the result of hydrothermal activity in the late Tertiary and may be the largest mass of rhodochrosite ever discovered (The Giant Crystal Project, 2011).  I treasure these slices for both their color and their uniqueness (Fig. 2). 

Fig. 2.  Rhodochrosite (stalactites) from Capillitas Mine in Argentina. Specimens ~4 X 4 cm.