|The $1.6 million dollar Rhodochrosite mineral from China|
on display at the Tuscon Gem and Mineral Show
photographer David J. Eicher, link is here which is the blog
of the State Geologist of Arizona
How does rhodochrosite relate to geological fluid dynamics? Searching for an answer to this on the WWW led me to an interesting WWW site on "giant crystals", which has some additional links. Rhodochrosite forms in regions of the crust where warm to hot water circulate to form hydrothermal systems. It often occurs in association with copper, lead, and silver sulfides. In some places, like stalactites in the Capillitas Mine in Argentina, it forms like stalactites, from water dripping through Mn-rich rocks (here's an earlier post on ice stalactites.) As far as I can tell, there is very little published about the Chinese rhodochrosites, and even less on this spectacular sample. Generally rhodochrosite is leached by acid to extract the manganese.
In hydrothermal systems, water is rarely pure, but is a salty brine. The water can come from a body of crystallizing magma at depth, or from metamorphic reactions deeper in the crust, or it can originate at the surface of the earth from meteorological processes and then trickle down into the crust where it is heated. There it can dissolve minerals in the rocks and become loaded with interesting cations, such as manganese. Upon circulating back toward the surface, the water can become saturated with these ions because it is cooling, or because it can boils, or because it intersects waters of different chemistry. These changes cause minerals to precipitate. Here's a good Britannica encyclopedia article about mineral deposits.