Empress of China on display at Tuscon Gem and Mineral Show; rhodochrosite

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

Rhodochrosite is a manganese carbonate mineral, MnCO3, and so it belongs to the family of minerals like calcite, CaCO3. It is a very soft mineral, with a hardness of 3.5-4 on a scale of 10, but it is about three and a half times as dense as ice. Calcium, magnesium and zinc frequently substitute for the manganese, changing the color to lighter shades of red or pink.  because it is soft, it's difficult to cut and is rarely used in jewelry.  It was first discovered in Romania, where it was found in silver mines.   It is the state mineral of Colorado because very large crystals are not common, and some of the best occur in the Sweet Home Mine in Colorado.  According to Wiki, the Incas believed that rhodochrosite is the blood of their former rulers, turned to stone, and thus it is sometimes called "Inca Rose."

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.