How Does Rhodochrosite Form
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Rhodochrosite (pronounced "rode-a-CROW-sight") is a mineral, typically translucent to subtranslucent, that is composed primarily of manganese carbonate (MnCO3). Its name comes from the Greek word for "rose-colored," although the mineral's color can range from pink, red or rosy red to yellow, gray or brown. Rhodochrosite has a brittle, conchoidal (irregular and glass-like) fracture, a glassy luster and a hardness of 3.5 to 4. Attractive, high-quality specimens are valued as display pieces; less commonly, this brittle and easily scratched mineral is used in jewelry. Rhodochrosite also serves as an ore of manganese.
Hydrothermal Deposition
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The Sweet Home Mine, located in Colorado's Alma mining district, was the source of valuable, museum-quality specimens of cherry-red, transparent, rhombohedral crystals of rhodochrosite. Originally a silver mine that operated from the 1870s to the 1960s, Sweet Home reopened in 1991 and operated for the next 13 years to supply the collectors' market with rhodochrosite (which had been regarded as gangue, or waste material, during the mine's silver-producing days).
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The rhodochrosite mined at Sweet Home is an example of a mineral formed by hydrothermal deposition. According to a team of researchers headed by geoscientist Volker Lüders, mineral deposits formed at this location roughly 26 to 25 million years ago when magmatic fluid (water that escaped from magmas) intruded into rock about 3 kilometers below ground surface. As the fluid slowly cooled and mixed with groundwater, ions that had been dissolved in it could no longer remain in solution. Manganese and carbonate ions combined to form rhodochrosite deposits. Other ions also combined, forming crystals of minerals such as quartz, fluorite, tetrahedrite, apatite, pyrite and sphalerite.
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Rhodochrosite from Sweet Home owes its distinctive redness and impressive crystal size (up to 16.5 centimeters across) to the hydrothermal fluid's slow cooling. Fast cooling creates small crystals intermingled with impurities, while slow cooling allows larger, purer crystals to form. Sweet Home rhodochrosite is predominantly manganese carbonate, with a relative lack of the impurities---magnesium, calcium and especially iron---that would have made the mineral pinker.
Hydrothermal Alteration
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Hydrothermal fluids can also create mineral deposits by reacting geochemically with the rocks they come in contact with, producing a zone of alteration. Rhodochrosite from the Wolf Mine in the Siegerland siderite (carbonate iron ore) district in Germany formed through this process. According to Mindat.org, the mineralization occurs in Devonian sedimentary rocks, probably as a result of basaltic melts that intruded in the vicinity. Additional minerals occurring in the alteration zone include the iron oxide limonite and ores of other metals. The richest deposits occurred at about 350 meters below ground surface.
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Wolf Mine rhodochrosites range in color from salmon or raspberry pink to orange or brownish. Crystals tend to clump tightly together, forming shapes reminiscent of stars or hedgehogs. Other, botryoidal (grapelike) and spherical crystal aggregates and coatings resemble raspberries. Crystal sizes are typically a few millimeters to about a centimeter, with some crystals reaching up to 2 centimeters.
Carbonate Mineralization
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At the Capillitas Mine in northeastern Argentina, rhodochrosite deposits take the unusual form of stalactites and stalagmites. A few million years ago, hydrothermal fluids deposited lead and zinc sulfides in the area, along with manganese carbonate. As water percolated through the sulfide deposits, it dissolved the accompanying carbonates. The resulting mineralized water eventually emerged into underlying caves. As each drop fell from the cave ceiling, it left behind a minute deposit of rhodochrosite. Another small quantity of rhodochrosite landed on the cave floor below. Over time, these deposits accumulated to form stalactites and stalagmites, sometimes fusing to form columns.
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Stalactites, stalagmites and columns from the Capillitas mine can grow to be several meters in length. They are often cut into sections, which are shaped and polished for display or used in jewelry or decorative items. The distinctive pink-and-white banded rhodochrosite from this locality looks a bit like bacon. The individual crystals that comprise these deposits rarely exceed a centimeter in size.
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