Rhodochrosite

Rhodochrosite is a manganese carbonate.  The name rhodochrosite comes from the Greek ρόδο, "rose", and χρώς, "coloring", referring to its color and was named in 1813 by Johann  Friedrich Ludwig Hausmann for the occurrence in what is now known as Romania.

Chemistry: MnCO₃

Class: Carbonate

Crystal system: Trigonal

Color: Pink, rose, red, yellowish-grey, brown, white, gray; colourless to pale rose in transmitted light. 

Luster: Vitreous, Pearly

Specific gravity: 3.7

Mohs Hardness: 3.5-4

Cleavage: Perfect On {1011}

Streak: white

Rhodochrosite's pink to red coloration is due to manganese (Mn²⁺). It forms a complete solid solution series with the iron carbonate siderite. Calcium, magnesium and iron can all substitute for the manganese in rhodochroiste giving a chemical formula of (Mn, Fe, Mg, Ca) CO₃. Increased iron shifts the coloration into the brown range. The red coloration occurs when the iron content is minimal.

Rhodochrosite commonly occurs as a primary gangue mineral in moderate- to low-temperature hydrothermal veins, also in high-temperature metasomatic deposits and sedimentary manganese deposits or as a late stage hydrothermal mineral in pegmatites. It may be formed as a biomineral  by some fungi (Lichenothelia).

Rhodochrosite tends to form in layers in the open spaces within sedimentary or metamorphic rocks.  It precipitates out of ascending fluids as vein and fracture-filling or out of descending solution as layered accumulations on the walls of cavities in the form of stalactites and stalagmites.  In either case, repeated episodes of crystallization build up layers, each of which can be a unique event that results in material with slightly differing composition and color.  Well-formed crystals are rare.  The deposits at the Sweet Home mine, near Alma, Colorado, are known for their excellent crystalline specimens

Rhodochrosite is rarely confused with other minerals.  Its pink color, perfect cleavage in three directions, low hardness and weak effervescence with cold dilute hydrochloric acid are all properties not commonly shared with other minerals.  Some confusion occurs between the pink-colored minerals with similar names - rhodochrosite and rhodonite.  Rhodonite is the manganese silicate which has a hardness of 5.5 to 6.5.

Rhodochroiste can be an ore mineral for manganese, however, it rarely forms in the concentrations necessary for economic extraction.  Most uses are for lapidary/jewelry purposes  and mineral specimens.  It is often slabbed (cut in thin sections) to emphasize its banding or concentric patterns and used in lapidary fabrication (ie boxes) and to make cabochons.  It is not often faceted into gemstones due to its softness and perfect cleavage.

It is found in a small number of locations worldwide where other manganese minerals are usually present.. Rhodochrosite for lapidary and mineral specimen use is only found in a few locations worldwide. These include Argentina, South Africa, Peru, Montana, Colorado, Russia, Romania, Gabon, Mexico, and Japan.

Rhodochrosite is Argentina's "national gemstone". Colorado officially named rhodochrosite as its state mineral in 2002.  It is sometimes called "Rosa del Inca", "Inca Rose" or Rosinca.