Azurite, Dixie-Apex Mine, Washington Co., Utah; bright blue microcrystals lining open spaces in copper ore; UMNH.MC.2157. Fito Torres / NHMU
Colorful Carbonates
By Dave Richerson
Rhodochrosite, N’Chwaning I Mine, Northern Cape, South Africa; another example showing the scalenohedral crystal form; UMNH.MC.0879. Fito Torres / NHMU
Do you know that you, your pets and other animals contribute to the formation of new minerals? We all breathe out carbon dioxide (CO2) that interacts with water vapor in the atmosphere to form a weak acid. That weak acid falls with rain on an ore deposit and slowly dissolves minerals in that deposit. The CO2 reacts with chemical elements in the ore to yield what we refer to as “carbonates.” Thus, the blue and green carbonate minerals azurite and malachite can form in copper ore deposits, pink to red rhodochrosite in manganese ore deposits and a wide range of colors of smithsonite in zinc ore deposits.
The carbonate you are probably most familiar with is CaCO3 in the form of cave deposits. The carbonated water slowly dissolves the common rock limestone. As this carbonated water continues to drip, stalactites, stalagmites, and helictites are built up on the ceiling, floor and walls of the cave. The minerals that result are calcite and aragonite. They are generally white, colorless, or stained yellowish or brownish with iron. However, in one cave environment in Argentina, manganese ore was dissolved such that the cave growths are made of beautiful pink rhodochrosite in concentric bands and layers.
Malachite, Katanga, Democratic Republic of the Congo; polished specimen showing where copper-rich water dripped to form side-by-side stalagmites in a copper ore deposit; UMNH.MC.0068R. Fito Torres / NHMU
Most carbonate minerals are formed in ore deposits. The famous copper mines in the Democratic Republic of Congo are the source of beautiful green malachite. Much of this malachite was deposited just like the calcite in caves, resulting in stalagmite-like structures with concentric layers of bright green malachite that are cut and polished to yield spectacular gemstones and decorator pieces.
Another copper carbonate is azurite, but it is dark blue rather than green like malachite. Many mines in Utah and Arizona have produced beautiful specimens of azurite. Examples of Utah localities are the Dixie-Apex Mine near Saint George, the Blue Crystal Mine in the Lisbon Valley and the Mammoth Mine near Eureka. Often the azurite is present as small crystals that deposited on any open spaces in the ore.
Zinc ore deposits also yield a carbonate mineral, ZnCO3. This mineral is named smithsonite after the same person from which the Smithsonian Institute received its name. The smithsonite is generally deposited in rounded crusts made up of many crystals side-by-side. In some mines, such as the Tsumeb Mine in Namibia, the smithsonite is present as crystals with a rhombohedral shape.
Smithsonite, Kelly Mine, Socorro Co., New Mexico; exceptional specimen 18 cm long showing that many crystals grew radially to form a rounded mass typical of smithsonite; this shade of blue is the most common color of smithsonite, but other shades of blue and green are also prized; UMNH.MC.0924. Fito Torres / NHMU
Two of the most famous localities for smithsonite are the Kelly Mine in New Mexico and the Refugio Mine at Choix, Sinaloa, Mexico. Smithsonite has also been found at a number of mines in Utah. An interesting characteristic of smithsonite is the range of colors. It can range from white to gray to pink to green and blue.
Although carbonates are often found in colorful crusts, they also can be present as sharp crystals. The two most common crystal forms are rhombohedrons and scalenohedrons (dog-tooth). The crystal form is determined by the crystal structure (arrangement of atoms) of the mineral. Calcite, rhodochrosite, smithsonite and siderite (iron carbonate, FeCO3) all crystallize in rhombohedrons or scalenohedrons.
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Azurite, Dixie-Apex Mine, Washington Co., Utah; bright blue microcrystals lining open spaces in copper ore; UMNH.MC.2157. Fito Torres / NHMU
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Smithsonite, El Refugio Mine, Choix, Sinaloa, Mexico; crust of less common pink color; UMNH.MC.0365A. Fito Torres / NHMU
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Smithsonite, Kelly Mine, Socorro Co., New Mexico; botryoidal (grape-shaped) balls of blue smithsonite; UMNH.MC.0509. Fito Torres / NHMU
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Malachite, Katanga, Democratic Republic of the Congo; unpolished specimen showing where copper-rich water dripped to build up concentric layers of malachite; UMNH.MC.0300. Fito Torres / NHMU
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Rhodochrosite, Sweet Home Mine, Alma Mining District, Park Co., Colorado; also showing rhombohedral shape, along with tiny quartz crystals; UMNH.MC.3970. Fito Torres / NHMU
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Malachite, Katanga, Democratic Republic of the Congo; polished specimen showing where copper-rich water dripped to form side-by-side stalagmites in a copper ore deposit; UMNH.MC.0068R. Fito Torres / NHMU
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Siderite, Allevard, Isère, France; crystals showing the rhombohedral crystal shape common to many of the carbonate minerals; UMNH.MC.0379. Fito Torres / NHMU
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Smithsonite, Kelly Mine, Socorro Co., New Mexico; exceptional specimen 18 cm long showing that many crystals grew radially to form a rounded mass typical of smithsonite; this shade of blue is the most common color of smithsonite, but other shades of blue and green are also prized; UMNH.MC.0924. Fito Torres / NHMU
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Rhodochrosite, Catamarca Province, Argentina; polished specimen of layered buildup of cave growth in a manganese deposit; UMNH.MC.3054. Fito Torres / NHMU
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Calcite, Frizington, England; specimen exhibiting the scalenohedral (dog tooth) crystal form common for calcite and other carbonates; UMNH.MC.2188. Fito Torres / NHMU
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Rhodochrosite, N’Chwaning I Mine, Northern Cape, South Africa; another example showing the scalenohedral crystal form; UMNH.MC.0879. Fito Torres / NHMU
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Aragonite forming helectite-like cave growth; Mercur Dome Mine, Mercur, Utah; UMNH.MC.2294. Fito Torres / NHMU
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