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? It's true. We all breathe out carbon dioxide (CO2) which interacts with water vapor in the atmosphere. The result is a weak acid, which falls with rain on ore deposits and slowly dissolves the minerals there.
When CO2 reacts with other elements in ore, it creates what we refer to as “carbonates” — and they come in a variety of vibrant colors! Blue and green carbonate minerals (like azurite and malachite) can form in copper ore deposits, while a pink or red carbonate called rhodochrosite forms where there is manganese. Deposits featuring zinc can produce a carbonate called smithsonite, which comes in a wide variety of colors.
You're likely most familiar with carbonates formed in cave deposits. As carbonated water slowly dissolves the common limestone, stalactites, stalagmites, and helictites are built up on the ceiling, floor and walls of the cave. The resulting minerals are called 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 so the cave growths are made of beautiful pink rhodochrosite.
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 home to 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 the green of malachite. Many mines in Utah, including the Dixie-Apex Mine near Saint George, the Blue Crystal Mine in the Lisbon Valley and the Mammoth Mine near Eureka, have produced beautiful specimens of azurite. Often these carbonates are small crystals.
Zinc ore deposits also yield a carbonate mineral named smithsonite (yes, it's named after the same person from which the Smithsonian Institute received its name). Smithsonite is generally deposited in rounded crusts made up of many crystals side-by-side. In some mines, like the Tsumeb Mine in Namibia, smithsonite is present as crystals with a rhombohedral shape (a six-sided object where every side is a perfect rhombus).
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. But 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, gray, pink, 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 (sometimes called dog-tooth) — it just depends on the crystal structure, or how the atoms arranged themselves. Calcite, rhodochrosite, smithsonite, and siderite all crystallize in one of these two structures.
Check out some of these beautiful examples below!
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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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