Carbonate Minerals

Generally, the carbonate minerals are found at or near the surface. They represent the Earth's largest storehouse of carbon. They all are on the soft side, from hardness 3 to 4 on the Mohs hardness scale.

Every serious rockhound and geologist takes a little vial of hydrochloric acid into the field, just to deal with the carbonates. The carbonate minerals shown here react differently to the acid test, as follows:

01
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Aragonite

Calcium carbonate
Photo (c) 2007 Andrew Alden, licesned to About.com

Aragonite is calcium carbonate (CaCO3), with the same chemical formula as calcite, but its carbonate ions are packed differently. (more below)

Aragonite and calcite are polymorphs of calcium carbonate. It is harder than calcite (3.5 to 4, rather than 3, on the Mohs scale) and somewhat denser, but like calcite, it responds to weak acid by vigorous bubbling. You may pronounce it a-RAG-onite or AR-agonite, though the majority of American geologists use the first pronunciation. It is named for Aragon, in Spain, where notable crystals occur.

Aragonite occurs in two distinct places. This crystal cluster is from a pocket in a Moroccan lava bed, where it formed at high pressure and relatively low temperature. Likewise, aragonite occurs in greenstone during the metamorphism of deep-sea basaltic rocks. At surface conditions, aragonite is actually metastable, and heating it to 400°C will make it revert to calcite. The other point of interest in these crystals is that they are multiple twins that make these pseudo-hexagons. Single aragonite crystals are shaped more like tablets or prisms.

The second major occurrence of aragonite is in the carbonate shells of sea life. Chemical conditions in seawater, notably the concentration of magnesium, favor aragonite over calcite in seashells, but that changes over geologic time. Whereas today we have "aragonite seas," the Cretaceous Period was an extreme "calcite sea" in which the calcite shells of plankton formed thick deposits of chalk. This subject is of great interest to many specialists.

02
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Calcite

Calcium carbonate
Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Calcite, calcium carbonate or CaCO3, is so common that it's considered a rock-forming mineral. More carbon is held in calcite than anywhere else. (more below)

Calcite is used to define hardness 3 in the Mohs scale of mineral hardness. Your fingernail is about hardness 2½, so you can't scratch calcite. It usually forms dull-white, sugary-looking grains but may take on other pale colors. If its hardness and its appearance aren't enough to identify calcite, the acid test, in which cold dilute hydrochloric acid (or white vinegar) produces bubbles of carbon dioxide on the mineral's surface, is the definitive test.

Calcite is a very common mineral in many different geologic settings; it makes up most limestone and marble, and it forms most cavestone formations like stalactites. Often calcite is the gangue mineral, or worthless part, of ore rocks. But clear pieces like this "Iceland spar" specimen are less common. Iceland spar is named after classic occurrences in Iceland, where fine calcite specimens can be found as big as your head.

This is not a true crystal, but a cleavage fragment. Calcite is said to have rhombohedral cleavage because each of its faces is a rhombus or warped rectangle in which none of the corners are square. When it forms true crystals, calcite takes platy or spiky shapes that give it the common name "dogtooth spar."

If you look through a piece of calcite, objects behind the specimen are offset and doubled. The offset is due to the refraction of the light traveling through the crystal, just as a stick appears to bend when you stick it partway into water. The doubling is due to the fact that light is refracted differently in different directions within the crystal. Calcite is the classic example of double refraction, but it's not that rare in other minerals.

Very often calcite is fluorescent under a black light.

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Cerussite

Lead carbonate
Photo courtesy Chris Ralph via Wikimedia Commons

Cerussite is lead carbonate, PbCO3. It forms by weathering of the lead mineral galena and may be clear or gray. It also occurs in massive (noncrystalline) form.

04
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Dolomite

Calcium-magnesium carbonate
Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Dolomite, CaMg(CO3)2, is common enough to be considered a rock-forming mineral. It is formed underground by the alteration of calcite.

Many deposits of limestone are altered to some extent into dolomite rock. The details are still a subject of research. Dolomite also occurs in some bodies of serpentinite, which are rich in magnesium. It forms at the Earth's surface in a few very unusual places marked by high salinity and extreme alkaline conditions.

Dolomite is harder than calcite (Mohs hardness 4). It often has a light pinkish color, and if it forms crystals these often have a curved shape. It commonly has a pearly luster. The crystal shape and luster may reflect the atomic structure of the mineral, in which two cations of very different sizes place stress on the crystal lattice. However, commonly the two minerals appear so much alike that the acid test is the only quick way to distinguish them. You can see the rhombohedral cleavage of dolomite in the center of this specimen, which is typical of carbonate minerals.

Rock that is primarily dolomite is sometimes called dolostone, but "dolomite" or "dolomite rock" are preferred names. In fact, the rock dolomite was named before the mineral that composes it.

05
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Magnesite

Magnesium carbonate
Photo courtesy Krzysztof Pietras via Wikimedia Commons

Magnesite is magnesium carbonate, MgCO3. This dull white mass is its usual appearance; the tongue sticks to it. It rarely occurs in clear crystals like calcite.

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Malachite

Copper carbonate
Photo courtesy Ra'ike via Wikimedia Commons

Malachite is hydrated copper carbonate, Cu2(CO3)(OH)2. (more below)

Malachite forms in the upper, oxidized parts of copper deposits and commonly has a botryoidal habit. The intense green color is typical of copper (although chromium, nickel, and iron also account for green mineral colors). It bubbles with cold acid, showing malachite to be a carbonate.

You'll usually see malachite in rock shops and in ornamental objects, where its strong color and concentric banded structure produce a very picturesque effect. This specimen shows a more massive habit than the typical botryoidal habit that mineral collectors and carvers fancy. Malachite never forms crystals of any size.

The blue mineral azurite, Cu3(CO3)2(OH)2, commonly accompanies malachite.

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Rhodochrosite

Manganese carbonate
Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

Rhodochrosite is a cousin of calcite, but where calcite has calcium, rhodochrosite has manganese (MnCO3).

Rhodochrosite is also called raspberry spar. The manganese content gives it a rosy pink color, even in its rare clear crystals. This specimen displays the mineral in its banded habit, but it also takes the botryoidal habit. The crystals of rhodochrosite are mostly microscopic. Rhodochrosite is far more common at rock and mineral shows than it is in nature.

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Siderite

Iron carbonate
Photo courtesy Geology Forum member Fantus1ca, all rights reserved

Siderite is iron carbonate, FeCO3. It's common in ore veins with its cousins calcite, magnesite, and rhodochrosite. It may be clear but is usually brown.

09
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Smithsonite

Zinc carbonate
Photo courtesy Jeff Albert of flickr.com under Creative Commons licence

Smithsonite, zinc carbonate or ZnCO3, is a popular collectible mineral with a variety of colors and forms. Most often it occurs as earthy white "dry-bone ore."

10
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Witherite

Barium carbonate
Photo courtesy Dave Dyet via Wikimedia Commons

Witherite is barium carbonate, BaCO3. Witherite is rare because it easily alters to the sulfate mineral barite. Its high density is distinctive.