The carbonate minerals have as their basic building block the carbonate ion complex, CO 3 , which has an overall -2 charge. It tends to form bonds with atoms having a small number of electrons in the valence shell. In the case of the first two minerals, calcium combines with CO 3 to form calcite, and magnesium does as well, with calcium, to form dolomite. Both calcium and magnesium have two electrons in their valence shell, which serve to satisfy the octet rule in combination with CO 3 . You will notice that the third carbonate mineral shown here, azurite, has copper atoms combined with the carbonate ion group. The valence of copper requires that a little something else be combined in to satisfy the octet rule, and that is the OH, or hydroxide ion. Here are the elements involved in the carbonates covered here:
Elements in carbonates
Chemical formula: CaCO 3 (calcium carbonate)
Structure: Calcite can be found in beautiful six-sided crystals (hexagonal crystal form), and as crystal masses called "dog's tooth spar."
Hammer behavior: Calcite possesses three planes of weakness within the crystalline structure. These planes are not at 90 degrees; if they were, cubic cleavage would be apparent. Instead, the angles are at 75 degrees, leading to rhombohedral cleavage. Rhombohedral cleavage (breaking up into rhombohedrons, if clean cleavage surfaces occur) may be thought of as forming a "skewed cube" appearance, as if you had put your hand on the corner of a cube and pressed it over. In some very pure calcite that is perfectly transparent, double refraction of light occurs, such that when you put a calcite rhombohedron on a sheet of paper with a mark, the mark will appear doubled.
Color: variable, with examples being colorless, white, yellow, gray, but can also appear with green, brown, red, blue tones.
Diaphaneity: transparent to translucent
Occurrence: Calcite is one of the most common minerals on Earth's surface. It is found primarily in sedimentary carbonate rock called limestone. Limestone is, in fact, made mostly of calcite, most often as an accumulation of the calcite shells of clams, snails, echinoderms, and other marine organisms. It can form as an inorganic precipitate associated with carbonate sedimentary rocks, especially in seams, and forms the material found in cave formations (caves form in limestone). Calcite is a common cement in many sedimentary rocks. It is also the material in marble, which is metamorphosed limestone. In marble, calcite crystals have been recrystallized to grow larger than in the original rock.
Use: Calcite (as source material in limestone) is used in fertilizers and soil conditioners, and is an important constituent in cement.
Special Property: Calcite will effervesce (fizz) when dilute (watered-down) hydrochloric acid is applied. This is often touted as a key to identifying calcite, but usually the rhombohedral cleavage makes it easy to identify.
Chemical formula: CaMg(CO 3 ) 2 (calcium-magnesium carbonate)
Structure: Similar to calcite's, with Magnesium in the crystalline structure, leading to a slightly different structure.
Hammer behavior: As with calcite, rhombohedral cleavage (three directions of cleavage, not at 90 degrees).
Color: white, gray, pink, brown
Hardness: 3.5 to 4
Occurrence: Dolomite is common in the sedimentary deposit called dolostone, which is similar, of course, to limestone. In fact, some dolomite is formed from original calcite in limestone, where the magnesium was added by chemical processes after burial of the original calcite material. Dolomite can also form in sedimentary environments where groundwaters rich in magnesium interact with limestone. Dolomite is also found in association with some ore deposits of metals.
Use: Dolomite is used as a soil additive, dietary supplements, and in construction.
Special Property: Notably, dolomite does not effervesce as strongly as calcite. Often it is only in powdered form that a "fizz" is apparent, and even then is often slight.
Chemical formula: 2CuCO 3 . Cu(OH) 2 (copper carbonate, and copper hydroxide)
Structure: Azurite has a very interesting molecular structure that results in brilliant blue color.
Hammer behavior: Azurite is found in irregular masses exhibiting a rather sharp-edged fracture.
Color: vivid royal blue
Streak: light blue
Hardness: 3.5 to 4
Occurrence: Azurite is often found with a companion mineral, malachite, also a copper carbonate. Azurite is deep blue; malachite is deep green, leading to a striking association in some specimens. Both of these minerals form from the weathering of copper ore.
Use: Azurite is a copper ore. It was also used as a traditional paint pigment, until synthetic coloring was developed.