Carbonates

Carbonate minerals are characterized by a composition that features the carbonate ion. Common carbonate minerals are divisible into calcite, aragonite, and dolomite groups.

Formation

Carbonates are one among several classes of mineral, which are the materials that make up rocks. They are natural substances with a definite chemical composition and an ordered internal arrangement. Minerals can be divided into chemical groups based on their atoms and into structural groups based on the atoms’ ordered arrangement.

All carbonates contain the carbonate ion as their defining anionic group. An ion is an atom that has lost or gained electrons and so has become chemically reactive. An ion that has lost electrons has a positive charge and is called a cation; an ion that has gained electrons has a negative charge and is called an anion. The number of electrons lost or gained represents the charge of the ion. The charge and the radius (size) of an ion determine how it is chemically bonded to another ion, how strong the bonding is, and the number of ions that can be coordinated to it or surround it.

The carbonate ion contains one carbon ion in the middle of a triangle formed by three oxygen ions, which occupy the corners of the triangle. The central carbon ion has a charge of +4 and the oxygen ions have a charge of -2. As a result, the carbonate ion group acts as if it were a single ion with an overall -2 charge. To form a carbonate mineral, the negative charge on the carbonate ion must be balanced by cations such as calcium, magnesium, strontium, manganese, and barium.

Carbonate Mineral Structure

Atoms in a mineral can be pictured as lying on imaginary planes, which are called atomic planes. These planes cut across other planes, forming definite and measurable intersection angles. The atomic planes that contain many atoms tend to develop into crystal faces. Minerals are crystalline, and the crystal faces of a mineral are related to the internal arrangement of atoms. The bonds between atoms of the same atomic plane tend to be stronger than the bonds across the atomic planes. Consequently, minerals commonly break or cleave along preferred planes, or cleavage planes, when struck by a hammer. A set of parallel cleavage planes yields one cleavage direction. Some minerals have more than one cleavage direction; others have none, since the bond strength between atoms is the same in all directions. Common carbonate minerals have three cleavage directions at oblique angles and cleave into skewed box-like shapes.

The atoms of a mineral are symmetrically related to one another. Since crystal faces are related to the internal arrangement of the atoms, these crystal faces are also symmetrically related. Scientists have found that all minerals can be grouped into thirty-two crystal classes based on their symmetry relations. A small group of atoms forms the basic building block of crystals. This building block is called a unit cell; it is an arrangement of the smallest number of atoms which, as a unit, may be repeated over and over again to form a visible crystal. The volume of a unit cell can be determined from the lengths of its lines and the angles subtended by them. Depending on the cell’s shape, these imaginary lines may be parallel to its edges or may pass through opposite corners, sides, or edges. The lines are called crystallographic axes. The thirty-two crystal classes of minerals can be grouped into six crystal systems based on the shapes of their unit cells.

The common carbonates belong to two of the six: the orthorhombic and hexagonal systems. The orthorhombic crystal system is characterized by having three crystallographic axes that are unequal in length and perpendicular to one another. The unit cell is a rectangular box with unequal sides. The hexagonal system has four axes, all of which pass through a common center. Three of these axes lie on the same plane, are of equal length, and are separated from one another by 120 degrees. The fourth axis is different in length from the others and is perpendicular to them. The hexagonal unit cell is a box with 60-, 90-, and 120-degree angles.

The three common carbonates—calcite, aragonite, and dolomite—belong to three structural types. Calcite and aragonite are polymorphs of the same compound; that is, the same chemical compound occurs in different structures. Aragonite is orthorhombic and less symmetrical than calcite, which is hexagonal. Calcite and dolomite are both hexagonal, but they do not have identical structures. In calcite, calcium atomic planes lie between carbonate ion planes. In dolomite, alternating calcium planes are occupied by magnesium atoms. As a result, calcite has a higher symmetry content than dolomite, although both belong to the same crystal system. Also, dolomite shows more internal order than calcite because it requires the positioning of more different atoms in specific atomic sites.

Different chemical compounds that have identical structures are said to be isostructural. All carbonates that contain divalent cations (+2 charge) whose ionic radii are less than or equal to that of the calcium ion are isostructural to calcite, which has a hexagonal structure. Siderite (iron carbonate), magnesite (magnesium carbonate), and rhodochrosite (manganese carbonate) are isostructural to calcite.

Calcite Group

Calcite is by far the most common of all carbonate minerals. It is commonly off-white, colorless, or transparent. It may exhibit different crystal shapes, but in all cases, a careful examination will reveal the hexagonal crystal structure. It is fairly resistant to abrasion, although it can be scratched by a steel knife. Calcite fizzes and dissolves in acid because the acid breaks down the carbonate ion and releases carbon dioxide. Calcite is the main component of a rock called limestone. Most limestone is at least partly biological in origin, made of calcium carbonate secreted by marine organisms. Limestone is soluble in acid, and most surface water is slightly acidic. Many caves and some sinkholes are found in regions where the rocks are limestone, and formed because calcite is dissolved by groundwater. Sinkholes are formed when the roofs of caves collapse.

Dolomite Group

In the dolomite group, alternating calcium atomic planes are occupied by other divalent ions, such as magnesium in dolomite, iron in ankerite, and manganese in kutnahorite. Since the magnesium, iron, and manganese ions are smaller than the calcium ion, the dolomite group structure is not as highly symmetrical as the calcite structure. Consequently, although both the calcite and the dolomite group minerals are hexagonal, they belong to different classes among the thirty-two classes of crystals. Of the dolomite group minerals, the mineral dolomite is by far the most common. It is fairly common in ancient carbonate rocks called dolostones. Dolomite is white to pinkish and is similar to calcite in many ways; however, it does not fizz readily when diluted acid is dropped on it.

For a long time it was a puzzle why dolostone was so abundant, as dolomite is not forming in many places today. We now know that most dolomite begins as limestone and is converted to dolomite later. In some cases the conversion happens when fresh water mixes with salt water as the rocks are forming, but more recent research suggests that most dolomite forms when warm fluids circulating through the rocks replace calcium with magnesium.

Aragonite Group

In the aragonite group, carbonate ions do not lie on simple atomic planes. Adjacent carbonate ions are slightly out of line; also, adjacent carbonate ions face in opposite directions. Divalent cations whose ionic radii are larger than or equal to the calcium ion form carbonate minerals that are isostructural to aragonite. Of the minerals in the aragonite group, aragonite is the most common. It is generally white and elongate. It is the carbonate mineral that readily precipitates from marine water, but it is not a stable mineral, and in time it changes to the more symmetrical and hexagonal structure of calcite. Aragonite is the least symmetrical polymorph of calcium carbonate, and is found in metamorphic rocks that were formed under high pressure and comparatively low temperature.

In its most abundant form, aragonite is synthesized by aquatic organisms that are common in shallow marine environments of warm latitudes, such as the Gulf Coast. As marine organisms die, their shells, which are made of calcite, settle at the bottom. These shells may be broken into smaller fragments by browsing organisms or wave action. When buried to form limestone, the aragonite quickly recrystallizes to calcite.

Identification of Carbonate Minerals

Carbonates are generally light-colored, soft, and easy to scratch with a knife, as compared with most other common rock-forming minerals. They form a white powder when scratched. They are harder than fingernails, however, and cannot be scratched by them. When acid is poured on carbonate powder, it fizzes, liberating carbon dioxide. Carbonates such as calcite do not even have to be powdered for the acid test, because they fizz readily. The cleavage planes and the angles between cleavages are another physical method by which carbonate minerals can be distinguished. Clear crystals such as those of calcite can produce double refraction of objects, another property that identifies carbonates without the aid of instruments.

Better mineral identification is done by scientists after a rock is cut to a small size, mounted on glass, and ground to a very thin section of rock (0.03 millimeters thick), which is then capable of transmitting light. The thin section is placed on a stage of a transmitted-light polarizing microscope. A lens below the stage polarizes light; it allows the transmission only of light that vibrates in one direction—east to west, for example. Another lens above the stage allows the passage only of light that vibrates in the other direction—north to south. When glass is placed on a stage and the lower polarizer is inserted across the light source, the color of the glass can be seen. When the upper polarizer is also inserted, however, the glass appears dark, because no light is transmitted. The optical properties of most minerals, including the carbonates, are different from those of glass. Other accessories are used in addition to the polarizing lenses in order to determine minerals’ optical properties. Magnification by microscope permits the better determination of minerals’ physical properties, such as their shape and cleavage. Special dyes can also be used to distinguish calcite from dolomite.

Determination of Crystal Structure

The crystal structure of carbonate crystals can be determined with the aid of a contact goniometer. Its simplest version is a protractor with a straight edge fastened in the middle. The goniometer is used to measure the angles between crystal faces, from which scientists can determine the crystal structure.

X-ray diffraction can ascertain the crystal structure of any substance, including carbonates. Diffraction peaks characteristic of each mineral can be displayed on a chart recorder when X rays bombard a sample. Each diffraction peak results from the reinforcement of X-ray reflections from mutually parallel atomic planes within the sample. Several diffraction peaks from one mineral indicate equivalent numbers of sets of atomic planes within the minerals. The difference in the peak heights corresponds to the density of atoms in the pertinent atomic planes. The detection device does not have to be a chart recorder; it can be a photographic paper or a digital recorder that can be appropriately interfaced to a computer for the quick identification of minerals.

Practical Applications

Carbonates are a common group of minerals that form in environments that range from arid lands to shallow seas. After silicates, they are the most abundant minerals on the surface of the earth. Hot springs are one place where calcite precipitates. Travertine, or tufa, is a banded rock that precipitates at the mouth of springs. Caliche, deposits of carbonate that precipitate from groundwater in arid climates, is a source of serious problems to irrigation farmers. Sinkholes that form through the collapse of roof rocks of near-surface caves in the limestone of warm and humid regions are a problem not only to farmers but also to homeowners. It is not unusual for part of a house, or the whole of it, to sink suddenly into a depression caused by the collapse of an underground cave.

Carbonates are important for their regulation of the pH, or acidity content, of ocean waters. Carbonate minerals dissolve when the acid content of water is raised and precipitate when the acid content is reduced. In this way, the pH of ocean water is regulated to a steady value of 8.1.

Carbonates are also known for their industrial applications, which range from dolomite tablets to building materials such as cement and mortar. One of the finest building rocks is marble, which is composed of carbonate minerals. Marble often is delicately banded with different colors. The banding arises because the minerals are lined up in directions perpendicular to the natural pressure under which an impure limestone was metamorphosed and converted to marble. If the original limestone was pure and composed entirely of calcite, the marble that is metamorphosed from it would be white and not banded. Polished marble is used as a building material, or often as decorative stone for doors or exteriors. Polished travertine is also used as building stone, but it is placed in the interiors of buildings because of banded porous zones that can accumulate rainwater. Regular limestone is used in buildings and, most commonly, in retaining walls alongside houses and roads. Most limestone is used for cement. Cement is up to 75 percent lime or calcium oxide, created by heating limestone to drive off carbon dioxide; the rest is silica and aluminum.

The most important natural application of carbonates is in regulating the earth’s climate. Carbonate rocks lock up carbon dioxide that would otherwise trap solar heat and warm the earth. Weathering and other processes also break down calcite and release carbon dioxide. The balance between carbonate formation and breakdown is critical in maintaining the earth’s natural climate balance.

Principal Terms

anion: a negatively charged ion

aragonite: a carbonate with the orthorhombic crystal structure of the calcium carbonate compound; it forms in marine water or under high-pressure, metamorphic conditions

calcite: a carbonate with the hexagonal crystal form of the calcium carbonate compound; a common mineral found in limestone and marble

cation: a positively charged ion

crystal: a solid with an internally ordered arrangement of component atoms

divalention: an ion with a charge of 2 because of the loss or gain of two electrons

dolomite: a double carbonate that includes magnesium and has a hexagonal structure; it is abundant in ancient rocks

ion: an atom that has lost or gained one or more electrons

isostructural: having the same structure but a different chemistry

polymorphs: different structures of the same chemical compound

Bibliography

Ahr, Wayne M. Geology of Carbonate Reservoirs. Hoboken, N.J.: John Wiley & Sons, 2008. Covers many principles of mineralogy with a focus on carbonates. Covers rock properties, petrophysical properties, stratigraphy, deposition, and diagenesis of carbonate reservoirs. A summary chapter on the geology of carbonate reservoirs ties together fundamental topics with specific field examples. Provides references and an extensive index.

Bathurst, Robin G. C. Carbonate Sediments and Their Diagenesis. 2d ed. New York: Elsevier, 1975. Chapter 6 discusses the chemistry and structure of the more common carbonate minerals.

Butler, James Newton. Carbon Dioxide Equilibria and Their Applications. Chelsea, Mich.: Lewis, 1991. Butler discusses in great detail the role of carbonates in the chemical equilibria of carbon dioxide. Includes a short but useful bibliography and an index.

Deer, W. A., R. A. Howie, and J. Zussman. An Introduction to the Rock-Forming Minerals. 2d ed. London: Pearson Education Limited, 1992. A work of reference useful for Earth science students. Carbonates are discussed in detail.

Klein, Cornelis, and C. S. Hurlbut, Jr. Manual of Mineralogy. 23d ed. New York: John Wiley & Sons, 2008. Details of carbonates are treated in chapter 10. Suitable for college-level students.

Loucks, Robert G., and J. Frederick Sarg, eds. Carbonate Sequence Stratigraphy: Recent Developments and Applications. Tulsa, Okla.: American Association of Petroleum Geologists, 1993. Contains several essays that address advances in the study of carbonates as they relate to stratigraphy, as well as their relevance to the petroleum industry. Bibliography and index.

Marino, Maurizio, and Massimo Santantonio. “Understanding the Geological Record of Carbonate Platform Drowning Across Rifted Tethyan Margins: Examples from the Lower Jurassic of the Apennines and Sicily (Italy).” Sedimentary Geology 225 (2010): 116-137. Provides an extensive overview of drowning processes and unconformities as well as specific examples to build on fundamental concepts. Although this is written to convey new research to graduate students and professionals, there is a great deal of background information to make this article accessible to undergraduates.

Mason, Brian, and L. G. Berry. Elements of Mineralogy. San Francisco: W. H. Freeman, 1968. An excellent and easy-to-read book on the study of minerals. Used by many colleges. Carbonates are discussed in Chapter 7.

Ninokawa, Aaron T., et al. "Multiple Carbonate System Parameters Indepently Govern Shell Formation in Marine Muscle." Communications Earth & Environment, 21 May 2024, https://www.nature.com/articles/s43247-024-01440-5. Accessed 25 July 2024.

Parker, Sybil P., ed. McGraw-Hill Encyclopedia of the Geological Sciences. 2d ed. New York: McGraw-Hill, 1988. Offers complete entries on all the common carbonate minerals, including aragonite, dolomite, limestone, and calcite. Written at a college level. Illustrated.

Prinz, Martin, George Harlow, and Joseph Peters, eds. Simon and Schuster’s Guide to Rocks and Minerals. New York: Simon & Schuster, 1978. Rocks and minerals are described and illustrated with color photographs in this easy-to-read book.

Swart, Peter K., Gregor Eberli, and Judith A. McKenzie, eds. Perspectives in Carbonate Geology. Hoboken, N.J.: Wiley-Blackwell, 2009. A collection of papers presented at the 2005 meeting of the Geological Society of America. The papers present studies on carbonate sediments and the comparison of modern and ancient sediments. Suited for the professional geologist or graduate student.

Zhang, Zhongshuo, et al. "Unraveling the Carbonate Issue Through the Regulation of Mass Transport and Charge Transfer in Mild Acid." Chemical Science, 2024, pubs.rsc.org/en/content/articlelanding/2024/sc/d3sc06583a. Accessed 25 July 2024.