Dolomite

Dolomite is a sedimentary rock formed from a reaction between carbonic acid with calcium and magnesium. It forms in ancient marine environments, facilitated by chemical transformations that replace other carbonate rocks. Dolomite also can be found in limestone deposits, an important source of petroleum reservoirs because of its naturally porous structure. Dolomite is used for a variety of industrial processes, and colored dolomite is highly prized as an ornamental and decorative stone; dull varieties are often used as a component in construction materials, including concrete.

Carbonate Minerals

Dolomite is a carbon-rich mineral classified as a type of sedimentary rock. Sedimentary rocks are formed from the accumulation of small mineral fragments, dust, and dissolved compounds that are joined through chemical bonding. When this fragmentary sediment is buried and compressed, it solidifies.

Dolomite (CaMg(CO₃)₂) belongs to the class of minerals known as carbonates because they contain carbonate ions (CO₃^2-) within their chemical structure. Carbonate is a salt, which is a type of compound that forms from a reaction between an acid and a base. Carbonate is created from reactions involving carbonic acid and basic substances in the environment. In the case of dolomite, these basic molecules are magnesium and calcium. Carbonate minerals tend to form in aqueous environments rich in living organisms.

The most common type of carbonate mineral is calcite (CaCO₃), which is a component of a variety of sedimentary rocks and is present on every continent. Calcite is commonly blended with limestone in naturally occurring deposits, which are harvested and processed to create building stone.

Dolomite is similar to calcite in that it also contains atoms of calcium bonded to carbonate ions. In addition, dolomite molecules contain atoms of magnesium, and the presence of magnesium affects dolomite’s color, chemical characteristics, and environmental occurrence.

Dolomite is further classified as a member of the anhydrous carbonates, which are carbonates that do not contain water molecules embedded in their crystal structure. Most anhydrous carbonates can also form into hydrated carbonates in environments where humidity and ambient moisture increase. Monohydrocalcite, for instance, is a hydrated form of calcite that is not stable at room temperature and exists only where temperatures are below freezing. Monohydrocalcite contains molecules of water bonded to carbon and calcium within the mineral matrix. Dolomite exists only in the anhydrous form, though there are hydrated carbonates that form from the combination of carbonate and magnesium.

Dolomite is closely related to ankerite, a carbonate mineral that consists of carbonate ions bonded to calcium, with magnesium, iron, and manganese integrated into the crystal lattice. Ankerite is basically dolomite in which a portion of the magnesium has been replaced by iron or manganese atoms. Ankerite and dolomite can appear in the same geologic deposits, and both minerals form from carbon- and calcium-rich aqueous solutions in which various concentrations of metallic elements contribute to the formation of dolomite in some areas and ankerite in others.

Formation of Dolomite

Dolomite exists in two basic forms: as a pure mineral crystal called simply dolomite and as a compressed component of limestone deposits called dolomitic limestone. As a component of limestone, some dolomite is present in a variety of building materials and sculptural stones, though this dolomite is generally indistinguishable from the surrounding limestone without chemical analysis. Dolomite is a common component of many types of sedimentary rocks, often occurring in mixed beds with other carbonates.

Dolomite mineral is an evaporite, which is a type of mineral that forms primarily in aqueous, saline environments, such as salt pans, salt marshes, and salinas or salt pools. In these environments, water, which is generally derived from a nearby marine environment, is enriched in chemicals from the surrounding sediment. Solar radiation causes water to evaporate from the environment, transforming into gaseous water vapor at the water surface and moving into the atmosphere. As this occurs, the remaining material is concentrated into a mineral-rich residue; further evaporation leads to the formation of crystal deposits. In thousands of years, evaporite minerals can form into large deposits several hundred feet thick within the earth’s crust.

Dolomite is common in ancient rock deposits. Until recently, geologists were uncertain about the mechanisms of dolomite formation because they could not find dolomite forming on the surface of the earth. Geologists have been trying to discover the reason for this phenomenon, often called the dolomite problem, for more than two hundred years.

Some geologists have suggested that most dolomite is first deposited as a different type of carbonate mineral, like calcite or aragonite. Exposure to warm, magnesium-rich saline waters is then believed to initiate a process called diagenesis, which results in a physical alteration of the rock’s chemical structure, giving rise to dolomite. Diagenesis is similar to metamorphosis, which is the transformation of one type of mineral to another at high temperatures and pressures. By contrast to metamorphosis, diagenesis occurs at lower temperatures and pressures, such as those that commonly exist on the surface of the earth.

In 2011, Australian geologists discovered dolomite minerals within mats of red algae (Hydrolithon onkodes), a species common in tropical reef habitats. Red algae assist in reef building by helping to cement corals to the sea floor, thereby enabling the coral to resist the hydraulic pressure from incoming waves. This is accomplished because red algae deposit calcium around the base of the coral colonies as they metabolize food ingested from the surrounding water.

Using X-ray diffraction, geologists determined that the cells of red algae contain dolomite, magnesium, and calcite. The organism’s conversion of calcite has long been understood as the mechanism behind the organism’s function in cementing coral beds, but cellular dolomite is a new discovery and now believed to provide a solution to the dolomite problem.

Geologists theorize that the buildup of dolomite is caused by the spread and eventual death of red algal species on the floor of reef ecosystems. As the organisms die, their bodies decompose, leaving deposits of dolomite, calcite, and magnesite. Given this recent discovery, it appears that dolomite may actually form primarily in shallow, marine reef systems rich in red algae and coral.

After dolomite mineral has been deposited in its crystalline form, it may be buried and blended with other minerals to form dolomitic limestone. This occurs when deposits of limestone and dolomite are buried and compressed, a process called sedimentation. In thousands or millions of years, the compressed minerals form into large, solid deposits. The development and decline of marine ecosystems thereby constitute an avenue for the generation of a variety of calcium-rich minerals and rocks, including dolomite, calcite, and mixed-rock types like dolomitic limestone.

Dolomite is commonly found in rocks that are more 100 million years old. For many years, researchers hoped to understand how dolomite grows in nature to aid in its production. In 2023, researchers determined that dolomite could be grown in a lab if defects in its mineral structure were removed as it grows. These defects prevent additional layers of dolomite from forming, which majorly slows the rock's growth, sometimes by 10 million years.

Structure of Dolomite

Carbonate minerals tend to form in sheets, because carbonate molecules bond in such a way that they tend to form discrete molecular layers. In calcite, sheets of carbonate molecules alternate with sheets of calcium. In dolomite, magnesium atoms form into a single layer, alternating with layers of calcium and carbonate. Atoms of calcium and magnesium form into alternating layers because the atoms differ in size and are more stable when organized into alternating sheets, rather than in mixed layers containing both types of atoms.

Dolomite crystals typically form in a rhombohedral pattern, which is composed of rectangles of crystals layered onto one another. Dolomite also engages in “twinning,” whereby two crystals share certain points of their lattice, growing from a shared connection, called a twin boundary or a composition surface. Because of twinning, the crystals often develop saddle-shaped depressions on one side of the crystal. Saddles are not present in all crystals, and some form more traditional, pointed rhombohedrons.

Luster describes the way that a substance or object interacts with incoming light. The luster of a particular crystal is related to the transparency of the materials constituting the crystal and the degree of light refraction caused by the presence of different elements within the lattice. Many dolomite crystals develop a pearly luster, which consists of translucent crystals, sometimes appearing milky with a pearlescent shine. Other dolomite crystals display a vitreous luster, which is described as glass-like and transparent, and some dolomite crystals display a dull luster, a translucent, nonreflective surface.

Dolomite forms in a variety of colors, ranging from white and off-white to green and pink. Dolomite with a yellow hue may reflect the presence of additional magnesium within the lattice. Dolomite crystals with a brownish or blackish hue result when iron becomes blended with the dolomite during formation or subsequent compaction. Pure dolomite crystals are generally pink in hue and mostly translucent, reflecting the properties of magnesium and calcium and their effect on light refraction and absorption.

Fracture is a measure of the way that crystals tend to break when subjected to force or pressure. The appearance and structure of a fractured crystal is related to characteristics involving the bonding of atoms within the crystal’s chemical structure. Dolomite crystals display conchoidal fracturing, which results in fractures that follow an even plane between crystals and generally result in smooth edges. This property of dolomite is largely caused by the layering of molecules within the mineral’s lattice, allowing for relatively even fractures to develop along the zones where layers meet.

Uses of Dolomite

Dolomite is primarily used as an ornamental stone because of its attractive crystals that are clear and colored. Smoothed and polished dolomite can therefore be used as decoration or in fabricating home furnishings and decorative stone accessories.

Dolomite also is an ingredient used in making concrete for industrial processes. Dolomitic limestone is used as a construction material and may be polished and used in a similar way to decorative marble.

In some cases dolomite is used as a precursor for deriving magnesium by subjecting the mineral to a chemical environment that separates the metals from the mineral structure. A significant amount of industrial magnesium is derived from processing dolomite.

Dolomite rock, especially that derived from ancient marine ecosystems, is known as a productive reservoir for fossil fuels. Dolomite deposits, and deposits of carbonate rock in general, tend to be porous and to fracture within the crust, leaving areas of reduced density that can serve as traps for the accumulation of petroleum and natural gas.

Petroleum may form in these same environments because of the richness of biological fauna and flora that live and die in shallow marine ecosystems. In millions of years, these decomposing organisms form into hydrocarbon residue that is buried within the surrounding carbonate rocks.

Mining dolomite quarries to obtain petroleum is a multimillion-dollar industry. Dolomitic limestone deposits are one of the leading sources for petroleum in some parts of the world. Dolomitic limestone is particularly likely to contain petroleum because it forms primarily from the chemical transformation, through biogenesis, of calcite limestone. When this occurs, the resulting rock is more porous, containing more pockets and small reservoirs in which petroleum oil can accumulate. Mining dolomitic limestone not only yields petroleum oil but also yields raw dolomite and limestone that can be further processed and sold for industrial applications.

Principal Terms

anhydrous: having little or no water in its physical structure

carbonate: ionic salt formed from the interaction of carbonic acid with basic elements

diagenesis: physical and chemical change occurring within a rock after solidification; results in the formation of an alternate mineral and occurs at temperatures and pressures below those required for metamorphosis

evaporite: type of mineral resulting from concentration within an evaporating aqueous medium

fracture: characteristic used for mineral classification based on the shape and texture of fragments produced when the mineral is subjected to force or pressure

luster: characteristic used for mineral classification based on the degree to which the mineral refracts light

petroleum: substance formed from the liquefied remains of fossilized organisms compressed beneath the earth’s crust and often found in deposits of dolomitic limestone

rhombohedral: crystal structure based on the combination of different angular lengths and resulting in a combination of rectangular shaped sections

salt: ionic compound formed from a reaction between an acidic and basic substance

twinning: process in crystal formation in which two separate crystals share one or more points of connection between their respective lattices

Bibliography

Braithwaite, C. J. R., G. Rizzi, and G. Darke. “The Geometry and Petrogenesis of Dolomite Hydrocarbon Reservoirs: Introduction.” In The Geometry and Petrogenesis of Dolomite Hydrocarbon Reservoirs, edited by C. J. R. Braithwaite, G. Rizzi, and G. Darke. Bath, England: Geological Society, 2004.

Flugel, Erik. Microfacies of Carbonate Rocks: Analysis, Interpretation, and Application. 2d ed. New York: Springer, 2010.

Hyne, Norman J. Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production.2d ed. Tulsa, Okla.: Pennwell Books, 2001.

King, Hobart M. "Dolomite." Geology.com, 2024, geology.com/rocks/dolomite.shtml. Accessed 25 July 2024.

Monroe, James S., Reed Wicander, and Richard Hazlett. Physical Geology. 6th ed. Belmont, Calif.: Thompson Higher Education, 2007. General-interest text discussing processes, methodology, and theoretical principles behind modern geological research. Includes a brief discussion of dolomite chemistry, mining, and industrial uses.

Plummer, Charles C., Dianne H. Carlson, and David McGeary.Physical Geology. 13th ed. Columbus, Ohio: McGraw-Hill Higher Education, 2009.

Smith, Derek. "'Dolomite Problem': Two-Hundred-Year Old Mystery Resolved." University of Michigan, Michigan News, 23 Nov. 2023, news.umich.edu/200-year-old-geology-mystery-resolved/. Accessed 28 July 2024.