Earth Science

Carbonate Rocks Fields of Study: Crustal and Surface Features; Rocks and Minerals; Sedimentation and Sedimentary Rocks Carbonate rocks make up a class of sedimentary rocks whose primary components are carbonate minerals, most commonly calcium carbonate (limestone) and calcium magnesium carbonate (dolomite). Carbonate rocks compose about one-quarter of the sedimentary rocks found on the earth's surface and have many industrial applications, particularly in construction and agriculture. Carbonate rocks are also extremely good petroleum reservoirs, which makes them a particularly well-studied type of mineral formation. Principal Terms allochem: large, coarse grains about the size of sand or gravel, found in carbonate rocks compaction: the process by which loose grains of sedimentary material are brought closer together diagenesis: any of the physical and chemical changes that take place in sedimentary material after it has been deposited and as it is converted into rock dolomite: a translucent rock mainly composed of a carbonate of calcium and magnesium; one of the two most common types of carbonate rock, the other being limestone facies: the characteristic set of features that distinguish a given rock, such as its physical appearance, composition, and method of formation karst: a geological formation formed when limestone dissolves; this process results in characteristic structures, such as sinkholes, towers, caves, and ridges limestone: a hard rock formed mostly out of calcite, or calcium carbonate; along with dolomite, one of the two most common forms of carbonate rock lithification: the process by which soft particulate sedimentation is transformed into hard, compact sedimentary rock matrix: fine particles, typically of mud, that occupy the spaces between larger grains of sediment petrographic: having to do with the branch of science that describes or analyzes rocks by microscopic study porosity: the extent to which a given material, such as rock, contains minute holes or gaps through which air or liquid can travel sedimentary: in geology, rock that has been formed out of particulate matter

deposited by wind or water on land or at the bottom of a body of water; through time, this material consolidates Properties and Formation of Carbonate Rocks Carbonate rocks are a class of sedimentary rock; that is, they are formed out of tiny particles of matter. Because they are easily transported by wind or water, these particles tend to settle together, either on land or at the bottom of a body of water. In time, this accumulated sediment becomes lithified, or transformed into a solid material. Unlike both igneous rock, which is formed when volcanic lava or magma solidifies, and metamorphic rock, which is formed under the influence of tremendous heat or pressure, the physical, chemical, and biological processes that result in the formation of sedimentary rocks take place at the surface of the earth. Three quarters of the earth's surface is covered with various types of sedimentary rock. Of these, about 20 to 25 percent are carbonate rocks. If more than one-half of the particles that make up a sedimentary rock are compounds of the element carbon, then the rock is known as a carbonate rock. The two most common types of carbonate rocks are limestone and dolomite. Limestone is composed mainly of either calcite or aragonite. These are both forms of calcium carbonate, a compound that has the chemical formula CaCO3, but they have different crystalline structures. Limestone can be formed in several ways. Some limestone is made up of organic materials, such as the bodies of marine organisms. When they are alive, these organisms absorb dissolved calcium carbonate from water and use it to form their shells and skeletons; when they die, these shells and skeletons disintegrate to form sediment that is consolidated into rock. The White Cliffs of Dover, a long series of chalk cliffs that line part of the English coast, are composed of limestone of organic origin; so are coral reefs. Some limestone is made up of inorganic materials. Stalagmites and stalactites, the tapered columns and icicle-like structures commonly found in caves, are examples of this type of limestone. They form when water drips through the walls, ceilings, and cave floors; as this happens, calcium carbonate that is dissolved in the water precipitates, or is deposited in solid form. Dolomite, also known as dolostone, is formed when some or all of the calcium ions in limestone are replaced by magnesium ions; this can happen, for instance, when seawater that contains magnesium trickles past limestone rock. This process forms a compound known as calcium magnesium carbonate, which has the chemical formula CaMg(CO3)2. Both limestone and dolomite appear in a huge variety of forms. Each has different physical properties depending on the type and amount of impurities it contains (substances such as quartz, clay, pyrite, and silica). Carbonate rocks are soluble in acid, and ordinary rainwater is slightly acidic. Therefore, in areas where the bedrock is largely composed of carbonate rocks like limestone and dolomite, a particular type of landscape known as karst terrain tends to form. Karst terrain is characterized by structures such as sinkholes, underground caves, shafts, towers, and rock faces that are pitted, ridged, or grooved. (In some places, enough rock dissolves to create underground passages through which groundwater can easily travel. This results in a karst aquifer, a source of water that can be tapped for household or commercial use.) Classification Systems for Carbonate Rocks Several different systems classify carbonate rocks. Three of the most common are

grain-size classification, the Folk system, and the Dunham system. Grain-size classification places carbonate rocks into different categories depending on the size of the majority of the grains, or particles, that can be seen within the rock. The name "calcilutite" is given to carbonate rocks with a grain-size of 62 micrometers, or about 0.0025 inches, or smaller; the name "calcarenite" is given to rocks with a grain-size of 62 micrometers to 2 millimeters, or about 0.0025 to 0.08 inches; and the name "calcirudite" is given to rocks with a grain-size larger than 2 millimeters (about 0.08 inches). The Folk classification, devised by University of Texas geoscientist Robert L. Folk, separates carbonate rocks according to the major type of grain they contain and the nature of material in which these grains are embedded. For example, a rock containing mostly grains of fossilized marine organisms (biological material) embedded in a matrix of micrite (a fine, clay-like substance formed of tiny crystals of calcium carbonate) would be called biomicrite. A rock containing mostly grains of intraclast (old pieces of eroded limestone) embedded in the same micrite matrix would be called intramicrite. Devised by Robert J. Dunham, the Dunham classification places carbonate rocks into five major groups based on texture and formation. Boundstones are rocks made of particles that are generally in contact with each other and that appear to have consolidated into a solid mass as they were deposited. All other rocks in the Dunham system are made of particles that are separated by some support material; unlike boundstone, these particles did not come into contact with each other as they were deposited, but consolidated because a framework of some kind formed between them. These rocks are subdivided into four groups-mudstone, wackestone, packstone, and grainstone-according to the type of support material they contain and the proportion of particles to support material. Field and Laboratory Analysis Geoscientists use a variety of techniques to study carbonate rocks both in the field and in the laboratory. In the field, a set of simple tools like a hand-held magnifying glass, a grain-size comparator (a small translucent chart with a printed scale that is held over a rock sample), a measuring tape, and a pen knife can be used to generate a detailed record of physical observation. Qualities that a researcher may note about a given sample of carbonate rock include, but are not limited to, color, degree of luster, degree of translucency, apparent age (whether the rock looks fresh or weathered), dominant grain size, uniformity of grain size, type of grains, shape of grains, orientation or angle of the grains, and the nature and amount of the matrix, or support framework, which exists between the grains. The overall context in which a rock is found is also important to note. For instance, the beds, or the horizontal layers in which sedimentary rock are typically stacked, may be tilted slightly as a result of the movement of the earth's plates; this tilt, known as an angle of dip, is another point of data a geologist will collect. To determine the relative hardness of a carbonate rock sample on a scale of one to ten (known as the Mohs scale of mineral hardness), researchers may scratch the rock's surface with a fingernail, a copper penny, or a small glass plate. To distinguish between limestone and dolomite rocks, a few drops of a solution of dilute hydrochloric acid can be dripped onto the sample. Dolomite rocks will react slowly and weakly with the acid, while limestone rocks will produce an immediate active fizzing reaction with large bubbles.

The physical characteristics a researcher observes about a given rock are known collectively as facies. Together, facies provide important insights into when, how, and out of what materials the rock was formed. However, carbonate rocks can be studied in other ways, such as through petrography, the study of rocks in thin slices under a microscope. This is an extremely important technique for describing the underlying structural features of carbonate rocks. The characteristics that are revealed in this way are known as microfacies. Features like grain size, grain composition, and the amount and nature of support material between grains may not be visible to the naked eye, so being able to examine a rock's microfacies is an essential step in classifying that rock. Economic Uses of Carbonate Rocks Carbonate rocks have a large number of human uses, many of which are important in the construction, manufacturing, and agricultural industries. Limestone and dolomite rocks that are harvested for industrial use typically have a rather high concentration of calcium carbonate or calcium magnesium carbonate (at least 80 percent as opposed to only 50 percent). Because they contain fewer impurities, these commercial-grade rocks have physical properties that are more predictable. Crushed limestone gravel is used to make concrete, mortar, brick, and tile, and it is also used as a road-surfacing material and to form foundations for the load-bearing parts of buildings. Larger, denser, more durable pieces of limestone are cut into sturdy blocks that can be used as building materials or to provide support for structures such as dams that are subject to water erosion. Pulverized limestone is added to the soil on many industrial farms, where it serves as a fertilizer and acid neutralizer. Both limestone and dolomite find their way into a wide variety of household products, including paper, paint, soap, pharmaceuticals, varnish, and glass. They are used as ingredients in feed for livestock, to treat water and sewage, to purify steel during its manufacturing process, and to minimize the spread of coal dust from mines. Although marble itself is not a carbonate rock, it is formed from the metamorphosis of limestone and dolomite rocks. Under great heat or pressure, the crystal grains in limestone and dolomite can grow in size and become more firmly interlocked. This process is what gives marble its characteristic hardness and makes it an ideal material for building and construction. Perhaps the most significant industrial application of carbonate rock does not involve the rock itself, but something it contains: petroleum. Many factors that come into play during the formation of carbonate rocks result in these rocks having a high degree of porosity. For instance, even after the grains that compose it are compacted and lithified, small gaps often remain between them. Sometimes, either the grains themselves or the mud or clay matrices in which they are trapped can begin to disintegrate and be carried away by water moving through the rock. This porous nature makes carbonate rocks extremely good natural reservoirs for petroleum deposits. Further Reading Ahr, Wayne R. Geology of Carbonate Reservoirs: The Identification, Description, and Characterization of Hydrocarbon Reservoirs in Carbonate Rocks. Hoboken, N.J.: John Wiley & Sons, 2008. A well-organized, advanced resource that demands some prior knowledge; suitable for upper-undergraduate and graduate-level readers and essential for petroleum geologists and engineers. Each chapter concludes with study

questions. Flügel, Erik. Microfacies of Carbonate Rocks. New York: Springer, 2004. Hundreds of photographic plates, figures, and diagrams crowd this clearly written, highly technical primer on the methods, interpretive processes and practical applications of microfacies analysis. Suitable for college students who have already acquired a basic knowledge of geology. Huddart, David, and Tim Stott. Earth Environments: Past, Present, and Future. Hoboken, N.J.: John Wiley & Sons, 2010. Together, chapters 11 and 18 provide motivated undergraduate students with the ideal launching pad for delving into the latest primary research. Hugget, Richard J. Fundamentals of Geomorphology. New York: Routledge, 2011. This accessible textbook, suitable for advanced high-school students who are prepared for some technical language, introduces basic concepts about the relationship between surface land features and underlying geological structures. See especially chapter 14, on karst landscapes. Middleton, Gerard V., ed. Encyclopedia of Sediments and Sedimentary Rocks. Boston: Kluwer Academic, 2003. An extremely comprehensive and well-written reference book, ideal for college-level students of earth science. Each substantial entry is broken down into logical subtopics and is carefully cross-referenced. Pohl, Water L. Economic Geology: Principles and Practice. Hoboken, N.J.: WileyBlackwell, 2011. Students interested in the industrial applications of carbonate rocks should consult chapter 3, which links the physical and chemical properties of various limestone and dolomite rocks to their human uses. Prothero, Donald R., and Frederic L. Schwab. Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy. New York: W. H. Freeman, 2004. Written in an engaging, conversational style, this student-friendly textbook is meticulously well organized and easy to follow but lacks color photographs. Scholle, Peter A., and Dana S. Ulmer-Scholle. A Color Guide to the Petrography of Carbonate Rocks: Grains, Textures, Porosity, Diagenesis. Tulsa, Okla.: American Association of Petroleum Geologists, 2003. Hundreds of full-color photographic plates make this both a visually appealing book and an invaluable reference guide for anyone wishing to study carbonate rocks under the microscope. Tucker, Maurice E. Sedimentary Rocks in the Field. Hoboken, N.J.: John Wiley & Sons, 2011. A practical field guide for students of geology interested in taking on an active research project. Covers tools and techniques for collecting samples and measuring and recording lithologies, textures, and other features of sedimentary rocks. M. Lee See Also Clastic Rocks; Coastal Processes; Deserts and Dunes; Fluvial Processes; Minerals; Paleoecology; Weathering.