Download Carbonate Rocks

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.