by Harold L. Levin
Chapter 4 - page 8
Rocks and Minerals: Documents that Record Earth's History
Chemical/biochemical Sedimentary Rocks
This group includes the evaporites, the carbonates (limestones and dolostone), and the
These rocks form within the depositional basin from chemical components dissolved in the
seawater. These chemicals may be removed from seawater and made into rocks by chemical
processes, or with the assistance of biological processes (such as shell growth). In some cases it
is difficult to sort the two out (in carbonates or some siliceous rocks, for example), so they are
grouped together as chemical/biochemical.
- Evaporites - The evaporites form from the evaporation of water (usually seawater).
- Rock salt - composed of halite (NaCl).
- Rock gypsum - composed of gypsum (CaSO4 . 2H2O).
- Travertine - composed of calcium carbonate (CaCO3), and therefore, also technically a carbonate rock; travertine forms in caves and around hot springs.
Travertine deposits in a cave. Photo courtesy of Pamela Gore.
- Carbonates - The carbonate sedimentary rocks are formed through both chemical and biochemical processes.
They include the limestones (many types) and dolostones.
- Minerals in Carbonate Rocks:
Carbonate Rock Names:
Micrite (microcrystalline limestone) - very fine-grained; may be light
gray or tan to nearly black in color. Made of lime mud, which is also called calcilutite.
Oolitic limestone (Look for the sand-sized oolites.)
- Calcite (CaCO3)
- Aragonite (CaCO3)
- Dolomite (CaMg(CO3)2)
Sand-sized oolites or ooids
Fossiliferous limestone (look for various types of fossils in a limestone matrix)
Coquina (fossil hash cemented together; may resemble granola)
Fossiliferous limestone. Photo courtesy of Pamela Gore.
Chalk (made of microscopic planktonic organisms such as coccolithophores; fizzes readily in acid)
Travertine (see evaporites)
Others - intraclastic limestone, pelleted limestone, etc.
Coquina. Photo courtesy of Pamela Gore.
Siliceous rocks - The siliceous rocks are those which are dominated by silica (SiO2).
They commonly form from silica-secreting organisms such as diatoms, radiolarians, or
some types of sponges. Chert is formed through chemical reactions of silica in solution
replacing limestones. Silica leaching from beds of volcanic ash also replaces limestone below the ash layers.
- Diatomite - looks like chalk, but does not fizz in acid. Made of microscopic
planktonic organisms called diatoms. May also resemble kaolinite, but
is much lower in density and more porous). Also referred to as Diatomaceous
- Chert - massive and hard, microcrystalline quartz. May be dark or light in color.
Often replaces limestone. Does not fizz in acid.
Organic Sedimentary Rocks (Coals)
This group consists of rocks composed of organic matter (mainly plant fragments)
which have accumulated in swamps or marshes.
Because of this, they lack minerals (which must be inorganic, by definition).
These are the coals. In order of
increasing depth of burial (temperature and pressure):
- Peat (porous mass of brownish plant fragments resembling peat moss).
- Lignite (crumbly and black).
- Bituminous coal (dull to shiny and black; sooty; layers may be visible).
- Anthracite coal (extremely shiny and black, may have a slight golden shine; low
density; not sooty; technically a metamorphic rock due to high temperatures and
pressures to which it has been subjected).
Coal is a fossil fuel that is the source of more than half of all electricity produced in the United States.
Power plants burn coal to make steam.
The steam turns turbines which generate electricity.
Electric utility companies use more than 90% of the coal mined in the United States.
Coal is also used in industry. Chemicals derived from coal are used in making plastics,
tar, synthetic fibers, fertilizers, and medicines.
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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
September 10, 2005