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Carbonate Sedimentary Rocks Help

By — McGraw-Hill Professional
Updated on Sep 1, 2011

Carbonate Sedimentary Rocks

Carbonate rocks all have carbon-related compounds in their composition. The two most important minerals found in carbonate rocks are:

  • Calcite (CaCO 3 )
  • Dolomite (CaMg(CO 3 ) 2 )

Carbonate sedimentary rocks are formed through chemical and biochemical processes. They include the limestones , which contain over 80% of the carbonates of calcium and magnesium, and dolostones . Limestone is made up of calcium carbonate (CaCO 3 ) from carbonate sands and mud, while dolostone is made up of calcium–magnesium carbonate (CaMg(CO 3 ) 2 ).

Dolomite formation is little different from some of the other evaporite and chemical sediments. Dolomite is formed by the reaction between sedimentary calcite or aragonite with magnesium ions in any seawater that trickles down through any sedimentary spaces. As the ions are exchanged, some of the calcium ions are switched with magnesium ions and calcium carbonate is then changed into dolomite.

Carbonate rocks are separated by their texture and contents. They include everything from fine mud to a mix-mash of fossils and debris. Table 7-3 lists the separate types and textures of carbonate sedimentary rock.

Table 7-3 Carbonate rocks are formed in a variety of ways.

Carbonate rock type Composition
Micrite (microcrystalline limestone) Very fine-grained; light gray or tan to nearly black
Oolitic limestone Sand-sized oolites
Fossil laden limestone Fossils in a limestone matrix
Coquina Fossil mash cemented together; may resemble granola
Chalk Microscopic planktonic organisms such as coccolithophores
Chert Silica skeletons of sponges, diatoms, and radiolarians
Crystalline limestone Larger grained than micrite
Travertine Stalactites and stalagmites (CaCO 3 )
Coal Converted land plant remains
Other Intraclastic limestone, pelleted limestone

 

Unlike igneous rock, carbonate sedimentary rocks have a fine-grained texture. There are a variety of different forms found. Some of these include the following:

  • Micrite (microcrystalline limestone) – very fine-grained; may be light gray or tan to nearly black in color; made of lime mud ( calcilutite ),
  • Oolitic limestone (look for the sand-sized oolites ),
  • Fossiliferous limestone (fossils in a limestone matrix),
  • Coquina (fossil hash cemented together; may resemble granola),
  • Chalk (made of microscopic planktonic organisms such as coccolithophores; fizzes readily in acid),
  • Crystalline limestone ,
  • Travertine (evaporates of calcium carbonate, CaCO 3 ) stalactites and stalagmites, and
  • Other – intraclastic limestone, pelleted limestone.

Practice problems of this concept can be found at: Sedimentary Rock Practice Test

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