6: Carbonate Sedimentary Rocks
Carbonate sedimentary rocks make up approximately 22% of the sedimentary record and are second in abundance to mudrocks. They are of great economic importance because they serve as both source and reservoir rocks in petroleum systems and are used in everything from metallurgy to cement. They are also of environmental importance because of their ability to buffer acidic water, association with karst topography, and influence on soil chemistry.
- List and identify the most important carbonate minerals.
- Explain the chemical reactions responsible for carbonate precipitation and how the amount of carbon dioxide in water influences carbonate precipitation.
- Explain how dolomite forms.
- Identify the main components of carbonate rocks and name the rocks using common classification schemes.
Chapter thumbnail shows details of ooids in the Silurian Mifflintown Formation, PA ( Michael C. Rygel via Wikimedia Commons ; CC BY-SA 4.0 ).
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- 6.1: Composition
- Carbonates are dominated by the calcium, magnesium, and carbonate ions. The four most common carbonate minerals are calcite, aragonite, dolomite, and siderite - all of them form under specific environmental conditions. Calcite and aragonite are pseudomorphs; calcareous organisms have changed what form they prefer through geologic time in response to the amount of dissolved magnesium in the ocean.
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- 6.2: Carbonate Precipitation
- Carbonate precipitation is encouraged by conditions that reduce the amount of dissolved carbon dioxide in the water. Specifically, this means that warm, shallow, agitated conditions tend to be most favorable for carbonate deposition. Additional influences include the activity of organisms, the amount of light, salinity, and clastic sediment input.
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- 6.3: Carbonate Components and Classification
- Carbonates are classified based on the type and relative abundance of grains (skeletal, coated, pellets, and/or intraclasts) and the amount of interstitial mud (micrite) versus pore space/cement. There are two primary classification schemes: the Dunham scheme is more useful for outcrop and hand sample and the Folk scheme is more useful for thin sections.