6.1: Composition
Mineralogy
Carbonate sedimentary rocks are dominated by the Ca 2 + , Mg 2 + and CO 3 2 - ions. Although there are many carbonate minerals, the four most important ones are:
Calcite (CaCO 3 ) is the more stable polymorph of calcium carbonate; its atoms are arranged into a rhombohedral architecture. Mg can substitute for up to 16% of Ca in calcite; specimens with <4% are referred to as low-Mg calcite and those with 4-16% are referred to as high-Mg calcite.
Under atmospheric temperatures and pressures, aragonite (CaCO 3 ) is a less stable polymorph of calcium carbonate and given time it will often turn into calcite. The atoms are arranged into an orthorhombic configuration. Many invertebrate shells that have the “mother of pearl” luster are composed of aragonite.
Dolomite (CaMg(CO 3 ) 2 ) has equal parts Ca and Mg and is thus a different mineral than high-Mg calcite which has a maximum of 16% Mg substituting in for Ca. Although extremely common in the geologic record, direct precipitation at the Earth’s surface is unlikely because it can only form at high temperature and/or under conditions where the Mg/Ca ratio greatly exceeds typical ocean chemistry (see Figure 6.3.1 ).
Siderite (FeCO 3 ) is common in reducing, freshwater conditions and is often nucleated around organic material that causes localized reduction. It is uncommon as an early formed mineral in marine environments because the iron will preferentially bond with the sulfur (and thus form pyrite) that is abundant in marine settings.
Calcite and aragonite seas
The crystal structure of aragonite does not allow Mg to substitute for Ca easily. Conversely, calcite allows Mg to readily substitute, but increased Mg concentrations make the calcite unstable. The end result is that when the ocean is relatively enriched in Mg (as is the case when there is rapid seafloor spreading), aragonite and high-Mg calcite become the most abundant types of inorganic carbonate in the oceans (“aragonite seas”). Conversely, “calcite seas” exist at times of low Mg concentration. However, most ancient aragonite transforms into calcite during diagenesis.
Carbonate Rocks
Rocks that are made up of >50% calcite/aragonite are classified as limestones and those with >50% dolomite are called dolomite (or dolostones). Although there are numerous other carbonate minerals, calcite, aragonite, and dolomite make up approximately 98% of them and are the most important ones for rocks names. Please also keep in mind that other minerals can occur in accessory amounts in carbonate rocks; quartz grains, chert, and glauconite are particularly common.