6.1: Composition

Mineralogy

Carbonates are dominated by the Ca²⁺, Mg²⁺ and CO₃^2- ions. Although there are many carbonate minerals, the four most important ones are:

Calcite (CaCO₃) is the most 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.

Figure \(\PageIndex{1}\): Atomic structure of calcite (rhombohedral) and aragonite (orthorhombic; Wikimedia Commons; CC BY-SA 4.0).

Under atmospheric temperatures and pressures, aragonite (CaCO₃) 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.

True dolomite (CaMg(CO₃)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 (FeCO₃) 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.

Figure \(\PageIndex{2}\): Carbonate minerals. A) This ammonite shell was probably originally composed of aragonite, but during diagenesis it was transformed into calcite which is more stable (James St. John via Wikimedia Commons; CC BY 2.0). B) Exceptional preservation allowed for pristine preservation of this nacreous aragonite ammonite shell (James St. John via Wikimedia Commons; CC BY 2.0). C) Photomicrograph of a carbonate (stained for calcite). showing rhombohedral dolomite crystals and stained (red) calcite crystals (Michael C. Rygel via Wikimedia Commons; CC BY-SA 4.0) D) Polychaete worm preserved in a siderite nodule (James St. John via Wikimedia Commons; CC BY 2.0)

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.

Figure \(\PageIndex{3}\): Alternation of calcite versus aragonite seas through geologic time (Wilson44691 via Wikimedia Commons; public domain)

Carbonate Rocks

Rocks that are made up of >50% calcite/aragonite are classified as limestones and those with >50% dolomite are called dolomite. 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.