Carbonate Sediments and Rocks

Question 1

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Answer 1

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Question 2

Explain the concepts of basic carbonate chemical equilibria.

Answer 2

Maintaining a degree of CaCO3 saturation in seawater, which varies with the water’s temperature and location.
Buffering the pH of seawater by counteracting the addition of CO2, thus maintaining the pH balance.
Maintaining the balance of CO2 between the oceans and the atmosphere.

Question 3

Describe the three most common carbonate minerals and their characteristics.

Answer 3

Calcite: It is a polymorph of CaCO3, often with Mg2+ or Fe2+ substitutions. It can be classified into low-Mg calcite, intermediate-Mg calcite, and high-Mg calcite based on its magnesium content.
Aragonite: It is another polymorph of CaCO3, usually found in orthorhombic crystal form. It has limited magnesium substitution and is commonly observed in mollusc shells, shallow-water carbonate mud, and sea-floor cements.
Dolomite: It has the formula CaMg(CO3)2 and occurs as rhombohedral crystals. It consists of alternating layers of MgCO3 and CaCO3, with possible iron (Fe) substitutions. Dolomite commonly forms cements or replaces calcite/aragonite in carbonate rocks.

Question 4

Describe the building components of carbonate sediments and rocks and their characteristics.

Answer 4

Orthochems: These are carbonate precipitates that form in place, such as cements. They often grow on a substrate and can be observed as aragonite needles or other crystal forms.
Allochems: These are transported carbonate grains that contribute to the formation of carbonate sediments and rocks. Allochems include bioclasts (complete or fragmented skeletons of organisms), pellets (grains composed of micrite), intraclasts (chunks of cohesive mud ripped from the sea floor), and ooids (small spheroidal grains formed in shallow, high-energy tropical marine environments).