Magnesium-rich calcite

About 25% of the carbonates deposited in shallow water are eventually eroded and carried downslope by bottom and turbidity currents to become part of the shelf and pelagic sediments. Shallow-water carbonates are also notable for their mineral composition. In addition to calcite and aragonite, some shallow-water calcifiers deposit hard parts containing high percentages of magnesium. These are referred to as magnesium-rich calcites. 

Sedimentary calcium carbonates are formed as the shells of marine plants and animals. Biologically produced CaCOs consists primarily of two minerals aragonite and calcite. Shallow-water carbonates, primarily corals and shells of benthic algae (e.g. Halimeda) are heterogeneous in their mineralogy and chemical composition but are composed mainly of aragonite and magnesium-rich calcite (see Morse and Mackenzie (1990) for a discussion). Carbonate tests of microscopic plants and animals, most of which hve in the surface ocean (there are also benthic animals that produce carbonate shells), are primarily made of the mineral calcite, which composes the bulk of the CaCOs... 

Rasa coal is exceptional because of its high sulfur content, most of which appears to be organic sulfur. The high organic sulfur content is characteristic of marine-influenced bituminous coals. Rasa coal appears to have been formed in a high-pH marine environment where bacteria thrived. This hypothesis is supported by the presence of substantial amounts of both calcite and dolomite in Rasa coal. The calcium- and magnesium-rich environment where Rasa coal formed is expected to have been alkaline. The... 

Carbon is the twelfth most abundant element in the Earths crust, although it accounts for only c.0.08% of the combined lithosphere (see Box 1.2), hydrosphere and atmosphere. Carbon-rich deposits are of great importance to humans, and comprise diamond and graphite (the native forms of carbon), calcium and magnesium carbonates (calcite, limestone, dolomite, marble and chalk) and fossil fuels (gas, oil and coal). Most of these deposits are formed in sedimentary environments, although the native forms of C require high temperature and pressure, associated with deep burial and metamorphism. 

Figure 7.39. Schematic diagram illustrating the effects of climate on meteoric diagenetic pathways for coeval carbonates originally composed of a polymineralic assemblage of calcite with various amounts of magnesium and strontium-rich aragonite. (After James and Choquette, 1984.)...

Almost all deep-sea carbonate-rich sediments are composed of calcite low in magnesium (> 99% CaCOa). This material is primarily derived from pelagic skeletal organisms. Coccolithophores... 

The uptake and incorporation of other elements, magnesium or strontium, in the calcite have been investigated and shown the bioavailability of these elements. In modem ocean waters, the content and range of magnesium in biotic and abiotic deposited minerals were virtually identical in studies by Carpenter and Lohmann (1992). However, there was a more rapid strontium uptake in the calcium-rich biomineral. The composition of the calcium carbonate mineral deposited is a function of kinetics and related to the metabolism of the organism. 

In making thermodynamic calculations to determine departure from equilibrium, thermodynamic data for pure stoichiometric calcite and dolomite were used. However, mineralogic and x-ray examination of aquifer material has shown that the calcite may have several mole percent magnesium the dolomite that occurs in the system is generally calcium-rich (13). Therefore, both of these minerals in the natural state have a higher free energy and hence a somewhat higher solubility than the pure minerals. Thus, part of the supersaturation that we have calculated may be more apparent than real. 

---