Calcite with magnesium

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.)...

Most of the Rotaliina group of foraminifera mineralizes with magnesian calcite. The magnesium content may vary but the c-axes of the crystals are oriented perpendicular to the inside surface of the shell wall. 

The starting material calcium carbonate occurs naturally as calcite and more rarely as aragonite and vaterite. In natural deposits calcium carbonate is often found as mixtures with magnesium carbonate. The mineral CaMg(C()3)2 is known as dolomite. 

Figure 29.2 shows the mineralogic results of the calculation. Dolomite dissolves, since it is quite undersaturated in the waste fluid. The dissolution adds calcium, magnesium, and carbonate to solution. Calcite and brucite precipitate from these components, as observations from the wells indicated. The fluid reaches equilibrium with dolomite after about 11.6 cm3 of dolomite have dissolved per kg water. About 11 cm3 of calcite and brucite form during the reaction. Since calculation... 

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