High-magnesium calcite

Baltres A. and Medesan A. (1978) High-magnesium calcite in fecal pellets of Artemia salina from Techirghiol Lake. Sedim. Geol. 20, 281-290. 

Figure 11.7 Scanning electron microscopy images of marine beachrock and meteoric cayrock cements. (A) Blades of high-magnesium calcite on constituent particles in beachrock. Northeast Sapodilla Cay, southern Belize Barrier Reef. (B) Dissolution cavity is lined with blockyand bladed low-magnesium calcite in cayrock. Cay Bokel, Turneffe Islands, Belize. (C) Low-magnesium calcite needle fibre (whisker) cement in cayrock. Harry Jones Point, Turneffe Islands, Belize.

We observe the opposite trend, expected from the multiphase hypothesis as a higher solid/solution ratio leads to a smaller mole fraction of MgCOg and, consequently, to a lower solubility (and pH) of the solution. In the first run the high Na2C03 added is offset by tne large amount of calcite added. In the dissolution process the magnesium content of the solid decreases and does so to a greater extent for smaller solid/solution ratios. 

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. 

Although calcium is more metallic in character than magnesium, compounds of the two elements share some similar properties. Calcium carbonate, CaC03, occurs naturally as chalk and limestone. Marble is a dense form of calcium carbonate that can be given a high polish it is often colored by impurities, most commonly iron cations (Fig. 14.26). The two most common forms of pure calcium carbonate are calcite and aragonite. All these carbonates are the fossilized remains of marine life. Calcium carbonate decomposes to calcium oxide, CaO, or quicklime, when heated ... 

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