Sedimentary Magnesite—Basis for Carbonate Deposition

There are two physical forms of magnesite cryptocrystalline or amorphous magnesite and crystalline, macrocrystalline, or bone magnesite. 

Carbonate compounds are relatively insoluble, and Table 2.1 lists the solubility constants for a number of geologically important carbonates. Since these minerals are relatively insoluble, carbonates are precipitated at relatively low carbonate and counterion concentrations. As an example, a solution containing 10-4 M Ca2+ will be precipitated by a concentration of CO2 in excess of 10-4 32 M. This occurs because the product of the two ionic concentrations exceeds the solubility constant for calcium carbonate see Equation (2.1)  

Since the solubility of calcium carbonate is considerably less than that of magnesium carbonate, evaporation and concentration of salt lakes and lagoons must have produced calcium carbonate deposits initially. The brine would gradually be depleted of calcium ion and enriched with magnesium. Eventually, a condition would be reached where the brine concentration of the magnesium ion and carbonate exceeds the solubility constant of magnesite, and precipitation would proceed see Equation (2.2)  

Sedimentary deposits of cryptocrystalline magnesite occur either in lagoons, salt lakes, or freshwater lakes (lacustrine). The genesis of magnesite in saltwater requires specific conditions for it to occur a reducing alkaline environment, a high concentration of magnesium sulfate, and a concentration 

TABLE 2.1 Solubility Constants of Geologically Important Carbonates 

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