Calcite, solubility under varying

Table 2 Calculated Calcite Solubility Under Varying Conditions Atmospheric CO Excluded...

In calcium-dominated wetlands, high concentrations of Ca can result in formation of complex calcium phosphate compounds of varying solubilities such as calcium phosphate, dicalcium phosphate, beta-tricalcium phosphate, octacalcium phosphate, and hydroxyapatite. Under these conditions, the phosphorus concentration in the soil pore water of calcareous soils is a function of Ca + activity. Solubility of these compounds decreases with an increase in Ca content. Insoluble beta-tricalcium phosphate is more likely to be found at a high pH. Thermodynamically, apatite is the most stable compound in soils. At relatively high phosphate concentrations, dicalcium phosphate or octacalcium phosphate may form and slowly transform to the more stable phase hydroxyapatite. These precipitation reactions can occur on the surfaces of calcite. The amount of exposed surface will determine the amount of phosphorus precipitated. In a Ca-saturated clay, calcium... 

Calcium has been recognized as a constituent of gallstones since the earliest chemical studies (see reference 118). It occurs as calcium carbonate (either calcite or aragonite), calcium phosphate (apatite), calcium bilirubinate, or the salts of bile acids and fatty acids. In normal hepatic bile, the calcium concentration is about 21 mg %. It is about 45 mg % in normal gallbladder bile and about twice that in the gallbladder bile of patients with cholesterol stones (119). The concentration of calcium is said to vary with the concentration of bile salts (3). Apparently, calcium in bile is bound in part to bile salt micelles, and under normal conditions this complex remains stable and soluble (120). However, under appropriate conditions of pH, calcium concentration, and anion concentration, calcium salts may precipitate. 

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