Effect of orthophosphate on the dissolution kinetics of biogenic magnesian calcites |
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| Authors: | Lynn M Walter Jeffrey S Hanor |
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| Institution: | Department of Geology, Louisiana State University, Baton Rouge, LA 70803, U.S.A. |
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| Abstract: | The dissolution behavior of two biogenic Mg-calcites, the echinoid, Tripneustes (12 mol% MgCO3), and the red alga, Neogoniolithon (18 mol% MgCO3 plus brucite), was studied using free-drift methods in distilled water and phosphate-spiked solutions at 25°C and PCO2 = 1 atm. Small concentrations of phosphate strongly inhibit dissolution rates. Inhibition increases with increased phosphate levels and with approach toward saturation. Near saturation, dissolution rates are reduced by 103 by the presence of 0.6 μmol adsorbed-P/m2. The magnitude of phosphate inhibition is similar to that observed for low-Mg calcite, and like calcite, the mechanism of inhibition is probably by adsorption of P at surface kink sites. Phosphate appears to inhibit removal of Mg and Ca equally during Mg-calcite dissolution. Rates of brucite dissolution are unaffected by phosphate.Mg-calcites containing >8.5 mol% MgCO3 should be thermodynamically unstable relative to aragonite in most natural waters. Recent work, however, shows that in contrast to its effect on the behavior of Mg-calcites. phosphate does not inhibit aragonite dissolution. The presence of phosphate might thus reverse the relative stability of these two minerals during diagenesis of shallow marine carbonate sediments. |
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