Magnesian calcite stabilities: A reevaluation |
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| Authors: | Lynn M Walter |
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| Institution: | Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 USA |
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| Abstract: | Equilibrium constants at stoichiometric saturation with respect to various magnesian calcite compositions were measured using free-drift dissolution rate data and inverse time plots to estimate equilibrium pH. The equilibrium constants determined for two ultrasonically cleaned and annealed biogenic magnesian calcites (12 and 18 mole % MgCO3) in CaCl2 + MgCl2 media at two Mg:Ca molar ratios (1:5 and 5:1) are about three times smaller than those previously reported by Plummer and Mackenzie (1974). These equilibrium constants are not affected by changes in initial pH value, solid:solution ratio, or solution Mg:Ca molar ratio when the ion activity product is expressed in the fractional exponent form. Other models for expression of the equilibrium ion activity product fail to yield consistent values in solutions of different Mg:Ca molar ratios.Experiments performed using crushed samples not ultrasonically cleaned and annealed yield equilibrium constants which vary with solid:solution ratio. Those performed at high solid:solution ratios yield values which approach those previously reported. Submicron size particles and crystal strain induced by crushing the biogenic carbonates may cause more rapid dissolution rates and, hence, overestimation of the solubility of samples not prepared so as to minimize these effects. Thus, the large range in reported solubilities of magnesium calcites may be a result of differences in sample preparation procedure.The results of these measurements shift the thermodynamic equivalence point of aragonite and magnesian calcite from 7.5 mole % MgCO3 up to 12 mole % MgCO3 and prompt a reassessment of models for carbonate diagenetic reactions in natural environments. |
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