The interaction of aqueous Mg²⁺ ions with growing calcite crystals was studied by closed system recrystallization of aragonite to calcite in the presence of aqueous CaCl₂-MgCl₂ solutions at 25-90°C. The measured heterogeneous coefficients for Mg²⁺ between calcite and solution (^lgr ^C_Mg2+) are independent of the solution's composition and rate of recrystallization. They are strongly dependent on temperature, being 0.0573 ± 0.0017 at 25°C, 0.0681 ± 0.0019 at 35°C, 0.0778 ± 0.0022 at 50°C, 0.0973 ± 0.0021 at 70°C, and 0.1163 ± 0.0034 at 90°C. If, as reported in literature, calcite containing 12-16 mole % magnesium has the same solubility as that of aragoni e, the new ^lgr^C_Mg values exclude its formation directly from sea water. However, in closed systems isolated from seawater, the transformation of aragonite to calcite should start spontaneously once the magnesium-calcium ratio of the interstitial fluids drops from 5 to 2 or less, no freshwater intervention in this process being necessary.

The kinetics of recrystallization in the absence of Mg²⁺ at 25 and 35°C are strictly diffusion controlled and are, thus, markedly affected by the mass of aragonite per unit-volume of solution taking part in the process. The kinetic effect of Mg²⁺ is still under study.

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