Comparative dissolution characteristics of carbonate minerals in the presence and absence of aqueous magnesium ion

Abstract

Short-term (5-25 hr.) steady-state pH was measured for the dissoln. of reagent grade calcite, Deep Spring Lake dolomite, and Florida Bay sediment (aragonite + 14 mole % Mg calcite) in distd. H2O and 0.1m MgCl2 at 25° and const. (1 and 10-2.51 atm.)Pco2. If the steady state is approached from undersatn. and unground samples are used, calcite and dolomite show const. ion activity products independent of Mg++ concn. The product values are for calcite 10-8.37, and for dolomite 10-17.0. The sediment did not exhibit a simple reversible behavior. Distd.-H2O soly. is partly controlled by the rate of recrystn. of high-Mg calcite to more stable CaCO3 forms and depends on the surface-area exposure to soln. Mg++ solute depresses the pH and ion activity more than predicted from pH in distd. H2O at the same pco2. Decreased soln. rates and Mg loss from high-Mg calcite can cause this. Inhibition of low-Mg calcite nucleation yields reversible equil. of Mg++ and grain surfaces. The surfaces are not nonexchangeable dolomite or exchangeable CaCO3-MgCO3. [on SciFinder(R)]