> I have simulated dissolution of dolomite by a pure water with initial logPCO2 = -1 in a CO2-closed system. As a mineralogical control I used SI(calcite) = 0. I expect that Mg and Ca increase in solution during congruent dissolution of dolomite, mantaining a molar ratio of 1, and when the equilibrium with calcite is achieved Ca does not vary anymore whereas Mg keeps on increasing. On the contrary, I observe that when SI(calcite) = 0 is attained Ca begins decreasing. Why? Which is the real process occurring in nature. Why? Because you are adding one calcium and two carbonate ions with dolomite and precipitating one calcium and one carbonate ion in calcite. You have to precipitate a little more calcium than you are adding to maintain K = a(Ca+2)a(CO3-2). However, in your calculations you are allowing dolomite to become supersaturated. Dissolution of dolomite should not continue once saturation is reached without some other reaction (cation exchange, gypsum dissolution, for example). Your problem definition should allow dolomite equilibrium in EQUILIBRIUM_PHASES (moles of dolomite 0, just like calcite). With both calcite and dolomite in equilibrium, the ratio of a(Ca+2)/a(Mg+2) is fixed. I'll let you do the derivation based on the equilibrium constant expressions for dolomite and calcite. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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