Re: PHREEQCI - MINTEQ

["David L Parkhurst" <dlpark@xxxxxxxx>]

> 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