I am doing masters research (with Heather Jamieson at Queen's University) on mineral-water interactions in diamond mine waste in Canada's north. I want to carry out reaction path modeling with PHREEQC, but I am having some trouble using the REACTION keyword data block. The main mineralogy of my waste rock is: chrysotile (~ 50 %) forsterite (~40%) calcite (~7%) gypsum (~ 3%) The trouble I am having is figuring out how to enter the relative stoichiometry of these minerals based on the modal abundances I have listed above. Would you be able to give me some hints on how I should approach this conversion? The absolute abundances are not very meaningful. What you need to know is the relative "reaction" rates. My first guess would be that calcite and gypsum react to equilibrium relatively rapidly. Rates of chrysotile and fosterite would be kinetically controlled. If you have water analyses of a water that has reacted with the waste rock, my suggestion is to use speciation modeling to determine the saturation state relative to calcite and gypsum, and then to perform inverse modeling to try to deduce a set of minerals and mineral transfers that account for the composition of the water that has reacted with the waste rock. The ratio of the mass transfers will be the relative rates of reaction of the minerals that do not approach equilibrium. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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