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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 Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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