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> I wonder if you can advice about the stability constant used in the
PHREEQC.dat database for hydroxyapatite (-3.421). In the data base
MINTEQ.dat this constant is much lower (given as -44.333 which corresponds
to -7.149 if reaction is written as in PHREEQC.dat). The constant of
MINTEQ.dat is similar to the constant given by Lindsay (1979) which is
-7.14 (using the same presentation of the dissolution reaction).
Data of my research with soils seems to fit well with the constant used in
PHREEQC.dat, but I had difficulties to point a reference for this constant.
I would appreciate if you could inform me of a reference for this constant.

The number comes from wateq4f (Ball and Nordstrom, U.S. Geological Survey
Open-File Report 91-183), which lists Brown (1960) as the source of the
data (with conversion of the reaction to H+ from OH-).

Brown, W.E., 1960, Behaviour of slightly soluble calcium phosphates as
revealed by phase-equilibrium calculations: Soil Science, V. 90, p. 51-57.


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

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