Dear All,
I am trying to calculate the solubility of ferberite(FeWO4)-huebnerite(MnWO4)
solid-solutions in hydrothermal fluids at elevated temperatures and
pressures. For this purpose, I am searching for a set of appropriate solution
parameters (e.g. regular or subregular Margules-type etc).
When I studied the available literature about ferberite-huebnerite solutions,
I came across several problems and inconsistencies. In a paper by Amosse &
Mathieu (1980), J. Chem. Thermodynam., v. 12, p. 683-689, enthalpies of
mixing along the FeWO4-MnWO4 join were directly determined by high-T solution
calorimetry. These authors report very large negative enthalpies of mixing,
which have a maximum value of -12.8 kJ/mol at X(Mn) =0.5. The data in that
paper appear to suggest a regular solution behaviour, so this enthalpy of
mixing would translate into a regular solution parameter of -51.2 kJ/mol.
This very negative value appears to be very unusual to me, if I compare it
with the Fe-Mn exchange energies in other systems, e.g. for FeO-MnO (a few
kJ/mol). The general observations on other tungstate and molybdate systems
outlined in a paper by Davies & Navrotsky (1983) would also contradict with
such a very negative interaction parameter. However, the estimate I would
calculate from correlations in that paper would be on the order of 22 kJ/mol.
This would, in turn, predict a solvus crest at 1335 K. and have the
implication that in Sn-W-depositing hydrothermal systems Fe-Mn substitution
in wolframite should be rather limited. However, this is in contrast to
observations on natural samples, which indicate very extensive Fe-Mn
substitution.
Does anybody know about a reasonable ferberite-huebnerite solid-solution
model in the literature? Despite extensive search I was not able to find
other data than the two studies mentioned above. So any suggestion or
reference would be greatly appreciated.
With kind regards
Thomas Wagner
--
Dr. Thomas Wagner
Institut für Geowissenschaften
Universität Tübingen
Wilhelmstr. 56
D-72074 Tübingen
Germany
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