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Dear All,

I am currently studying unusual high-Mg metapelites from the High 
Himalayan Crystalline Sequence containing the assemblage 
orthoamphibole + kyanite + cordierite + biotite + quartz  garnet  
staurolite  chlorite. The orthoamphibole is a gedrite, with a 
composition (Na0.5) (Mg3.1 Fe2.4) Al1.3 [Si6 Al2 O22] (OH)2.

In order to ensure a maximum consistency in the analysis of the P-T 
stability of such assemblages compared to other more "classical" 
Barrovian assemblages found in the same sequence, I'm trying to 
include the orthoamphibole in a KFMASH grid I calculated with 
THERMOCALC, according to the latest dataset of Holland & Powell 
(1998). Using a simple ideal activity model for the anthophyllite - 
Fe-anthophyllite - gedrite solution, two FMASH invariants involving 
orthoamphibole + staurolite + cordierite + garnet + kyanite and 
orthoamphibole + staurolite + cordierite + chlorite + kyanite appear 
to be unstable, as they occur at low pressures (around 3 kbar), in 
the stability fields of andalusite and sillimanite. In contrast, 
these two invariants are stable in the KFMASH grid of Xu et al. 
(1994), and they plot close to the ky = sill equilibria around 620 C 
/ 6 kbar. This discrepancy is most likely the consequence of a 
significant non-ideality in the orthoamphibole. Yet, I can't find 
informations about orthoamphibole non-ideality in the THERMOCALC 
documentation or in related papers (e.g. Will & Powell, 1992; Xu et 
al., 1994; Holland & Powell 1998).

I would consequently greatly appreciate if someone could tell me 
where to find such informations (activity coefficients, DQF 
parameters) allowing to calculate a non-ideal activity model for the 
anthophyllite - Fe-anthophyllite - gedrite solution. Better still, if 
someone has an already coded version of such an activity model for 
THERMOCALC, I would be extremely grateful if I could get a copy.


Bonjour chez vous !
                                                Vannay Jean-Claude



Holland, T. J. B. & Powell, R. 1998. An internally consistent 
thermodynamic data set for phases of petrological interest. Journal 
of Metamorphic Geology 16, 309-343.

Xu G., Will T.M. & Powell R. 1994. A calculated petrogenetic grid for 
the system K2O-FeO-MgO-Al2O3-SiO2-H2O, with particular reference to 
contact-metamorphosed pelites. Journal of Metamorphic Geology 12, 
99-119.

Will T.M. & Powell R. 1992. Activity-composition relationships in 
multicomponent amphiboles: An application of Darken's quadratic 
formalism. American Mineralogist 77, 954-966.