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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.