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Hi all. I am brand new to this mailing list so please excuse me if i break some parochial protocols. I am replying to the summary discussion on the occurrence of the assemblage Bt-absent Ms+Cld+Chl+Als by David Pattison, who wrote: >To summarize, this assemblage (Bt-absent Ms+Cld+Chl+Als) is well >documented in >both the kyanite and andalusite fields, in a couple cases straddling the >Ky-And boundary (Mengel & Rivers, Can Min 1994; forthcoming paper by Pentti >Holtta and co-authors), but never in the Sil field. I hate to be the bearer of the exception to the rule, but i have recently observed a thin section of a strongly sheared pelitic schist with the apparent asssemblage ms + ctd + chl + qtz + sillimanite! the rock comes from the Olary Domain of South Australia (those of you not familiar with this proterozoic terrane might like to read Clarke et al. (1987, J.met.geol) for a summary of metamorphism in the area). chloritoid occurs as porphyroblasts (subhedral) wrapped by an anastamosing prismatic and fibrolitic sillimanite (+ ms + qtz) shear fabric. the chloritoid also contains sygmoidal inclusions of prismatic sillimanite. I have interpreted these textural realtionships as indicating a stable association between chloritoid and sillimanite (along with accessory minerals) synchronous with deformation (resulting in the development of the dominant fabric). late chlorite rarely overgrows the chloritoid porphyroblasts. biotite may have been present in the rock, however, now only oxidised possible remnants of biotite are left. > The assemblage is >likely terminated by the CldAls=StChl reaction, which occurs at lower >temperature than the Al2SiO5 invariant pt. (eg, in grids of Spear & Cheney >1989; Powell & Holland 1990). I have reviewed the petrogenetic grid of powell and holland (1990) and the recalculated grid with the 2001 tutorial update of the holland and powell (1998) dataset. In both instances, the stable assemblage ctd+sill is not predicted. I welcome any and all comments on this matter. regards Gordon Webb