Study of pelitic schists in the Rangeley area, Maine, by means of petrographic, x-ray, and electron-microprobe techniques enables definition of the isogradic reaction relating the Staurolite and lower sillimanite zones. The reaction is a discontinuous one and can be shown on an AFM projection as the tie line change from Staurolite + chlorite to sillimanite + biotite. This topology change, in conjunction with the mineralogical data provides the equation:
Staur + Mg-Chte + Na-Musc + (Garn?)→Sill + Bio + K-richer Musc + Ab + Qtz + H2O.
This reaction should result in a sharp isograd in the field but in fact is found to be spread out over a zone which is called the transition zone. It is proposed that this zone results from buffering of fH2O by means of the equation above. Buffering of fH2O by continuous reactions also appears to be taking place in the lower sillimanite zone.
Metamorphic conditions for the isogradic reaction are believed to be near 4 kb and 600°C. Pfluid was probably equal to Ptotal but somewhat greater than PH2O.