Jane, In principle, the phengite barometer is buffered in your rocks, but the experiments are unreversed. Two-feldspar would apply in principle, but the system is not well calibrated at T < 600°C. Without obtaining Fe3+/Fe2+ on both the muscovite and biotite, any application of thermobarometry involving those phases must be considered suspect. Be sure to check for Ba in the micas and Kfeldspar. I would recommend finding some other assemblages! Green and Usdansky (1986) published a correction to their equation. However, we still cannot reproduce their T results with their equations. My impression is that their system gives temperatures that are too low. eric >Dear colleagues > >I have been studying gneisses from Erzgebirge, which have mineral >assemblage quartz-Kfs-Pl-Ms-Bt. To constrain P-T conditions of their >metamorphism, I can use phengite barometry - however, determination >of temperatures seems to be difficult. At least muscovite is outside >the compositional range defined by Hoisch (1989) biotite-muscovite >thermometer, being too Mg-rich. > >I will be grateful to any advice or comment from those who have >experience with this and other thermobarometers (e.g. Green and >Usdansky 1986 based on Na-K exchange reaction between Pl and Ms, >and more recent ones), which could be used for these rocks. > >Thank you in advance! > >Dr. Jana Kotkova >Czech Geological Survey >Klarov 3 >118 21 Praha 1 >Czech Republic --