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