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Dear Howard: Are you sure that Na is lossed during the analysis ? I mean, maybe the pyroxene has a Na content below the values that you expect when compared with the Si values... that is to say that ideal stoichiometry is not reached in high-pressure pyroxenes ! (Cawthorn, R. G. & Collerson, K. D. (1974) - The recalculation of Pyroxene End-Member Parameters and the Estimation of Ferrous and Ferric Iron Content from Electron Microprobe Analyses. American Mineralogist, Vol. 59, pp. 1203-1208.) Besides, Na is somewhat problematic when analysed on the microprobe, because of its low atomic number... I hope this will help you ! Cheers Nuno Leal -----Mensagem original----- De: Metamorphic Studies Group [mailto:[log in to unmask]] Em nome de day Enviada: 24 de janeiro de 2016 03:08 Para: [log in to unmask] Assunto: Na loss in pyroxene Colleagues Does anyone know of accessible studies of sodium loss (or not) from omphacitic pyroxene during microprobe analysis? Thanks Howard Howard W. Day, Professor Emeritus The Dept. Formerly Known as Geology University of California Davis One Shields Ave. Davis CA 95616