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Dear Juergen and all, concerning Fe3+, first of all there are experiments on the (high-T!) oxydation of biotite (e.g., Virgo and Popp 2000 Am Min, plus more recent ones). I agree with you that weathering is an important issue, and may be difficult to detect. However, let's consider the problem the other way around: Fe3+ exists and has the right size to enter the octahedral site. Why couldn't it stay in biotite, like in other minerals? In any case, I believe that the occurrence of low-T retrograde oxidation can be verified analytically, by cross-checking EMP, Mossbauer, SIMS and single-crystal XRD (what we did in the 2003 paper quoted below). Note also that low-T Fe3+ uptake need not necessarily take place by an oxy-exchange, but might involve K loss in the interlayer. This could be another way of testing for weathering. Concerning titanium, for the sake of completeness I add a few works that have dealt with and reviewed this issue in various ways: Waters, D.J. & Charnley, N.R., 2002. Local equilibrium in polymetamorphic gneiss and the titanium substitution in biotite . American Mineralogist, 87, 383-396. Cesare B., Cruciani G. and Russo U. (2003) Hydrogen deficiency in titanium-rich biotite from anatectic metapelites (El Joyazo - SE Spain): implications for high-temperature petrogenesis American Mineralogist, 88, 583-595. Cesare, B., Satish-Kumar, M., Cruciani, G., Pocker, S., Nodari (2008) Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (SE India): Petrology and further insight into titanium substitutions. American Mineralogist, 93, 327-338 Tajcmanova L., Connolly JAD, Cesare B. (2009) A thermodynamic model for titanium and ferric iron solution in biotite. Journal of Metamorphic Geology, 27, 153-165 I hope this helps! Regards, Bernardo ******************************************************************************* Bernardo Cesare Dipartimento di Geoscienze, Universita' di Padova Via Giotto, 1, I-35137 PADOVA ITALY Tel: ++39-049-8272019 Fax: ++39-049-8272010 email: [log in to unmask] web site: www.dmp.unipd.it/bernardo/bernardo.html web site: www.micrROCKScopica.org ******************************************************************************* "se non tiri, non fai gol..." (R. Braga, 2009)