>Hi Dirk,
>
>You may read the section about phengite in the Trotet et al. (2001) paper,
>published in the latest EJM. They discuss the influence of retrogression on
>high-pressure phengites leading to higher Si-contents (!) via pyrophyllite
>substitution: K+Al - Vacancy+Si. This departure (as they call it) from the
>Ms-Cel binary is indicated by interlayer cation deficiency (K<1pfu) and by
>a difference between the Cel-components calculated by Si-content and by
>Mg+Fe+...(octahedral site) respectively.
>To my opinion phengite barometry is not easy anyway. It is for example
>strongly dependent on H2O-activity in the fluid; may vary with F and Ba
>contents of phengite; may vary with K-Na ratio and Fe-Mg ratio. How sure
>can we be about the thermodynamic database in the range of 400 - 600DegC,
>where phengite is stable but experiments are ssslllooowww?
>
>
>cheers horst
>
>
>Trotet F, Vidal O & Jolivet L (2001) Exhumation of Syros and Sifnos
>metamorphic rocks (Cyclades, Greece). New constraints on the P-T path.
>European Journal of Mineralogy, 13:901-920.
>
>
>------------------------------
>Horst Marschall
>Mineralogisches Institut
>Universitaet Heidelberg
>INF 236
>69120 Heidelberg
>++49-(0)6221-546021
>[log in to unmask]
>------------------------------
>
>
***************************************************
Dirk Hellwig
Department of Geosciences
Philipps-University Marburg
Hans-Meerwein-Strasse
35032 Marburg/Germany
[log in to unmask]
phone: +49-6421-2823055
fax: +49-6421-2828919
http://staff-www.uni-marburg.de/~hellwig/
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