Dirk,
Massonne subsequently published experiments on the ferrophengite
-muscovite join, I think in Eur. J. Mineral. some 8 years ago or so. My
concerns about the phengite experiments are as follows:
(1) the use of metastable starting materials rather crystalline starting
materials composed solely of the minerals involved in the reactions.
(2) the lack of full reversals starting with more celadonitic and less
celadonitic micas.
(3) the characterization of run products and mineral compositions only by
bulk XRD rather than by electron microprobe and/or analytical TEM.
(4) the failure to evaluate the level of Fe3+ in experimental (or natural)
phengites.
Applications to rocks require the full buffering assemblage inferred
by Massonne, although other reactions may be calculated if the experimental
data are correct. Years ago Ernst and Velde separately showed that
phengite stability is not strongly affected by dilution of H2O in the
fluid. A greater problem is in application to more oxidized rocks where
ferric iron probably substitutes in the micas. By the way, it is difficult
to analyze fine grained phengite, which is quite prone to beam damage and
loss of K during the analysis. The element Ba should also be included in
the package. In rocks that are not thoroughly recrystallized, phengite
may be intergrown submicroscopically with other sheet silicates.
eric
>Dear all,
>the pressure dependent celadonite (reverse Tschermak-) substitution in
>white mica (Si content) is often determined to apply the phengite
>geobarometer of Massonne&Schreyer (1987, Contrib. Min. Pet. 96, 212-224).
>Does somebody recall work performed on/ papers dealing with the influence
>of metamorphic fluids on the celadonite-substitution or on the (Fe2+,
>Mg)3Al-2 exchange between muscovite and phengite, maybe even on a
>quantitative basis. Any leaching effects?
>Your comments are, as always, appreciated,
>cheers
>Dirk
>***************************************************
>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/
>***************************************************
Eric Essene
Professor of Geology
Department of Geological Sciences
2534 C.C. Little Bldg.
425 E. University Ave.
University of Michigan
Ann Arbor MI 48109-1063 USA
fx: 734-763-4690
ph: 734-764-8243
|