All,
As a general comment, most biotite has significant Fe3+ that is
being unmeasured or uncorrected.
cheers,
eric
On Jul 2, 2009, at 12:23 PM, Pavel Pitra wrote:
> Hi Martin,
>
> for the moment there is no good MnNCKFMASHTO a-x file available,
> although, as far as I know, some people around Roger Powell are
> working on it.
>
> Then, if one wants to deal with this system, one has to do some
> choices. If Mn is an important component, then it certainly is
> important in garnet and you then have to choose the garnet model
> with Mn and without Fe3. Such approach has been adopted e.g. by
> Hasalova et al. (including RP) (2008) and this point is discussed
> there on p. 63.
> As far as biotite is concerned, given its systematically low
> contents of Mn, it is certainly much more important to keep the Ti-
> bearing model.
> If one is interested in low T conditions (and the beginning of
> garnet growth), other Mn-bearing phases have to be included and
> chlorite comes to mind as the most important candidate (there is no
> model for Mn-bearing ilmenite). This is what we have done in Le
> Bayon et al. (2006) - the model is described there (and I can send
> you the file).
> Mn can be also included by ideal mixing (although it probably is not
> ideal ;)) into cd, st and ctd.
>
> this is what we are currently using - it sort of works...
>
> Good luck,
>
> Pavel
>
>
> Hasalová, P., Stípská, P., Powell, R., Schulmann, K., Janousek, V. &
> Lexa, O., 2008. Transforming mylonitic metagranite by open-system
> interactions during melt flow. Journal of Metamorphic Geology,
> 26(1), 55-80.
>
> Le Bayon, B., Pitra, P., Ballčvre, M. & Bohn, M., 2006.
> Reconstructing P-T paths during continental collision using multi-
> stage garnet (Gran Paradiso nappe, Western Alps). Journal of
> Metamorphic Geology, 24(6), 477-496.
>
> ----
>> Unfortunately that will not work. The MnNCKFMASHTO file contains
>> the a-X relationships for garnet and biotite, amongst others, which
>> includes 'O' in their formulation. I cannot simply delete the
>> oxides. Moreover, I cannot use the MnNCKMASH datafile for biotite
>> because Ti is then missing!
>>
>>
>> Dr Martin J. Rigby
>> Department of Geology
>> University of Pretoria
>> Pretoria 0002
>> Republic of South Africa
>
>
> --
> Pavel PITRA
> Geosciences Rennes email: [log in to unmask]
> Université Rennes 1 tel: (++33) 2.23.23.65.06
> Campus de Beaulieu - Bat. 15 fax: (++33) 2.23.23.60.97
> F - 35 042 RENNES CEDEX
> FRANCE http://www.geosciences.univ-rennes1.fr/
> http://www.geosciences.univ-rennes1.fr/rubrique.php3?id_rubrique=71
>
>
