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Bruce, Dug, and all,
    A higher grade reaction that is similar to Dugald's is:
        almandine + O2 = Al2SiO5 + quartz + magnetite.
The locus of this reaction was calculated by Anovitz et al. in GCA in their
paper on the Cp of almandine.  It is located at relatively low fO2 and will
act as a sensor or a limit for fO2 in metapelites and other rocks with
garnet.  There will be similar reactions for any ferromagnesian silicate in
metapelites: e.g., staurolite, chloritoid, carpholite, chlorite,
stilpnomelane, biotite (cf. Eugster and Wones), and even muscovite if one
could distinguish Fe3+ from Fe2+.
eric










While not doubting the veracity of Dugald's calculations, I do feel it is
>worth emphasising that, for almost any rock with this assemblage, biotite
>and garnet will be major repositories of Fe2 and so this assemblage will
>itself be the buffer for the oxygen fugacity. As a result it will
>potentially be stable over a very large P-T range. Dugald's calculations
>indicate quite a low-T under fairly reducing conditions (and in agreement
>,this is quite a common greenschist facies assemblage), while the upper
>limit is almost certainly given by the degenerate thermal stability limit
>of muscovite + quartz in the upper amphibolite facies. Any additional
>presence of graphite at lower-T's will tend to drive the reaction as
>written to the left, destroying magnetite on heating. So a useless
>thermobarometer but a valuable fO2 buffer!
>
>Bruce Yardley
>
>
>At 13:11 22/09/01 -0400, you wrote:
>>At 10:18 PM 16/09/2001 +0530, Mallickjarun Joshi wrote:
>>>   Dear All, Could anybody tell me the pressure temperature conditions for
>>>the reaction which if I remember correctly is in the following  manner:
>>>biotite+garnet+O2=muscovite+magnetite+quartz
>>
>>The P-T curve for this reaction would be univariant in the system KFASH
>>(i.e., for the annite and almandine endmembers) provided that oxygen
>>fugacity is either specified or buffered. Using the nickel-bunsenite
>>buffer, we can write an equivalent reaction involving Ni and NiO so that
>>f(O2) is internally buffered and need not be specified:
>>
>>annite + almandine + 2 NiO = muscovite + 2 magnetite + 3 Qtz + 2 Ni
>>
>>Using the thermodyamic database of Holland and Powell (1998 J Metam Geol),
>>the P-T curve has a negative slope and is slightly convex downwards,
>>passing through 10 kbar at 260 deg C and 1 kbar at 500 deg C. Mu+Mt+Qtz is
>>stable on the low-T side of this curve. Adding Mg,Mn,Ca to the system would
>>displace the curve to lower P-T and expand it to a zone of finite width
>>(i.e., the assemblage Bt+Grt+Mu+Mt+Qtz would be stable across a range of P
>>and T).
>>
>>>I would also welcome any
>>>information on a reaction that consumes garnet and produces
>>>muscovite+magnetite+quartz in the garnet zone.
>>
>>If the foregoing considerations are correct, we should expect garnet to be
>>produced and muscovite+magnetite+quartz to be consumed in the garnet zone.
>>
>>Cheers, Dugald
>>
>>Dugald M Carmichael                    Phone/V-mail: 613-533-6182
>>Dept of Geological Sciences and Geological Engineering
>>Queen's University                              FAX: 613-533-6592
>>Kingston  ON  K7L3N6             E-mail: [log in to unmask]
>>
>
>
>--------------------------------------------
>Professor Bruce Yardley
>School of Earth Sciences
>University of Leeds
>Leeds LS2 9JT
>UK
>
>Tel. 0113 233 5227        Fax  0113 233 5259
>---------------------------------------------
>
>GEOFLUIDS now exists!    http://www.blackwell-science.com/gfl