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]
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