THE P-T-FO2 STABILITY OF DEERITE, FE122+FE63+[SI12O40](OH)10

New equilibrium experiments have been performed in the 20-27 kbar rang e to determine the upper thermal stability limit of endmember deerite, Fe122+Fe63+[Si12O40](OH)10. In this pressure range, the maximum therm al stability limit is represented by the oxygen-conserving reaction: d eerite(De) = 9 ferrosilite(Fs) + 3 magnetite(Mag) + 3 quartz(Qtz) + 5 H2O(W) (1). Under the oxygen fugacities of the Ni-NiO buffer the break down-reduction reaction: De = 12 Fs + 2 Mag + 5 W + 1/2 O2 (10) takes place at lower temperatures (e.g. DELTAT = 63-degrees at 27 kbar). The experimental brackets can be fitted using thermodynamic data for ferr osilite, magnetite and quartz from Berman (1988) and the following 1 b ar, 298 K data for deerite (per gfw): V-degrees = 55.74 J.bar-1, S-deg rees = 1670 J.K-1, DELTAH(f)degrees = -18334 kJ, alpha = 2.5 x 10(-5) K-1, beta = -0.18 x 10(-5) bar-1. Using these data in conjunction with literature data on coesite, grunerite, minnesotaite, and greenalite, the P-T stability field of endmember deerite has been calculated for P (s) = P(H2O). This field is limited by 6 univariant oxygen-conserving dehydration curves, from which three have positive dP/dT slopes, the o ther three negative slopes. The lower pressure end of the stability fi eld of end-member deerite is thus located at an invariant point at 250 +/- 70-degrees-C and 10 +/- 1.5 kbar. Deerite rich in the end-member can thus appear only in environments with geothermal gradients lower t han 10-degrees-C/km and at pressures higher than about 10 kbar, which is in agreement with 4 out of 5 independent P-T estimates for known oc currences. The presence of such deerite places good constraints on min imum pressure and maximum temperature conditions. From log f(O2)-T dia grams constructed with the same data base at different pressures, it a ppears that endmember deerite is, at temperatures near those of its up per stability limit, stable only over a narrow range of oxygen fugacit ies within the magnetite field. With decreasing temperatures, deerite becomes stable towards slightly higher oxygen fugacities but reaches t he hematite field only at temperatures more than 200-degrees-C lower t han the upper stability limit. This practically precludes the coexiste nce deerite-hematite with near-endmember deerite in natural environmen ts.