The redox stability of moissanite (SiC) compared with metal-metal oxide buffers at 1773 K and at pressures up to 90 kbar
Ulmer, Gene C.; Grandstaff, David E.; Woermann, Eduard; Göbbels, Matthias; Schönitz, Mirko; Woodland, Alan B.
published: May 1, 1998
ArtNo. ESP154017202008, Price: 29.00 €
Moissanite (SiC) has been accepted as naturally occurring in some mantle and meteorite samples. The thermochemical data bases have allowed calculations of the stability of SiC, moissanite, at 1 bar in such reactions as: SiC + O2 = C + SiO2 (MsCQ) and SiC + Mg2SiO4 + O2 = C + 2MgSiO3 (MsOCP). The 102 values of these reactions at igneous petrogenetic temperatures and 1 bar are as much as six to eight orders of magnitude too reduced for Earth upper mantle models that place the log 102 value between Delta (FMQ) = + 2 and Delta (FMQ) = - 3. Utilizing a ZrO2 membrane technique, in both piston-cylinder and in multi-anvil experiments, the 102 of the (MsOCP) buffer was compared with various metal-metal oxide buffers at 15 kbar and at 90 kbar at 1773 K. Results can be summarized as: (IW) > (Cr- Cr203) > (Mn - MnO) > (MsOCP) > (V -VO) > (Ti -TiO) (highest fO2) (lowest fO2) This order of redox potentials does not appear to change from 1 bar to 90 kbar. While we cannot measure the absolute values of these buffers at high pressure, we have demonstrated that high pressure does not stabilize SiC anywhere near the value of the (IW) buffer. The oxygen fugacity of the MsCOP assemblage is - 1-2 log units lower than the (Mn-MnO) buffer on the geotherm, at least to depths equivalent to 90 kbar at 1773 K. The thermodynamic calculations are thus closely verified by the results of the experiments. The implications for the state of the mantle are discussed.