Ferrous/ferric (Fe2+/Fe3+) partitioning among silicates in metapelites

Fe3+ and XFe3+, defined as Fe3+/(Fe2+ + Fe3+) on a molar basis, are now recognised as key parameters in phase equilibrium modelling. A hindrance is that it is only possible to routinely measure total Fe, and not Fe3+ and Fe2+, in minerals using the electron microprobe. Charge balance techniques can be used to estimate Fe3+ and Fe2+ for some minerals, but not for those that contain vacancies. Whilst other analytical techniques can determine XFe3+ in minerals, these are not commonly applied by metamorphic petrologists. Therefore, researchers must rely on estimates. This study collates wet chemical, Mossbauer spectroscopy, and X-ray absorption near edge structure (XANES) spectroscopy analyses of XFe3+ in metapelitic minerals and rocks from the literature. The resulting database of 77 studies contains 591 samples, of which 261 have XFe3+ determined for the whole rock. There are XFe3+ measurements for 483 biotites, 192 white micas, 78 chlorites, and 32 staurolites. Average (+/- 1 sigma) XFe3+ values in whole rock, biotite, white mica, chlorite, and staurolite are 0.23 +/- 0.16, 0.11 +/- 0.08, 0.55 +/- 0.18, 0.08 +/- 0.07, and 0.07 +/- 0.06, respectively. The average (+/- 1 sigma) number of Fe3+ cations in biotite, white mica, chlorite, and staurolite is 0.28 +/- 0.19 (22 O + Ti cations per formula unit, pfu), 0.17 +/- 0.13 (22 O pfu), 0.31 +/- 0.27 (28 O pfu), and 0.20 +/- 0.17 (46 O pfu), respectively. The mean whole rock XFe3+ is similar for metapelites containing ilmenite and magnetite, as well as those that report no Fe-oxide, but is considerably higher for hematite-bearing rocks. Whilst there is little variation with pressure and temperature, there is an increase in the number of Fe3+ cations and XFe3+ of both white mica and biotite with the type of Fe-oxide present. Our observations are compared with the predictions of phase equilibrium modelling using thermodynamic dataset 6.2 (Holland and Powell, J Metamorph Geol 29:333-383, 2011) and the solution models of White et al. (J Metamorph Geol 32:261-286, 2014a) for Fe3+ and XFe3+ in these minerals. The predicted XFe3+ and number of Fe3+ cations in biotite, chlorite, and staurolite broadly match natural observations, but for white mica the predicted mean XFe3+ is underestimated by 0.2-0.4 and the number of Fe3+ cations by 0.05-0.2. Whilst modelling correctly predicted increases in the XFe3+ of white mica and biotite with whole rock XFe3+, it also predicted variations in mineral XFe3+ as a function of pressure and temperature which are not observed in the natural samples.