Denis Rancourt
Rancourt, D.G., Mercier, P.H.J., Cherniak, D.J., Desgreniers, S., Kodama, H., Robert, J.-L. & Murad, E.
Clays Clay Miner. 49, 455–491 (2001). https://doi.org/10.1346/CCMN.2001.0490601
Abstract--A synthetic octahedral-site-vacancy-free annite sample and its progressive oxidation, induced by heating in air, were studied by powder X-ray diffraction (pXRD), Mossbauer spectroscopy, nuclear reaction analysis (NRA), Raman spectroscopy, X-ray fluorescence (XRF) spectroscopy, gas chromatography (GC), thermogravimetric analysis (TGA), differential thermal analysis (DTA), scanning electron microscopy (SEM), and size-fraction separation methods. For a set heating time and as temperature is increased, the sample first evolves along an annite--oxyannite join, until all H is lost via the oxybiotite reaction (Fe2+ + OH- ~ Fe3+ + 02- + Hout). It then evolves along an oxyannite-ferrioxyanuite join, where ideal ferrioxyannite, KFe3+s/3[~mA1Si3012, is defined as the product resulting from complete oxidation of ideal oxyannite, KFe3+2FeZ+A1Si30~2, via the vacancy mechanism (3 Fe2+ ~ 2 Fe3+ + I61[~ + Fe1"). A pillaring collapse transition is observed as a collapse of c near the point where Fe2+/Fe = 1/3 and all OH groups are predicted and observed to be lost. Quantitative analyses of H, using NRA, GC, and Raman spectroscopy, corroborate this interpretation and, in combination with accurate ferric/ferrous ratios from Mossbauer spectroscopy and lattice parameter determinations, allow a clear distinction to be made between vacancy-free and vacancybearing annite. The amount of Fe in ancillary Fe oxide phases produced by the vacancy mechanism is measured by M6ssbauer spectroscopy to be 1 1.3(5)% of total Fe, in agreement with both the theoretical prediction of 1/9 = 11.1% and the observed TGA weight gain. The initiation of Fe oxide formation near the point of completion of the oxybiotite reaction (Fe2+/Fe = 1/3) is con-oborated by pXRD, TGA, Raman spectroscopy, and appearance of an Fe oxide hyperfine field sextet in the M6ssbaner spectra. The region of Fe oxide folTnation is shown to coincide with a region of octahedral site vacancy formation, using a new Mossbauer spectral signature of vacancies that consists of a component at 2.2 mm/s in the I6~Fe3+ quadrupole splitting distribution (QSD). The crystal chemical behaviors of annite--oxyannite and of oxyannite ferrioxyannite are best contrasted and compared to the behaviors of other layer-silicate series in terms of b vs. [D] (average octahedral cation to O bond length). This also leads to a diagnostic test for the presence of octahedral site vacancies in hydrothermally synthesized annite, based on a graph of b vs. Fe2+/Fe. The implications of the observed sequence of thermal oxidation reactions for the thermodynamic relevance of the oxybiotite and vacancy reactions in hydrothermal syntheses are examined and it is concluded that the oxybiotite reaction is the relevant reaction in the single-phase stability field of annite, at high hydrogen fugacity and using ideal starting cation stoichiometry. The vacancy reaction is only relevant in a multi-phase field, at lower hydrogen fugacity, that includes an Fe oxide equilibrium phase (magnetite) that can effectively compete for Fe, or when using non-ideal starting cation stoichiometries.