Mechanisms and Crystal Chemistry of Oxidation in Annite: Resolving the Hydrogen-Loss and Vacancy Reactions

D. G. Rancourt1, P. H. J. Mercier1, D. J. Cherniak2, S. Desgreniers1, H. Kodama3, J.-L. Robert4 and E. Murad5
1 Department of Physics, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
2 Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York 12180, USA
3 Agriculture and Agri-Food Canada, 960 Carling Ave., Ottawa, Ontario, Canada K1A 0C6
4 Centre de Recherches sur la Synthèse et Chimie des Minéraux, CNRS, F-45071 Orléans Cèdex 2, France
5 Bayerisches Geologisches Landesamt, Aussenstelle Marktredwitz, Leopoldstrasse 30, Postfach 389, D-95603 Marktredwitz, Germany
E-mail of corresponding author: dgr@physics.Uottawa.Ca

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), Mössbauer 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+ + O2− + H↑). It then evolves along an oxyannite-ferrioxyannite join, where ideal ferrioxyannite, KFe3+8/31/3AlSi3O12, is defined as the product resulting from complete oxidation of ideal oxyannite, KFe3+2Fe2+AlSi3O12, via the vacancy mechanism (3 Fe2+ ⇌ 2 Fe3+ + [6]□ + Fe↑). A pillaring collapse transition is observed as a collapse of c near the point where Fe2+/Fe = ⅓ 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 Mössbauer spectroscopy and lattice parameter determinations, allow a clear distinction to be made between vacancy-free and vacancy-bearing annite. The amount of Fe in ancillary Fe oxide phases produced by the vacancy mechanism is measured by Mössbauer spectroscopy to be 11.3(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 = ⅓) is corroborated by pXRD, TGA, Raman spectroscopy, and appearance of an Fe oxide hyperfine field sextet in the Mössbauer spectra. The region of Fe oxide formation is shown to coincide with a region of octahedral site vacancy formation, using a new Mössbauer spectral signature of vacancies that consists of a component at 2.2 mm/s in the [6]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.

Key Words: Annite • Biotite • Crystal Chemistry • Differential Thermal Analysis • Ferrioxyannite • Gas Chromatography • Mössbauer Spectroscopy • Nuclear Reaction Analysis • Oxyannite • Oxybiotite • Scanning • Electron Microscopy • Thermogravimetric Analysis • X-ray Fluorescence Spectroscopy

Clays and Clay Minerals; December 2001 v. 49; no. 6; p. 455-491; DOI: 10.1346/CCMN.2001.0490601
© 2001, The Clay Minerals Society
Clay Minerals Society (www.clays.org)