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Mechanisms and crystal chemistry of oxidation in annite; resolving the hydrogen-loss and vacancy reactions

D. G. Rancourt, P. H. J. Mercier, D. J. Cherniak, S. Desgreniers, H. Kodama, J. L. Robert and E. Murad
Mechanisms and crystal chemistry of oxidation in annite; resolving the hydrogen-loss and vacancy reactions
Clays and Clay Minerals (December 2001) 49 (6): 455-491


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 (Fe (super 2+) +OH (super -) <-->Fe (super 3+) +O (super 2-) +H). It then evolves along an oxyannite-ferrioxyannite join, where ideal ferrioxyannite, KFe (super 3+) (sub 8/3) [] (sub 1/3) AlSi (sub 3) O (sub 12) , is defined as the product resulting from complete oxidation of ideal oxyannite, KFe (super 3+) (sub 2) Fe (super 2+) AlSi (sub 3) O (sub 12) , via the vacancy mechanism (3 Fe (super 2+) <-->2 Fe (super 3+) + (super [6]) []+Fe). A pillaring collapse transition is observed as a collapse of c near the point where Fe (super 2+) /Fe + 1/3 and all OH groups are predicted and observed to be lost. Quantitative analyses of H, using NRA, GC, and Raman pectroscopy, 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 vacancy-bearing annite. The amount of Fe in ancillary Fe oxide phases produced by the vacancy mechanism is measured by Mossbauer 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 (Fe (super 2+) /Fe = 1/3) is corroborated by pXRD, TGA, Raman spectroscopy, and appearance of an Fe oxide hyperfine field sextet in the Mossbauer spectral. The region of Fe oxide formation 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 (super [6]) Fe (super 3+) 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. Fe (super 2+) /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.

ISSN: 0009-8604
Serial Title: Clays and Clay Minerals
Serial Volume: 49
Serial Issue: 6
Title: Mechanisms and crystal chemistry of oxidation in annite; resolving the hydrogen-loss and vacancy reactions
Affiliation: University of Ottawa, Department of Physics, Ottawa, ON, Canada
Pages: 455-491
Published: 200112
Text Language: English
Publisher: Clay Minerals Society, Clarkson, NY, United States
References: 92
Accession Number: 2002-023392
Categories: Geochemistry of rocks, soils, and sedimentsSedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
Secondary Affiliation: Rensselaer Polytechnic Institute, USA, United StatesAgriculture and Agri-Food Canada, CAN, CanadaCentre de Recherches sur la Synthese et Chimie des Mineraux, FRA, FranceBayerisches Geologisches Landesamt, DEU, Federal Republic of Germany
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200209
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