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

Abstract

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), M6ssbauer spectroscopy, nuclear reaction analysis (NRA), Raman spectroscopy, X-ray fluorescence (XRF) spectroscopy, gas cba-omatography (GC), thennogravimetric 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 2+ + OH-~ Fe 3+ + 0 2-+ Hd'). It then evolves along an oxyannite-feiTioxyanuite 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 Fe z+ ~ 2 Fe 3+ + I61[~ + Fe1"). A pillaring collapse transition is observed as a collapse of c near the point where FeZ+/Fe = 1B 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 M6ssbauer 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 (FeZ+/Fe = 1B) 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 fen-ioxyannite 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 ...

Microbial Reduction of Structural Fe(III) in Illite and Goethite

“…The broad distribution with an average field of 43-44 T was assigned to goethite of lower crystallinity. This assignment is consistent with Ranourt et al (23) who assigned a distribution of HFDs in annite samples to a mixture of hematite and poorly crystalline Fe oxides. The decrease in the intensity of the broad component after the bioreduction indicated that the poorly crystalline fraction of goethite was preferentially reduced by the bacteria.…”

Reversed phase equilibrium constraints on the stability of Mg-Fe-Al biotite

“…Mossbauer data were collected at the Department of Physics, Ottawa University, employing the techniques described by Rancourt et al (2001).…”

“…These calculations are relatively insensitive to various assumptions regarding the energetics of, or the precise amount of this small amount of oxyannite component, the substitution mechanisms of which have been studied in detail by Rancourt et al (2001).…”

Upper limit of the tetrahedral rotation angle and factors affecting octahedral flattening in synthetic and natural 1M polytype C2/m space group micas

“…In annite, for example, the ideal end-member stoichiometry * E-mail: dgr@physics.uottawa.ca K(Fe 2+ 3 )(AlSi 3 )O 10 (OH) 2 is never attained because of mismatch constraints between the larger VI (Fe 2+ 3 ) octahedral and smaller IV (AlSi 3 ) tetrahedral sheets, which require some Fe 3+ to be present even under highly reducing conditions (Hazen and Wones 1972;Redhammer et al 1993;Rancourt et al 1994aRancourt et al , 2001Mercier et al 1996Mercier et al , 1999. The latter situation is believed to occur from the geometrical limit of !…”

“…Because accurate Fe-site populations were required, all folded raw spectra were corrected for thickness effects as described previously (Rancourt 1989(Rancourt , 1998Rancourt et al 1994aRancourt et al , 1994bRancourt et al , 2001. To treat thickness effects properly, each raw spectrum was Þ rst Þ tted with an arbitrary number of Voigt lines to obtain a statistically ideal Þ t. Intrinsic absorber cross-sections were then extracted from the Voigt Þ ts and used to generate thin-limit spectra (TLS) that are completely corrected for absorber thickness effects, both spectral shape and relative and absolute intensities.…”