Iron-57 conversion-electron Mössbauer spectroscopic study of the initial stages of the oxidation of biotite

Abstract

The technique of 67Fe conversion-electron Mossbauer spectroscopy is shown to be a viable method for the study of the early stages of solid-state reactions which start in regions close to solid surfaces. In particular, increases in the Fe3+ : Fe2+ ratio can be detected in a region confined t o within 300 nm of the outer surfaces of a biotite flake which has been subjected to mild oxidation in air. These changes would have gone undetected if conventional transmission Mossbauer methods had been used alone. The reactionswhich occur in the surface regions of biotites during heat treatment in air are discussed. THE use of 57FeMossbauer spectroscopy in the study of minerals and solid-state reactions is now well estab-1ished.l-3 Most Mossbauer spectra are accumulated by monitoring the y-radiation transmitted through a ' thin ' specimen and in this way information relating to bulk properties of the material under examination is obtained. In situations where the process of sample preparation and consequent alteration or destruction of the specimen is impracticable, ' thick ' iron-containing specimens can be studied in a reflection geometry by monitoring the back-scattered Xor y-ray photons which result from the decay of the I = $ nuclear spin state of 57Fe in the sample. This technique has been applied to mineral specimens including lunar samples 4-6 and in the study of corrosion products in sita.7 More attractive to the surface chemist is the possibility of increasing the surface sensitivity of 57Fe Mossbauer spectroscopy by counting the back-scattered conversion and Auger electrons.8-10 In this way the depth probed is liiiiited to ca. 300 nm.9911 The sensitivity of the method is such that (using existing equipment) ca. 80 A of natural iron (ca. 2% 57Fe) can be detected and, with

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

“…This is consistent with all of the Fe being oxidized by the oxybiotite reaction, without the vacancy reaction being required for further oxidation as would be the case in more Fe-rich samples. All authors who have studied thernaal decomposition of biotite in air or in vacuum (Farmer et al, 1971;Ferrow, 1987;Gfittler et al, 1989;Hogg and Meads, 1975;Ivanitskiy et al, 1975;Rancourt et al, 1993a;Rimsaite, 1970;Robert, 1971;Sanz et al, 1983;Tricker et al, 1976;Vedder and Wilkins, 1969) have found the first Fe oxidation reaction to be the oxybiotite reaction. Farmer et al (197l) concluded that the vacancy reaction (reaction 2) only occurred at higher temperatures than the temperatures where the oxybiotite reaction acted and only in biotites that were sufficiently Fe rich.…”

“…The oxybiotite reaction is also used to interpret studies involving synthetic oxidation induced by heating biotite samples in air, where it is generally believed to be the dominant mechanism (Farmer et al, 1971;Ferrow, 1987;Gtittler et al, 1989;Hogg and Meads, 1975;Ivanitskiy et al, 1975;Rancourt et at., 1993a;Rimsaite, 1970;Robert, 1971;Sanz et al, 1983;Smith et al, 1980;Tricker et al, 1976;Vedder and Wilkins, 1969). This has been demonstrated most convincingly in the remarkable early work of Robert (1971) who resolved and examined the inter-relationships between (both thermally and chemically induced) ferrous Fe oxidation, structural H loss, interlayer cation exchange and hydration, and dehydroxylation (2OH ~ [] + 02-+ H20]'), using a combination of methods including pXRD, TGA, DTA, elemental analysis, wet chemical ferrous Fe determination, optical microscopy, SEM and quantitative IR spectroscopy of both natural and deuterated samples.…”

“…RT STFe MOssbauer spectrum for sample 500-5-W-m, collected on the _+ 11 mm/s velocity range: (a) folded raw data with ideal Voigt-line fit and residual, used to extract the resonant absorber cross-section; (b) normalized resonant absorber cross-section extracted from the raw folded data; and (c) QSD+HFD fit of the calculated thickness corrected spectrum, showing the two assumed contributions, a QSD doublet that models the intra-mica Fe 3+ and a HFD sextet that models studies where biotite samples were heated in air (e.g. Farmer et al, 1971;Ferrow, 1987;Giittler et al, 1989;Hogg and Meads, 1975;Ivanitskiy et aL, 1975;Rancourt et al, 1993a;Rimsaite, 1970;Robert, 1971;Sanz et al, 1983;Tricker et al, 1976;Vedder and Wilkins, 1969). The second main feature is that all of the structural [6]Fe 2+ is oxidized to t6]Fe 3+ when the sample is heated for 5 h at 450~ or above (Tables 4 and 6).…”

The role of structural iron oxidation in the weathering of trioctahedral micas by aqueous solutions

“…Theoretical calculations of the electric field gradients arising from various anionic and cationic site occupancies (Goodman, 1976(Goodman, , 1980Mineeva, 1978), however, show that several structural arrangements can produce the broad non-Lorentzian peaks that were the original impetus for site assignment work. Indeed, Mineeva (1978) suggested that a large Original data from Annersten (1974), Bowen et al (1969), Chandra and Lokanathan (1982), Goodman and Wilson (1973), HBggstrBm et al (1969a, 1969b, Hogg and Meads (1970), Krause et al (1987), Levillain et al (1981), and Tricker and Winterbottom (1976).…”

“…^Relative to metallic Fe (Preston et al, 1962). Most of the MOssbauer studies of mica weathering have involved the oxidation of micas by thermal treatments (Pollak et al, 1962;Hogg and Meads, 1975;Tricker and Winterbottom, 1976;Ouseph and Groskreutz, 1977;Gendler et al, 1978;Tripathi et al, 1978;Chandra and Lokanathan, 1982;Ferrow and Annersten, 1984), and have shown drastic changes in the values for isomer shifts and quadrupole splittings with Fe oxidation. These changes are associated with charge-balancing reactions such as deprotonation and dehydroxylation that may not occur in aqueous systems.…”