A Mössbauer study of thermal decomposition of biotites

Abstract

SUMMARY. The effect of heat treatment in air at temperatures from 200 ~ to iooo ~ of two Cornish biotites has been studied using the M6ssbauer effect in 57Fe. One of the micas was also studied after heating in vacuo.Progressive changes in the M6ssbauer spectra of the materials after heat treatment can be correlated with the following processes: Oxidation of Fe 2+ ions in isolated octahedral sites to give Fe 3+ in the co-ordination Fe3+(OsOH); oxidation of Fe ~+ ions in adjacent a-octahedral (cis OH-) sites to give FeS+(O~); and oxidation of Fe z+ in adjacent a and b (trans OH-) sites to give either Fe3+(O~OH) or Fe3+(O6) depending on whether a shared hydroxyl group remains intact or otherwise (these processes occur at temperatures below 5oo ~ Also, at higher temperatures, dehydroxylation of Fe3+(O~OH) to give more Fe 3+ in the Fez+(Os) configuration. Finally, in the temperature range 9o0 to IOOO ~ structural breakdown yielding a-Fe~Oz as the iron-containing phase.The results and interpretation are in good agreement with thermogravimetric data and with a previous infra-red study. The mechanism of charge diffusion in the lattice during oxidation is discussed in the light of the results.ALTHOUGH much work has beend one on the thermal decomposition of hydrous silicates in general, there has been little published on biotites in particular, most of the mica work having been confined to muscovite. Brindley (1963) has summarized much of the work prior to that date and has emphasized the importance of X-ray techniques in elucidating the broader outlines of the various changes, but the more detailed mechanisms require other techniques for their elucidation. Hodgson, Freeman, and Taylor (I965a) have studied the thermal decomposition of crocidolite and amosite (I965b) by differential thermal analysis and thermogravimetry, and have interpreted their results in terms of the original detailed investigation of amphiboles by Addison et al. (1962). Gibb and Greenwood (I965) have also investigated these two amphiboles using the MSssbauer effect and their results generally support the results of previous workers. Vedder and Wilkins (1969) have studied the thermal decomposition of muscovites and biotites by infra-red spectroscopy, and the interpretation of the results to be presented here has been made along the lines of their arguments.A M~Sssbauer study of high-temperature reactions in kaolinite and halloysite has been undertaken by MacKenzie (1969) and M~Sssbauer work on oxidation of biotite by weathering has been published by Rice and Williams (I969).

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

“…The latter temperature of complete oxidation, for a 5 h exposure, is significantly lower than that found in natural biotite samples heated for comparable durations (e.g. Farmer et al, 1971;Gtittler et al, 1989;Hellner and Euler, 1957;Hogg and Meads, 1975;Ivanitskiy et aL, 1975;Rancourt et al, 1993a;Robert, 1971;Sanz et al, 1983;Vedder and Wilkins, 1969). This difference can be attributed to two special characteristics of our sample: its large Fe content and its small particle size.…”

“…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).…”

Oxidation and Reduction of Structural Iron in Chlorite at 480°C

“…The two processes may be separated by thermal oxidation of the iron succeeded by acid dissolution of the hydroxide interlayer (Ross and Kodama, 1974;Goodman and Bain, 1979). During dry thermal oxidation, the layer charge remains constant by the release of one hydrogen atom per oxidized Fe(II), as suggested in the oxidation of biotite (Vedder and Wilkins, 1969;Farmer et al, 1971;Veith and Jackson, 1974;Hogg and Meads, 1975;Tripathi et al, 1978;Bagin et al, 1980), chlorite (Makumbi and Herbillon, 1972;Goodman and Bain, 1979), and smectite Heller-Kallai, 1976a, 1976b). For chlorite, the mechanism of this hydrogen release is obscure, as is the reversibility of the oxidation step.…”