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

Abstract

Abstract--An iron-rich chlorite, ripidolite, was oxidized by air-heating at 480~ i.e., below the dehydroxylation temperature and subsequently reduced in hydrogen at the same temperature. On the basis of chemical, differential thermal, infrared, MSssbauer, and X-ray powder diffraction analyses, Fe(II) seems to be present only in the 2:1 layer of the original chlorite in a type of site similar to that of Fe(II) in biotite, with OH in cis-positions. These data also suggest that octahedral A1 and Fe(III) are located in the hydroxide sheet of the original chlorite. The structural changes of the mineral due to the oxidation and the subsequent reduction appear limited to minor structural rearrangements and, perhaps, to the introduction of OH in

Supergene Vermiculitization of A Magnesian Chlorite: Iron and Magnesium Removal Processes

“…These results contradict those obtained on experimentally altered chlorites (Makumbi and Herbillon, 1972;Ross, 1975;Ross andKodama, 1974, 1976;Senkayi et aL, 1981;Borggaard et aL, 1982) which suggested an initial oxidation of Fe 2+ with subsequent loss of Fe 3+ and Mg from the hydroxide sheet. Figure 6 indicates that the Fe 2+ content differs markedly between two end-members, i.e., chlorite and vermiculite.…”

“…The development of a 3616-cm ~ band in the interstratified phase in sample D, together with 912-and 520-cm bands, can be assigned to A1-A1OH-stretching (3616 cm x) and bending (912 cm -~) vibrations (Lietzke and Mortland, 1973;Farmer, 1974), whereas the 520-cm -~ absorption band can be attributed to octahedral A1-O-Si vibrations (Hayashi and Oinuma, 1965). The absorption band at 435 cm -1 can be related to octahedral Fe--O-Si vibrations (Borggaard et aL, 1982). These five additional bands are apparently sensitive to the vermiculitization process, and their evolutions lead to the following:…”

“…Experimental data from numerous investigators (e.g., Ross, 1975;Ross andKodama, 1974, 1976;Senkayi et al, 1981) have demonstrated that vermiculitization of chlorite can be readily achieved by a thermal treatment of the chlorite above its dehydroxylation temperature (610~ followed by an acid dissolution of its hydroxide sheet. These authors (as well as Makumbi and Herbillon, 1972) concluded from infrared, differential thermal, and more recently, M6ssbauer analyses (Goodman and Bain, 1979;Borggaard et al, 1982), that the experimental vermiculitization was initiated by the oxidation of ferrous iron through a reaction scheme of the form: [Fe2+OH] + -~ [Fe3+O]+ + H § + e-in the structural unit, followed by the loss of ferric iron and magnesium from the hydroxide sheet.…”

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

“…It is also believed to occur in the laboratory oxidation or re-equilibration of other Fee+-bearing and OH-bearing minerals (e.g. Borggaard et al, 1982;Clowe et al, 1988;Lear and Stucki, 1985;Veith and Jackson, 1974) and to represent a main mechanism of H incorporation in nominally anhydrous minerals such as pyroxenes (Skogby and Rossman, 1989;Skogby, 1994). On the other hand, in several studies of natural and laboratory weathering of biotite, the vacancy mechanism is seen to dominate (e.g.…”

Crystal Structure Refinement and Mössbauer Spectroscopy of an Ordered, Triclinic Clinochlore

“…Further, trivalent cations tend to concentrate into the smaller M4 site. If the cations are disordered over the 2 sites, the symmetry can be monoclinic, whereas ordering of trivalent cations into M4 causes a difference (D) in the mean cation oxygen dis- (Taylor et al 1968;Coey 1975;Ericsson et al 1977;Goodman and Bain 1978;Blaauw et al 1980;Borggaard et al 1982;Kodama et al 1982;Ballet et al 1985;Raclavsk~i and Raclavsk# 1988;Aramu et al 1989;Christofides et al 1994) that are generally assigned to the M1 and M2 sites, but are recognized to be potentially indistinguishable from M3 and M4 doublets. In many of these studies (for example, Christofides et al t994), the authors were primarily interested in Fe(IU)/Fe(II) ratios, and so did not pursue more elaborate fitting schemes to determine Fe occupancy of the interlayer sites.…”