Electron Hopping and Temperature Dependent Oxidation States of Iron in Ilvaite Studied by Mössbauer Effect

Abstract

The Mössbauer spectrum of ilvaite was measured between 115 K and 898 K, and the energy dispersive X-ray powder pattern was measured between 300 K and 1 123 K. Below 500 K the Mössbauer spectrum varies strongly with temperature while the X-ray spectrum remains unchanged. The results are interpreted by electron exchange between nearly identical Fe-sites in ilvaite.

High pressure X-ray diffraction study of ilvaite CaFe 2 2+ Fe3+[Si2O7/O/(OH)]

“…This is in fairly good agreement with activation energies determined by electrical conductivity measurements (0.24 eV, Gerard and Grandjean (1975), 0.15 eV, Heilmann et al (1977), 0.13 eV, Coey et al (1984)). Ghazi-Bayat et al (1992) have shown that in pure synthetic ilvaite the monoclinic to orthorhombic phase transition occurs at higher temperatures than in natural crystals i.e.…”

“…In addition, evidence for a mixed valent state of iron due to thermally activated electron hopping between Fe z+ and Fe 3+ at the 8d sites has been reported by several groups (Gerard and Grandjean 1971;Heilmann et al 1977;Amthauer and Evans 1978;Nolet and Burns 1979;Evans and Amthauer 1980). All these authors agree on an averaged hyperfine interaction of the Fe-ions at the 8d-sites above 400 K resulting in one quadrupole split doublet with averaged parameters in the M6ssbauer spectra.…”

A Mössbauer Investigation of Glauconite and Its Geological Significance

“…Spectra taken under the same conditions in this study ( Figure 2) showed the presence of a similar absorption shoulder. By analogy with patterns found for ilvaite (Heilman et al, 1977) and some synthetic spinels (Lotgering and Van Diepen, 1976), the flat shoulder suggests that some Fe in glauconite may have a variable oxidation state. To test this possibility, the analyses were repeated with the absorber held at 80 + I~ At this lower temperature the absorption shoulder disappeared, and the peaks showed a more symmetrical configuration (see Figure 3) as previously noted by Rolf et al (1977).…”

Polarized single crystal absorption spectroscopy of the Pnam-P2 1/a transition of Ilvaite Ca(Fe2+, Fe3+)Fe2+Si2O8(OH) as measured between 300 K and 450 K

“…Starting from room temperature an electron-hopping process within the 8d-sites was envisaged by Gerard and Grandjean (1971), Grandjean and Gerard (1975), Heilmann et al (1977), and Yamanaka and Tak6uchi (1979) which results in the loss of separated Fe z+-Fe ~ + sites within the double chains. This behaviour was compared with the Ver-5.…”