X-ray absorption study at the Fe K-edge of garnets from the Ivrea-Verbano zone

Abstract

K-edge X-ray absorption spectra of Fe in garnet samples from the Ivrea-Verbano zone were collected using synchrotron radiation. From XANES analysis, the prevalent oxidation state of Fe has been determined as 2+ in all studied samples. Coordination numbers and Fe-O bond lengths derived from the EXAFS analysis are compatible with a dodecahedral environment of Fe atoms and seem to be nearly independent of the variable Fe/Ca ratio of the cations sharing the dodecahedral site in these garnets. This suggests that, since at least up to 0.5 Ca atoms p.f.u, no sensible deformation of the dodecahedron geometry is sensed by the Fe cations, iron might strongly compete with Ca atoms in controlling the entry of rare earth elements in the dodecahedral site of garnets falling within this compositional range. Comparison of the EXAFS results with the data from single crystal X-ray diffraction structure refinements indicates a first shell neighbour distance accuracy of +0.02 ~, using theoretical EXAFS phases and amplitudes. The Debye-Waller factors derived from the EXAFS analysis indicate a higher degree of disorder on the four longer Fe-O bond distances, in comparsion with the other four shorter distances of the height-coordinated cation; this could be related to the nonrigid polyhedral behaviour of the dodecahedral site.

57Fe-Mössbauer investigation on garnets from the Ivrea-Verbano Zone

“…The isomer shift and quadrupole splitting values are almost invariable and suggest that the Fe-O distances are nearly the same in all the samples and in the end member, and this is in agreement with the distances found by Fe K-edge XAS studies on the same samples (Quartieri et al, 1993). Both spectroscopic techniques suggest that, at least in the range between 0.1 and 0.5 Ca atoms p.f.u., the presence of the larger Ca cation does not modify appreciably the geometry of the dodecabedral sites occupied by iron.…”

“…The anisotropic recoil-free fraction of Fe 2+ in garnets is attributed to the strong anisotropic vibrational behaviour of this cation in the eight-coordinated site of the cubic garnet structure. This assumption is also confirmed by the recent X-ray absorption studies performed on the same samples (Quartieri et al, 1993): the Debye-Waller factors derived from the XAS analysis indicate a higher degree of disorder on the four longer Fe-O bond distances, in comparison with the other four shorter distances of the eighth-coordinated iron cation.…”

“…A tentative interpretation of the spectra based on the fit where the areas of the ferrous peaks were constrained to be equal (model I), resulted in the apparent presence in some samples of Fe 3+ in tetrahedral coordination. This model is, however, inconsistent both with the chemical analyses, especially the high Si content of all the samples (Table 1), and with the X-ray absorption results (Quartieri et al, 1993): the absence in the XAS spectra of the intense pre-edge feature typical of the tetrahedral Fe 3+, rules out the hypothesis of the presence in our samples of iron substituting for Si in the tetrahedral site. A similar shortcoming was noted by Luth et al (1990) for a series of mantle-derived garnets and is thought to be an artefact due to the use of constrained areas for the peaks of the ferrous iron doublet.…”

“…comm.) and by XAS (Quartieri et al, 1993). For comparison, the data relative to a garnet sample much richer in Fe 3+ (BRIC, Basso et al, 1981) are also reported in Table 3.…”

Local environment and valence state of iron in microinclusions in fibrous diamonds: X-ray Absorption and Mössbauer data

“…The low pre-edge/white line ratio suggests that Fe 2+ coordination is higher than octahedral, and may indicate that in these diamonds, iron is present in cubic or in cubooctahedral (dodecahedral) coordination. These spectra resemble iron-bearing garnets [Quartieri et al, 1993, Wilke et al, 2001]. …”