Mossbauer spectroscopy of Zn-poor and Zn-rich rhodonite

Abstract

Room-temperature Mössbauer spectra of fi ve specimens of typical, Zn-poor rhodonite and fi ve of Zn-rich rhodonite ("fowlerite") have been obtained to determine whether the spectra refl ect structural differences between the two varieties. All Fe is divalent and in the high spin state, and the spectra support the interpretation that Fe is present in all fi ve M sites. Assignment of Mössbauer doublets to specifi c sites is made principally on the basis of isomer shift values and relationships between quadrupole splitting and polyhedral distortion parameters determined from previously reported crystal-structure refi nements. Trends in Mössbauer parameters show correlations with trends in steric details of the sites, both among individual M sites within Zn-rich and Zn-poor varieties and between corresponding M sites of rhodonite and fowlerite. In particular, the Mössbauer parameters for the geometrically similar M1 and M2 sites are similar, and those of M4 and M5 refl ect the tendency of the former toward more tetrahedral character and the latter toward less distortion in fowlerite. Iron occupancy is largely a function of polyhedral volume, although there is a secondary preference for octahedral coordination. Structural differences previously determined between rhodonite and fowlerite, which are driven principally by high concentrations of Zn in M4 and Ca in M5, are structurally pervasive and are maintained even when Fe substitutes for Zn or Ca.

Redox behavior of magnetite in the environment

“…Positive correlations were observed between the center shifts and quadrupole splitting for the clay and oxide samples was in agreement with previous findings for structural Fe 2+ (180,181). From the data sets, we concluded that the hyperfine parameters for sorbed Fe 2+ overlap with those found for structural Fe 2+ in many commonly occurring minerals.…”

“…This trend is not related to the CF values observed in spectra however, as these trends exist between multiple samples collected, and the CF is only used to interpret individual samples. Positive CS-QS trends between samples have been observed previously for structural Fe 2+ in clays as well as several other sample types, although the underlying cause remains unknown (180,181). What can be said is that the positive correlation between the two parameters suggests that the change in binding environment which affects the CS is also affecting the…”

“…To compare hyperfine parameters, the values are typically plotted with QS vs. CS ( Figure 6.3) (8,180,181). Two clear groupings exist within this data set; the oxide and clay samples cluster together in a narrow CS and QS range, while the cell data samples have significantly larger CS and QS values.…”