AN Fe MÖSSBAUER STUDY OF THE OLIVINE SOLID SOLUTION SERIES: IMPLICATIONS FOR METEORITE CLASSIFICATION AND DECONVOLUTION OF UNEQUILIBRATED CHONDRITE SPECTRA

Introduction: Mössbauer spectroscopy has been used to characterise and quantify the mineralogy of chondritic meteorites by detecting Fe bound in a crystal lattice [1]. The technique investigates the oxidation state and electronic configuration of Fe atoms in minerals [2]. Mössbauer spectra for meteorites frequently show overlapping or poorly separated absorption peaks for olivine and pyroxene, which are difficult to separate during the fitting process [3]. This is particularly true for studies of unequilibrated chondrites where their absorptions are often treated as a single peak and their mineral abundances considered together [4]. Some discrepancy has also been observed between pyroxene abundances as measured by Mössbauer spectroscopy and X-ray diffraction in carbonaceous chondrites [5]. Pyroxene appears to have been overestimated in samples analysed by the Mössbauer technique, possibly due to overlapping of peaks associated with olivine. The large spread of olivine compositions in unequilibrated chondrites suggests that some of the more fayalitic compositions may have been mis-identified as pyroxene. It was proposed that if one of these components could be isolated, the fitting procedure for meteorites could be improved and more accurate mineral abundances obtained. As well as improving interpretation of meteorite Mössbauer spectra, such a method could also point the way for a rapid method of characterising silicate chemistry and identifying the degree of equilibration in chondrites. Olivine has a simple chemistry and displays a complete solid solution series from forsterite to fayalite. This study aims to use Fe Mössbauer spectroscopy to analyse a number of mono-mineralic olivine samples within this range and determine how the Mössbauer parameters vary with changing proportions of Fe (recorded as olivine fayalite mol%). Bancroft et al [2] carried out a similar procedure on four samples of olivine of varying Fa mol%, but with the purpose of determining cation distribution in olivine. Methodology: Six synthetic olivine samples ranging from pure fayalite, Fe2SiO4 (Fa100), to forsteritic olivine, (Mg80,Fe20)2SiO4 (Fa20), were obtained for analysis. Approximately 100mg of powder was placed in a lead sample holder. The spectra were recorded at 298K with an Fe Mössbauer spectrometer using a Co/Rh source. Spectra were fitted with a constrained non-linear least squares fitting programme of Lorentzian functions. Three parameters reflecting the energy state and chemical environment of electrons at the nucleus are observed by Mössbauer analysis, two of which, chemical isomer shift and quadrupole splitting, are important for investigating olivine. The former is used to tell the oxidation state of Fe while quadrupole splitting gives information about the symmetry of the structure [1]. Results: The Mössbauer spectrum recorded for olivine shows a well-defined doublet (two-line absorption) of strong intensity as seen in Figure 1. The quadrupole splitting values for each of the synthetic olivine standards were plotted against the Fa mol% (Figure 2).