Berry, FJ

Bland, PA

Menzies, ON

Spiral - Imperial College Digital Repository

AN 57Fe MÖSSBAUER STUDY OF THE OLIVINE SOLID SOLUTION SERIES: IMPLICATIONS FORMETEORITE CLASSIFICATION AND DECONVOLUTION OF UNEQUILIBRATED CHONDRITESPECTRA.  O. N. Menzies1, P. A. Bland1 and F. J. Berry2. 1Planetary and Space Sciences Research Institute, TheOpen University, Milton Keynes MK7 6AA, UK, 2Department of Chemistry, The Open University, Milton KeynesMK7 6AA, UK.Introduction: Mössbauer spectroscopy has beenused to characterise and quantify the mineralogy ofchondritic meteorites by detecting Fe bound in acrystal lattice [1]. The technique investigates the oxi-dation state and electronic configuration of Fe atomsin minerals [2]. Mössbauer spectra for meteoritesfrequently show overlapping or poorly separated ab-sorption peaks for olivine and pyroxene, which aredifficult to separate during the fitting process [3].This is particularly true for studies of unequilibratedchondrites where their absorptions are often treated asa single peak and their mineral abundances consid-ered together [4]. Some discrepancy has also beenobserved between pyroxene abundances as measuredby Mössbauer spectroscopy and X-ray diffraction incarbonaceous chondrites [5]. Pyroxene appears tohave been overestimated in samples analysed by theMössbauer technique, possibly due to overlapping ofpeaks associated with olivine. The large spread ofolivine compositions in unequilibrated chondritessuggests that some of the more fayalitic compositionsmay have been mis-identified as pyroxene. It wasproposed that if one of these components could beisolated, the fitting procedure for meteorites could beimproved and more accurate mineral abundances ob-tained. As well as improving interpretation of mete-orite Mössbauer spectra, such a method could alsopoint the way for a rapid method of characterisingsilicate chemistry and identifying the degree ofequilibration in chondrites.Olivine has a simple chemistry and displays acomplete solid solution series from forsterite to faya-lite. This study aims to use  57Fe Mössbauer spectros-copy to analyse a number of mono-mineralic olivinesamples within this range and determine how theMössbauer parameters vary with changing propor-tions of Fe (recorded as olivine fayalite mol%). Ban-croft et al [2] carried out a similar procedure on foursamples of olivine of varying Fa mol%, but with thepurpose of determining cation distribution in olivine.Methodology: Six synthetic olivine samplesranging from pure fayalite, Fe2SiO4 (Fa100), to for-steritic olivine, (Mg80,Fe20)2SiO4 (Fa20), were ob-tained for analysis.  Approximately 100mg of powderwas placed in a lead sample holder. The spectra wererecorded at 298K with an 57Fe Mössbauer spec-trometer using a 57Co/Rh source. Spectra were fittedwith a constrained non-linear least squares fittingprogramme of Lorentzian functions. Three parame-ters reflecting the energy state and chemical environ-ment of electrons at the nucleus are observed byMössbauer analysis, two of which, chemical isomershift and quadrupole splitting, are important for in-vestigating olivine. The former is used to tell the oxi-dation state of Fe while quadrupole splitting givesinformation about the symmetry of the structure [1].Results: The Mössbauer spectrum recorded forolivine shows a well-defined doublet (two-line ab-sorption) of strong intensity as seen in Figure 1. Thequadrupole splitting values for each of the syntheticolivine standards were plotted against the Fa mol%(Figure 2).Figure 1. 57Fe Mössbauer spectrum for mono-mineralic olivine (Mg60,Fe40)2SiO4 showing a well-defined doublet.Figure 2. Quadrupole splitting values for a range ofolivine standards, including olivine from the SanCarlos meteorite and earlier data from Bancroft et al1967. Also plotted are two unequilibrated ordinarychondrites.2.82.852.92.9533.050 20 40 60 80 100Olivine SeriesBancroft et al,1967MeteoritesQuadrupole splitting mm/sOlivine Fayalite mol%Farmington L5Dhurmsala LL6San Carlos olivine0.90.920.940.960.981-6 -4 -2 0 2 4 6Absorption %Velocity mm/sLunar and Planetary Science XXXII (2001) 1622.pdfAN 57Fe MÖSSBAUER STUDY OF THE OLIVINE SOLID SOLUTION SERIES:  O. N. Menzies, P. A. Bland and F. J. BerryQuadrupole splitting values are highest for pure for-sterite and decrease linearly as Fe2+  substitutes forMg2+ in the olivine structure. Quadrupole splitting canbe used as a measure of the degree of asymmetry of acompound. The negative slope of the graph can beexplained by the fact that the more asymmetric formof olivine, forsterite, has a high quadrupole splittingvalue and as Fe2+  progressively substitutes for Mg2+,the structure becomes more symmetrical and thevalue of this parameter decreases.The relationship is confirmed by plotting Ban-croft's data points onto the same graph (Figure 2).Chemical isomer shift values have been plottedagainst the Fa mol% and also yield a linear correla-tion although not as marked as that for quadrupolesplitting.  Discussion: A standard olivine sample of knowncomposition (Fa10), from the meteorite San Carlos,was analysed and found to plot slightly above the linein Figure 2. Similarly, Bancroft’s sample of Fa12.5plots in this location. As pure forsterite contains noiron it could not be sampled by this technique to de-termine whether there is a departure from the linearrelationship observed between quadrupole splittingand olivine Fa mol% in the synthetic samples at theforsteritic end.To explore the application to meteorites, twosamples of equilibrated ordinary chondrites wereanalysed, in this case, with known olivine Fa mol%values [6],[7]. The samples were Farmington (L5)and Dhurmsala (LL6). The Mössbauer spectrum ofDhurmsala with clearly defined olivine peaks isshown in Figure 3. The olivine in these meteoritesplot on the trend defined by the standard olivinessuggesting that Mössbauer spectroscopy could beused in baseline meteorite classification.Figure 3. 57Fe Mössbauer spectrum for Dhurmsala.The two highest intensity peaks are those associatedwith olivine. (Note the velocity difference betweenthis and previous Figure 1.)Conclusion: The study shows that Mössbauerspectroscopy can be related to the mineralogy of oli-vine. The quadrupole splitting parameter shows aclear trend with changing olivine Fa mol%. The oli-vine of equilibrated meteorites fits this trend and withmore analyses it may be possible to determine thedegree of equilibration within the ordinary chondrites.This data will thus be important in deconvolutingspectra from unequilibrated meteorites.References:  [1] Berry, F. J. (1983). PhysicsBulletin 34, 517-519; [2] Bancroft, G. M., Maddock,A. G. and Burns, R. G. (1967). G.C.A. 31, 2219-2246; [3] Bland, P. A., Sexton, A. S., Jull, A. J. T.,Bevan, A. W. R., Berry, F. J., Thornley, D. M., Astin,T. R., Britt, D. T. and Pillinger, C. T. (1998). G.C.A.62, 3169-3184; [4] Hoffman, E., Seifu, D. and Oliver,F. W. (2000). Met. And Planet. Sci. 35, 431-434;  [5]Bland, P. A. Pers. comm. [6] Keil, K. andFredriksson, K. (1964). J. Geophys. Res. 69, p3487.[7] Fredriksson et al., in Ahrens, L.H. (1968), Per-gamon. P487.0.9650.970.9750.980.9850.990.9951-15 -10 -5 0 5 10 15Absorption %Velocity mm/sLunar and Planetary Science XXXII (2001) 1622.pdf

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An 57Fe Mössbauer study of the olivine solid solution series: Implications for meteorite classification and deconvolution of unequilibrated chondrite spectra.

An 57Fe Mössbauer study of the olivine solid solution series: Implications for meteorite classification and deconvolution of unequilibrated chondrite spectra.

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