Berry, FJ

Bland, PA

Cressey, G

Menzies, ON

Spiral - Imperial College Digital Repository

   A QUANTITATIVE MODAL MINERALOGY STUDY OF ORDINARY CHONDRITES USING X-RAY DIFFRACTION AND MÖSSBAUER SPECTROSCOPY.  O. N. Menzies1, P. A. Bland1 , G. Cressey2 and F. J. Berry3. 1Planetary and Space Sciences Research Institute, The Open University, Milton Keynes MK7 6AA, UK., o.n.menzies@open.ac.uk, p.a.bland@open.ac.uk, 2Department of Mineralogy, Natural History Museum, Cromwell Rd., London SW7 5BD, UK., g.cressey@nhm.ac.uk, 3Department of Chemistry, The Open University, Milton Keynes MK7 6AA, UK., f.j.berry@open.ac.uk.  Introduction: It has previously been demonstrated that the modal mineralogy of primitive, carbonaceous chondrites can be quantified using a combination of X-ray diffraction and Mössbauer spectroscopy [1,2]. We have shown how Mössbauer spectroscopy can be used to quantify the Fe-bearing mineralogy of the unequili-brated ordinary chondrites (UOCs) [3]. We report here new data for nine ordinary chondrites using X-ray dif-fraction to quantify their modal mineralogy. We also observe mineralogical variations with equilibration. The fine-grained nature of primitive unequilibrated chondrites, combined with their complex heterogeneic mineralogy, makes phase quantification difficult to achieve by conventional means, i.e. using an electron microprobe or by simple point counting. Normative calculations of ordinary chondrites have been made in an attempt to correlate meteorite mineralogy with as-teroid composition from reflectance spectra [4], al-though for clay-containing UOCs the assumption of an anhydrous mineralogy is inappropriate. To date no quantitative modal analysis of ordinary chondrites has been published. Such mineralogical information would be an invaluable tool to elucidate the geological history of primitive meteorites and ultimately to interpret early solar system processes.  Mössbauer spectroscopy can be used as a quantita-tive tool to yield wt% values of Fe-containing minerals [5]. It is particularly useful because the individual min-eral components are analysed in a non-destructive manner, the technique is insensitive to crystallinity and grain-size variations and can detect the presence of Fe-bearing minerals of abundance >1 wt% [1]. X-ray diffraction (XRD) has long been used to identify crystalline phases in rocks and meteorites [6]. Our analytical set-up incorporates a curved position-sensitive detector, allowing fast and efficient, quantita-tive assessment of multiphase samples [7], even for mixtures containing clay [8], which is of huge benefit in the analysis of primitive meteorites. Combining Mössbauer spectroscopy with XRD, allows for the identification of all phases, apart from non-crystalline, non-Fe-containing minerals. Methodology: Six unequilibrated and three equili-brated ordinary chondrites were chosen for this study.  For Mössbauer spectroscopy approximately 100mg of finely powdered sample was placed in a lead sample holder. Spectra were recorded at 298K with an 57Fe Mössbauer spectrometer, using a 57Co/Rh source and fitted with a constrained non-linear least squares fitting programme of Lorentzian functions. For XRD the powdered sample was placed in a shallow plastic well with a volume of 180mm3 and ir-radiated for 600 minutes. Details of the sample prepa-ration procedure are outlined in [8,9] and its applica-tion to carbonaceous chondrites in [1,2].  The CuKα1 (germanium III monochromated) radia-tion interacts with Fe in the sample causing emission of Fe fluorescence with the result that the diffraction peaks are superimposed on a gradually rising back-ground of Fe fluorescence. The percentage of a phase (taken from a database of mineral standards) present in an original sample was estimated by visually comparing the peak heights [8]. The standard was then increased or decreased by the necessary factor and effectively stripped out of the original sample. As the peaks for each Fe-containing phase were erased, the background fluorescence flat-tened and the number of identified phases eventually equalled 100%. The proportions of phases obtained by this method were percentages of fit. These were then corrected for matrix absorption to give accurate wt% proportions of all crystalline phases in the sample.                       2θ (CuKα1) Figure 1. XRD pattern for Bishunpur, LL3.1. The experi-mental pattern is that of the bulk sample run for 600mins. The calculated pattern is the composite of all phases be-lieved to be present in the sample and the residual is the experimental with the calculated subtracted. 0 Intensity EXPERIMENTAL CALCULATED RESIDUAL Lunar and Planetary Science XXXIII (2002) 1500.pdfQUANTITATIVE MODAL MINERALOGY OF ORDINARY CHONDRITES :  O. N. Menzies, P. A. Bland, G. Cressey and F.J. Berry.                                  A series of ordinary chondrites from throughout the 2θ (CuKα1)  Figure 3. Changing modal mineralogy with petrologic type or equilibration. Note that H, L and LLs are included so this is intended to give a picture of what minerals are present and their abundances, not to show clear trends. 0 20 40 60 80 100 LL3.1 LL3.4 H3.4 L3.4 L3.7 H3.8 LL4 L5 H6 % Phase Petrologic Type Phyllosilicate Troilite Fe-Ni Metal Olivine Pyroxene Magnetite Feldspar IntensityBishunpur LL3.1Soko Banja LL4M anych LL3.4EquilibrationLunar and Planetary Science XXXIII (2002) 1500.pdfFigure 2. Experimental XRD patterns for three LL chon-drites, stacked in order of increasing equilibration. Note theenhanced intensity and narrower widths of major diffrac-tion peaks with equilibration.  The technique has difficulties in correctly identify-ing the proportion of Fe-Ni metal. The metal, being denser and composed of crush-resistant grains, tends to sink to the bottom of the sample holder regardless of what preparation techniques are employed. Therefore, the X-rays see a less than representative proportion of metal. To overcome this problem, the metal was mag-netically separated and weighed. Assuming the propor-tions of silicates and sulphides were accurately deter-mined, the metal values were corrected and the actual wt% values of the bulk meteorite re-proportioned ac-cordingly. Results and Discussion: Figure 1 shows an XRD pattern for Bishunpur, LL3.1. The experimental pattern of the bulk sample is shown alongside the calculated pattern, which is the composite of all standard phases believed to be present in the sample. The residual pat-tern is the experimental minus the calculated pattern and should be a flat line when all phases are accounted for. The fluorescence background is proportional to the amount of Fe in the sample and it is possible to use this to calculate the total wt% of Fe. Each sample analysed is within 2% of its published total wt% Fe value. This also means that the results are directly comparable to those obtained by Mössbauer spectroscopy [1,2].  XRD identifies and provides quantification for oli-vine, pyroxene, troilite and feldspar. Fe-Ni metal can be identified but requires magnetic separation for quan-tification, for reasons described above.  Mössbauer spectroscopy is useful for identifying phases which cannot be readily recognised using XRD alone, as the peaks are hidden by other common phases or are indistinct. These include magnetite and a para-magnetic phase which is probably an Fe-containing phyllosilicate. equilibration series was sampled in order to assess changing mineralogy with parent-body metamorphism. Figure 2 shows the XRD patterns for three LL chon-drites in order of increasing equilibration. It can be seen that diffraction peaks narrow and increase in in-tensity as crystallinity develops. We also observe a homogenisation of olivine towards more forsteritic compositions with equilibration. Figure 3 uses stacked columns to represent the changing mineralogy with petrologic type. It should be noted that H, L and LL chondrites are included on the same graph, so trends must be interpreted with caution. Clearly a large proportion of phyllosilicate is present in the unequilibrated chondrites, which varies inversely with the proportion of ferromagnesian silicate. Feld-spar is present only from petrologic type 3.4. It is also interesting to note that Bishunpur appears to have a substantially large proportion of Fe-Ni metal for an LL chondrite. Conclusions: This is the first quantitative study of the modal mineralogy of ordinary chondrites.  We be-lieve that the fact that Mössbauer spectroscopy and X-ray diffraction data are mutually supportive allows us to be confident in our assessment of the modal miner-alogy of these unequilibrated meteorites.  References: [1] Bland P.A. and Cressey G. (2001) LPS XXXII, #1853. [2] Bland P.A. and Cressey G. (in press) MAPS. [3] Menzies O.N. et al. (2001) MAPS, 36, A131, [4] McSween H.A. et al. (1991) Icarus, 90, 107-116. [5] Bland P.A. et al. (1998) GCA, 62, 3169-3184. [6] DuFresne E.R. and Anders E. (1961) GCA, 26, 1085-1114. [7] Cressey G. and Schofield P.F. (1996) Powder Diff., 11, 35-39. [8] Batchelder M. and Cressey G. (1998) Clays, Clay Min, 46, 183-194. [9] Cressey G. and Batchelder M. (1998) Int. Un. Cryst. News., 20, 16-17.   

A quantitative modal mineralogy study of ordinary chondrites using X-ray diffraction and Mössbauer spectroscopy.

Abstract

Similar works

Full text

Available Versions

Spiral - Imperial College Digital Repository