M1, M2 site populations and distortion parameters in synthetic Mg-Fe orthopyroxenes from M�ssbauer spectra and X-ray structure refinements

Abstract

Abstract. Synthetic orthopyroxenes free of trivalent cations MgxFez -xSi206 (x = 0; 1; 1.6) were investigated using both M6ssbauer spectroscopy (MS) and single-crystal X-ray diffraction (XRD) in order to compare the results obtained on the same samples by the two techniques. The Mg-Fe distribution over M1 and M2 octahedral sites was determined on compounds with x = 1 and x = 1.6, unheated and annealed at 650, 850 and 1040 ~ C for 29 to 42 hours to obtain different degrees of order. The data obtained by the two techniques are in very good agreement, thus suggesting, for these compositions, the absence of systematic errors. The investigation of a synthetic compound with x=0 (ferrosilite) confirmed that the accuracy of both methods is very good. The observed small discrepancies between the results are not significant if compared with the relevant standard deviations which, for both methods, were derived by leastsquares calculations. The best fit is obtained for Fe(M1)/ Fe(tot) parameters that, in both methods, are directly calculated without using data from chemical analysis. For the unheated samples the values by MS and XRD are respectively: 0.185(9) and 0.180(4) for x = 1.6; 0.282(8) and 0.285(2) for x=l; 0.503(7) and 0.502(1) for x=0. For all the investigated samples the correlation between the geometrical distortion parameters, calculated by Xray refinement, and the quadrupole splitting values, measured by M6ssbauer spectroscopy, agrees with the theoretical model of Ingalls (1964).

Fe-Mg order-disorder in orthopyroxenes

“…A persistent feature for all spectra is the asymmetry of quadruple doublets, namely, the unequal amplitude and width for the two components of M2 doublet. This feature was also observed in previous studies on orthopyroxene (Virgo and Hafner, 1969;Burnham et al, 1971;Saxena and Ghose, 1971;Anovitz et al, 1988;Domeneghetti and Steffen, 1992;Angel et al, 1998). There are three known causes for such asymmetry.…”

“…The X Fe M1 /X Fe M2 ratio for each run, listed in Table 2, was taken as the ratio of the sum of area intensities of the outer two peaks (M1) to that of the inner two peaks (M2), assuming as is common practice an identical recoil-free fraction for M1 and M2 sites (Hawthorne, 1983(Hawthorne, , 1988Domeneghetti and Steffen, 1992). Uncertainties on the ratios were estimated by propagating the uncertainties on the area intensity of every peak from the fitting.…”

“…Several studies have been conducted to investigate Fe-Mg order-disorder in orthopyroxenes using conventional Möss-bauer spectroscopy (Virgo and Hafner, 1969;Saxena and Ghose, 1971;Besancon, 1981;Anovitz et al, 1988;Domeneghetti and Steffen, 1992;Skogby, 1992). The Fe-Mg distribution coefficients from these studies have been recalculated based on the reported site occupancies to account for the minor elements following the same procedure as in this study.…”

“…K D as a function of temperature and orthopyroxene composition from Mössbauer measurements. Data sources: VF69, Virgo and Hafner (1969); SG71, Saxena and Ghose (1971); B81, Besancon (1981); Aϩ88, Anovitz et al (1988); S92, Skogby (1992); DS92, interpolated from Domeneghetti and Steffen (1992). The dashed lines (two lines in each panel) are calculated from the calibration of Stimpfl et al (1999) based on XRD data.…”

Iron site geometry in orthopyroxene: Multiple scattering calculations and XANES study

“…However, when heated at high temperature and at controlled oxygen fugacity to prevent iron oxidation, the orthopyroxenes exhibit a redistribution of the Fe 2 + atoms and the M1 site is progressively enriched in Fe 2 + as a function of temperature (Virgo and Hafner 1969). This effect, well studied in many papers by combined M6ssbauer spectroscopy and XRD (e.g., Domeneghetti and Steffen 1992) has been evidenced also by XAS on a OPX sample heated at four different temperatures and characterized both by M6ssbauer and XANES spectroscopy (Anovitz et al 1988;Mottana et al 1991). A variation of the intensity of the main absorption peaks in the OPX-XANES spectra has been fourfd and interpreted as due to the changes in the octahedral site occupancy (Mottana et al 1991).…”