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

Abstract

Synthetic orthopyroxenes free of trivalent cations Mg x Fe 2-x Si2O6 (x=0; 1; 1.6) were investigated using both Mossbauer 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=1; 0.503(7) and 0.502(1) for x=0. For all the investigated samples the correlation between the geometrical distortion parameters, calculated by X-ray refinement, and the quadrupole splitting values, measured by Mossbauer spectroscopy, agrees with the theoretical model of Ingalls (1964).