Abstract
The Fe-Mg distribution over the M1 and M2 octahedral sites in five natural orthopyroxenes has been determined by single-crystal X-ray diffraction (XRD) and Mossbauer spectroscopy (MS). The results form the two methods are in good agreement for two Fe/Mg intermediate composition samples, provided that the MS data are thickness-corrected. For Fe-poor and one AL-rich sample, the results diverge; XRD gives a more ordered Fe-Mg distribution than MS, with up to 0.05 more Fe atoms per formula unit in the M2 position. The thickness effect on MS for ordered orthopyroxene is shown to cause considerable distortion of the spectra, resulting in an underestimate of the degree of ordering. For normally thick absorbers (5 mg Fe/cm 2 ), MS data will allocate approximately 2% too much of total Fe to the M1 position. However, the Fe distribution obtained from MS data can be corrected by extrapolation to zero thickness according to the formula Fe(M1)/Fe(tot) corr =[Fe(M1)/Fe(tot) means -0.004085t]/(1-0.003031t), where t is the Fe «thickness» of the absorber in mg Fe/cm 2
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