The Fe-Mg distribution over the MI and M2 octahedral sites in five natural orthopyroxenes has been determined by single-crystal X-ray diffTaction (XRD) and Mössbauer spectroscopy (MS). The results from the two methods are in good agreement for two Fe/Mg intermediate composition sampies, provided that the MS data are thickness-corrected. For two Fe-poor and one AI-rich sampie, 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 MI position. However, the Fe distribution obtained from MS data can be corrected by extrapolation to zero thickness according to the formula Fe(M I)/Fe(tot)cOIT = [Fe(MI)/Fe(tot)meas-0. 004085t]/(1-0.00303lt), where t is the Fe « thickness » of the absorber in mg Fe/cm 2 .

The crystal chemistry of clinopyroxene, orthopyroxene and olivine from a crushed fragment of the H5 Zaoyang chondrite has been investigated by X-ray structure refinement and detailed microprobe analysis. The meteoritic pyroxenes have cell and polyhedral volumes which compare well with such data from terrestrial pyroxenes that typically crystallize at low-pressure. Fe 2+ and Mg are rather disordered in M1 and M2 sites of clino-and orthopyroxenes; the closure temperatures of the exchange reaction are 600 and 512~ respectively, which is consistent with a reasonably fast cooling rate, estimated to be of the order of l~The closure temperature for the intercrystalline Ca-Mg exchange reaction for clino-and orthopyroxenes is 900~ as calculated from clino-and orthopyroxene intergrowth.The cooling rates obtained from Fe 2 +-Mg intracrystalline partitioning suggest a cooling of the order of degrees per day at temperatures of 600-500~ due to a strong loss of heat by irradiation.

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A suite of orthopyroxenes from spinel lherzolite xenoliths associated with basauites occurring in the Victorian (Australia) post-Pliocene 'Newer Volcauics' province was investigated by means of a crystal chemical methodology which provides accurate site occupancy and site configuration parameters.The.4+M1 configuration is essentially constrained by A1 w rather than Fe 2 +. In additmn,' " Fe 3 +, Cr 3 + and TI are confined to M1 (Molin, 1989) and A1 Iv to TB. M2 is controlled by Fe~2 ~ Mgxt2, constrained by (Fe 2+ + Ca)M2 > 0.14 atoms per formula unit (p.f.u.). Cation substitution in TB and M2 constrains the sum of the volumes of the respective polyhedra VTB+ VM2 to remain essentially constant. Therefore, M2 favours the retention of the large Fe 2+ up to melting-point, causing nonideality of this iron-depleted orthopyroxene. As a consequence, the investigated orthopyroxene can be considered an ultimate Fe 2+ carrier during partial mantle melting.