An XAFS study of the crystal chemistry of Fe in orthopyroxene

X-ray absorption fine-structure (XAFS) spectra at the Fe K edge in Bamble enstatite (Mgo88Feo,,)Si03 were analyzed in conjunction with theoretical XAFS spectra to determine the bonding configuration of Fe in this structure. The structural analysis involved determination of the Fe distribution between the octahedral M1 and M2 sites, and Fe-O bond lengths in the M2 site, into which Fe strongly partitions. Our analysis yielded bond lengths for Fe in the M2 site of 1.97(2), 1.98(2), 2.09(2), 2.16(2), 2.43(2), and 2.65(10) A, in agreement with bond lengths determined from X-ray and neutron-diffraction analysis of the two orthopyroxene end-members. The average Fe-O bond length in the M2 site is 2.22(2) A, longer than that of the Mg end-member (2.151 A) but approximately the same as that of the Fe end-member (2.228 A) of the orthopyroxene solid-solution series. Octahedral distortion of the M2 site may be greater than that of either the Fe or Mg endmember. The presence of a minor amount of Fe3+ was inferred by our analysis of the M I site and was also suggested by our bond-valence calculations, which yielded a charge of 2.07 for Fe in the M2 site and a charge of 2.78 for Fe in the Ml site. Simple calculations using our data and those of other studies show that the average Fe-O bond length in the M2 site is constant along the Fe-Mg join in the orthopyroxene solid-solution series.