Experimental and theoretical XANES study of the effects of Fe-Mg solid solution in the enstatite-ferrosilite series

Synthetic orthopyroxenes in the join enstatite - ferrosilite (Mg2Si2O6-Fe2S i2O6) have been studied by XANES with the aim to interpret the variations f ound in the spectra and evaluate the effects of the Mg-Fe substitution in t he orthopyroxene (OPX) structure. The experimental Fe- and Mg-K edge XANES spectra show variations of the peaks intensity as a function of the chemica l composition of the samples along the EN-FS join. In the region of the Fe edge, the intensity ratio between peak A and B (IB/IA) varies linearly (R = 0.99) with the Mg/Fe substitution. Multiple scattering calculation have be en performed both at the Fe and Mg K-edge, to study the geometrical modific ations of the M1 and M2 octahedral sites induced by the Mg-Fe substitutions . The spectra, obtained locating Fe (or Mg) in both M1 and M2 and weighing the contributions using the occupancy data for the M1 and M2 sites determin ed by XRD, allow to discriminate the effects to the total spectrum due to F e (Mg) located in each sites separately.