XANES study of titanium coordination in natural diosidic pyroxenes.

A series of diopsidic pyroxenes, having Si and Al deficiency in the tetrahedral sites, were selected for an X-ray absorption near-edge study (XANES) at the Ti and Fe K-edge using synchrotron radiation. The samples were previously characterized by single-crystal X-ray diffraction and the geometrical features were interpreted on the basis of Ti4+ → Si substitution. The anal. of the XANES spectral features confirms that Ti4+ preferentially enters the tetrahedral site in two of the diopsides (D3 and D13), characterized by the complete absence of Al3+. In the other three diopside samples (D38, D39, D102) Ti4+ appears to be distributed among the tetrahedral and the octahedral sites. The Fe K-edge spectra of the five pyroxenes do not show any evidence of tetrahedrally-coordinated iron; the energies of the XANES features, compared with those of the ref. compds., suggest that the prevalent oxidn. state of iron in the diopsides investigated is 2+.