XANES STUDY OF TITANIUM COORDINATION IN NATURAL DIOPSIDIC PYROXENES

A series of diopsidic pyroxenes having Si and Al deficiency in the tet rahedral sites were selected for an X-ray absorption near-edge study a t the Ti and Fe K-edge using synchrotron radiation. The samples were p reviously characterized by single-crystal X-ray diffraction and the ge ometrical features were interpreted on the basis of Ti4+-->Si substitu tion. The analysis of the XANES spectral features confirms that Ti4+ p referentially 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-coord inated iron; the energies of the XANES features, compared with those o f the reference compounds, suggest that the prevalent oxidation state of iron in the diopsides investigated is 2+.