FIVEFOLD-COORDINATED TI4- A NEW OCCURRENCE SHOWN BY TI K-EDGE XANES SPECTROSCOPY( IN METAMICT ZIRCONOLITE AND TITANITE )

The coordination environments of Ti in two fully metamict zirconolite samples and two partially metamict titanite samples were determined us ing high-resolution, X-ray absorption near-edge structure (XANES) spec troscopy at the Ti K edge. Fivefold-coordinated Ti is the dominant Ti species in the zirconolite samples (similar to 80 +/- 10% of the total Ti atoms). This unusual Ti coordination is also possible in the titan ite samples. No significant evidence for Ti-[4] was found in any of th e samples studied. Comparison with other amorphous materials, such as other metamict minerals (aeschynite and pyrochlore) and titanosilicate glasses and melts, suggests that fivefold coordination is rather comm on for Ti4+ in aperiodic structures. However, the metamict state is ch aracterized by the presence of unusual trigonal bipyramids around Ti-[ 5](4+).