Fivefold-coordinated Ti4+ in metamict zirconolite and titanite: A new occurrence shown by Ti K-edge XANES spectroscopy

Abstract The coordination environments of Ti in two fully metamict zirconolite samples and two partially metamict titanite samples were determined using high-resolution. X-ray absorption near-edge structure (XANES) spectroscopy at the Ti K edge. Fivefold-coordinated Ti is the dominant Ti species in the zirconolite samples (~-80 ± 10% of the total Ti atoms). This unusual Ti coordination is also possible in the titanite samples. No significant evidence for [4]Ti was found in any of the samples studied. Comparison with other amorphous materials, such as other metamict minerals (aes- chynite and pyrochlore) and titanosilicate glasses and melts, suggests that fivefold coordination is rather common for Ti4+ in aperiodic structures. However, the metamict state is characterized by the presence of unusual trigonal bipyramids around |5|Ti4+.

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