Turquoise zircon: new diffraction and optical data – an overview of V4+ crystal chemistry and optical spectroscopy in minerals

Vanadium-doped zircon exhibits a deep turquoise color that drew attention to this ceramic pigment since its discovery in 1946. Questions about vanadium valence and location in the zircon structure gave rise to many diffraction and spectroscopy studies. An overall convergence emerges from this literature about the occurrence of tetravalent vanadium. In contrast, the accommodation of V4+ at the Si tetrahedral site or at the Zr cubic site was for a long time disputed, until convincing diffraction and spectroscopic data pointed to V4+ hosted at a peculiar four-fold coordinated interstitial site of the zircon structure. The literature on the optical spectroscopy and crystal chemistry of V4+ in zircon (low to in-termediate V contents) and isostructural phases (hafnon, coffinite and thorium germanate) was integrated with four industrial pigments (representing high V concentrations). In addition, the optical and crystal chemical features of tetravalent vanadium occurring in minerals and synthetic analogues (cavansite, minasgraite, forsterite, apophyllite, goslarite) was overviewed. The zircon pigments were characterized by XRF, XRPD and DRS to get chemical composition, unit-cell and structural parameters (e.g. metal-oxygen bond lengths, polyhedral volumes, and distortion parameters), energy of the main optical bands and crystal field strength (10Dq). The V4+ incorporation into the zircon structure is testified by a progressive increase of the unit-cell volume with the V concentration. XRF analysis indicates a deficiency in Si suggesting a Si‒V balanced substitution. Such a substitution is supposed to occur (1) at the Si tetrahedral site 4b (point symmetry -4m2 or D2d) through a Si by V4+ substitution or (2) at the interstitial site 16g (point symmetry ..2 or C2) where the V4+ can be hosted, and the lattice electroneutrality is maintained by the formation of a Si vacancy. The optical spectra of V-doped zircon exhibit three main bands in the 4000‒22000 cm-1 range, which can be attributed to electronic transitions of V4+ in tetrahedral coordination. The spectral features are different from those of minerals containing the VO22- vanadyl ion (cavansite, minasgraite) where V4+ is in a five-fold, square planar coordination. The crystal field strength of tetravalent vanadium in zircon and isostructural phases scales with the mean V‒O distances at the interstitial tetrahe-dron, as obtained by diffraction (in zircon on average 1.894 Å). This observation needs caution, since 10Dq is challenging to be calculated, because of the three-fold splitting of the 2T2 band due to the low point symmetry of the interstitial site. The optical spectra of vanadium-doped zircon appear to depend in a complex way on the amount of V4+ actually incorporated, local environment at the interstitial site, Zr/Si ratio in zircon, and particle size distribution. References Niesert A., Hanrath M., Siggel A., Jansen M. and Langer K. (2002) – Theoretical study of the polarized electronic absorption spectra of vanadium-doped zircon. Journal of Solid State Chemistry, 169, 6-12. Siggel A. and Jansen M. (1990) - Röntgenographische Untersuchungen zur Bestimmung der Einbauposition von Seltenen Erden (Pr, Tb) und Vanadium in Zirkonpigmenten. Zeitschrift für anorganische und allgemeine Chemie, 583, 67-77.