Short-range order in tourmaline: a vibrational spectroscopic approach to elbaite

Polarized Fourier-transform infrared and Raman spectra were acquired on an elbaite sample previously characterized by electron- and ion microprobe analysis, X-ray diffraction and structure refinement. Spectra from the two vibrational spectroscopy techniques reveal a close similarity in the OH-stretching region, with three main absorption bands strongly polarized in the c-axis direction. By means of bond-valence theory arguments, the observed OH bands are interpreted and assigned to specific local cation arrangements around the O1 (a W) and O3 ( V) anion sites. In combination with the relatively simple composition of the studied sample, bond-valence constraints are used to identify stable anion-cation arrangements, which permit the occurrence of short-range ordering to be assessed. Evidence for nearly complete short-range order at the O1 site, with the stable arrangements (Y)(LiAlAl)(0.6)-(W)(OH)(0.6) and (Y)(LiLiAl)(0.4)-(W)(F)(0.4), are presented. These two local arrangements can be further expanded to obtain the larger ordered clusters [(W)(OH)-(Y)(LiAl2)-(V)(OH)(3)-(Z)(Al)(6)](0.6) and [(W)(F)-(Y)(Li2Al)-(V)(OH)(3)-(Z)(Al)(6)](0.4).