Short-range order in Li-Al tourmalines: IR spectroscopy, X-ray single crystal diffraction analysis and a bond valence theory approach

The short-range order in Li-Al-(OH-, F-) tourmalines with (Y)[Li/Al] approximate to 1 and different Na/Ca ratio was investigated by means of bond valence theory, experimental IR spectroscopic data and the results of X-ray single crystal diffraction. The stability of the arrangements coordinating W- and V-crystallographic sites occupied by OH-, F- and O2- ions was refined. A unified model of assignment of absorption bands in the IR spectra to the local arrangements (clusters) was suggested taking into account the first and the second OH(-)coordination spheres. The types of local cation arrangements around the W- and V-anion sites, alongside with clusters ratio and their distribution were brought out. The short-range order in Li-Al tourmalines controlled not only by local restrictions of the bond valence theory, but also by the long-range order was described.