Octahedrally coordinated vacancies in tourmaline: a theoretical approach

Bond-valence theory is used to explore the local arrangements involving vacancies at the Y and Z sites in the tourmaline structure. The local bond-valence requirements of all possible local arrangements around the O8, O7, O6, O3 and O1 anion-sites have been determined for Y- and Z-site vacancies locally associated with Li1+, Mg2+, Al3+, Fe2+, Fe3+, B3+ and Si4+. The results show that arrangements involving tetrahedrally coordinated R-T(3+)-cations and octahedrally coordinated R-Y(2+)- and R-Z(2+)-cations around O8, O7 and O6 can be ruled out, together with arrangements involving vacancies and Li-Y(1+). As the occurrence of a Y-site vacancy ((Y)square) is more in accord with the valence-sum rule than the occurrence of a Z-site vacancy ((Z)square), (Y)square is more likely to occur than (Z)square in tourmaline. Local arrangements involving vacancies around O1- and O3-sites can be stable for OH-, but not for O2-. Of particular interest in this regard are the arrangements [R-Y(3+) R-Y(3+) (Y)square]- (O1)(OH-) and [R-Z(3+) R-Z(3+) (Y)square]-(O3)(OH-), which yield the smallest deviations from the valence-sum rule. Coupling these stable arrangements with 2 x [Si-T(4+) R-Z(3+) (Y)square]-(O6)(O2-) forms larger vacancy clusters: [(Y)(R3+)(2)-(O1)(OH-)-(Y)(square)-(O3)(OH-)-(O6)(O2-)(2)-(R-Z(3+) Si-T(4+))(2)]. In tourmaline, vacancies are more favoured to occur at Y rather than at Z, in tandem with OH- at O3 and O1, R3+ at Y and Z and Si4+ at T.