Octahedrally coordinated vacancies in tourmaline: a theoretical approach

Abstract

AbstractBond-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 TR3+-cations and octahedrally coordinated YR2+- and ZR2+-cations around O8, 07 and O6 can be ruled out, together with arrangements involving vacancies and YLi1+. As the occurrence of a Y-site vacancy (Y☐) is more in accord with the valence-sum rule than the occurrence of a Z-site vacancy (Z☐), Y☐ is more likely to occur than Z☐ 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 [YR3+YR3+Y☐]−O1(OH−) and [ZR3+ZR3+Y☐]−O3(OH−), which yield the smallest deviations from the valence-sum rule. Coupling these stable arrangements with 2 × [TSi4+ZR3+Y☐]−O6(O2−) forms larger vacancy clusters: [Y(R3+)2−O1(OH−)−y(☐)− O3(OH−)−O6(O2−)2−(ZR3+TSi4+)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.