Octahedrally coordinated vacancies in tourmaline: a theoretical approach

摘要

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 Li~(1+), Mg~(2+), Al~(3+), Fe~(2+), Fe~(3+), B~(3+) and Si~(4+). The results show that arrangements involving tetrahedrally coordinated ~TR~(3+) -cations and octahedrally coordinated ~YR~(2+)- and ~ZR~(2+)-cations around O8, O7 and O6 can be ruled out, together with arrangements involving vacancies and ~YLi~(1+). 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 O~(2-). Of particular interest in this regard are the arrangements[~YR~(3+) ~YR~(3+) ~Y□]-~(O1) and [~ZR~(3+) ~ZR~(3+) ~Y□]-~(O3)(OH~-), which yield the smallest deviations from the valence-sum rule. Coupling these stable arrangements with 2 x [~TSi~(4+) ~ZR~(3+) ~Y□]-~(O6)(O~(2-) forms larger vacancy clusters: [~Y(R~(3+))_2 -~(O1)(OH~-)-~Y(□)-~(O3)(OH~-)-~(O6)(O~(2-))_2-(~ZR~(3+) ~TSi~(4+))_2]. In tourmaline, vacancies are more favoured to occur at Y rather than at Z, in tandem with OH~-—at O3 and O1, R~(3+) at Y and Z and Si~(4+) at T.