Crystal chemical relationships in the tourmaline group: Structural constraints on chemical variability

This paper explores some aspects of the crystal chemistry and structural constraints on tourmaline by examining 127 samples from the literature. According to the bond-valence model, the tourmaline structure shows lattice-induced strain at each polyhedron. The overall effect is an expansion of the triangular (BO3) group and compression of the tetrahedron. The X polyhedron can be either compressed or expanded: compression increases with vacancy content, whereas expansion is typical of Ca-rich tourmaline. The Y octahedron changes extensively from compressed through an unstrained to expanded state as a function of increasing Li content. The Z octahedron is almost unstrained in crystals with Sigma R-Z(2+) < 0.40 apfu, whereas it is compressed in crystals with Sigma R-Z(2+) > 0.40 apfu. The configuration of the six-membered tetrahedral ring is strongly affected by < Y-O >, which is the most important parameter linked to the deviation of the tetrahedral ring from hexagonal symmetry. The whole structure is stable when the channels through the Z octahedron framework are able to accommodate the Y cations. As < Y-O > becomes larger, the less puckered the tetrahedral ring and the more the O7 atom is displaced away from Z. Consequently, the difference between < Y-O > and < Z-O > cannot be too large, otherwise <-O > will be too small to be commensurate with shifting of the O7 atom. One possible mechanism to reduce the difference between < Y-O > and < Z-O >, is the disordering reaction Al-Y + R-z -> R-Y + Al-Z, which increases < Z-O> and decreases < Y-O >. In ideal dravite, schorl, and "tsilaisite," < Y-O > and < Z-O > are incommensurate.