CRYSTAL CHEMICAL RELATIONSHIPS IN THE TOURMALINE GROUP: STRUCTURAL CONSTRAINTS ON CHEMICAL VARIABILITY

Abstract

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 Σ2R2+ < 0.40 apfu, whereas it is compressed in crystals with zR2+ > 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 <z-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 YAl + zR →+ R + zAl, which increases <z-o> and decreases <y-o>. In ideal dravite, schorl, and "tsilaisite," <y-o> and <z-o> are incommensurate.