Crystal chemistry of the elbaite-schorl series

The crystal-chemistry of 13 elbaite-schorl tourmaline crystals from the Cruzeiro pegmatite (Minas Gerais, Brazil) was studied with a multi-analytical approach (SREF, EMPA, SIMS, MS). Effective cation radii at the Y and Z sites and site populations were refined by a minimization procedure. The results indicate that the crystals belong to the alkali group. Elbaite crystals are O2--free at the W and V sites and show OH content at the O2 site (up to 0.2 apfu). Conversely, schorl crystals always show O2- at the W site. The main substitutional mechanism is the dehydroxylation type: Fe-Y(2+) + Fe-Y(3+) + O-w -> Li-Y + Al-Y + (W)(OH+F). The T site is characterized by Si-T -> Al-T substitution. < X-O > is linearly correlated with vacancy content in crystals with (OH + F) ! 4, whereas it is almost constant in crystals with OH at the 02 position. Along the series, < Y-O > is inversely correlated with Al-Y. The Z site is almost fully occupied by R3+ (with Al-Z largely dominant) and the Fe-Z(tot) <-> Al-Z substitution explains the inverse correlation of < Z-O > with Al-Z. In the elbaite compositional range, lattice parameters are functions of < Y-O >, whereas in the schorl range they are essentially functions of < Z-O >. Alone, the whole elbaite-schorl series, both chemical substitutions and size increase of Y are far larger than those of Z. In spite of this, lattice parameters increase with < Y-O > as much as with < Z-O >. This is due to the role of the [ZO(6)] polyhedra, which extend along a and c to form the skeleton of the tourmaline structure. Therefore, any change in the size of Z leads to a change in the whole structure.