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: Y Fe 2+ + Y Fe 3+ + WO → Y Li + Y Al + W (OH+F). The T site is characterized by T Si → T Al substitution. is linearly correlated with vacancy content in crystals with (OH + F) ≤ 4, whereas it is almost constant in crystals with OH at the O 2 position. Along the series, is inversely correlated with Y Al. The Z site is almost fully occupied by R 3+ (with Z Al largely dominant) and the Z Fe tot ↔ Z Al substitution explains the inverse correlation of with Z Al. In the elbaite compositional range, lattice parameters are functions of , whereas in the schorl range they are essentially functions of . Along 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 as much as with . 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.