Compositional variation of tourmaline in the granitic pegmatite dykes of the Cruzeiro mine, Minas Gerais, Brazil

Schorl - dravite and schorl - elbaite from different zones of the rare-element-enriched complex granitic pegmatite dykes of the Cruzeiro mine, in Minas Gerais, Brazil, were analyzed by electron and ion (H, Li, B) microprobes. General mechanisms of substitution (common to schorl and to elbaite independently of their position in the pegmatites) drive compositions to "proton-deficient schorl", "alkali-free schorl", and "alkali-free elbaite" end-members, each involving Al enrichment. Among the specific mechanisms (i.e., those characterizing tourmaline within each separate zone), "octahedral-defect" and "alkali-free proton-rich" types are particularly important in OH-rich, pocket elbaite. The substitution X□ + OH ↔ XNa + + O 2-, driving compositions to an "alkali-free proton-rich elbaite", is proposed to explain both the lack of Na and the excess of (OH + F). Bond-valence calculations based on results of unpublished structure-refinement data allow the OH excess to be assigned to the O(2) site. The composition of the tourmaline in the Cruzeiro suite (high Al content; Mg, Fe, Zn, Mn, and Li covariation; antithetic behavior of OH and F) records the evolution of the pegmatite-forming melt.