Tourmaline from the rare-element Berger pegmatite (The Southern Tin Belt, central Namibia) chemical variation and implications for the pegmatitic evolution

Resumo

The Berger pegmatite (Southern Tin Belt, Namibia) is a zoned rare-element pegmatite, mined for tin and gem tourmaline. This mineral is an ubiquitous constituent in the pegmatite, occurring in the intermediate and core zones, as well as in the replacement units. Tourmaline chemistry changes depending on the pegmatite facies. Schorl is abundant in the intermediate zone. Multicolored elbaite occurs in the core zone, frequently showing watermelon texture. In that case, pink elbaite shows up to ~35% and ~9% of rossmanite and liddicoatite components, respectively. In the replacement bodies greenish to bluish tourmalines of the schorl-elbaite series are common. The broad compositional range shown by the tourmalines correlates well with the pegmatite zoning. The main mechanism that accounts for the chemical variation of tourmalines during crystallization seems to be YFe2+-2YAl1YLi1. The presence of Ca in the pink elbaites of the watermelon textures may be explained by the CaLi(NaR2+)-1 exchange vector. The chemical variation in tourmaline is consistent with an inward crystal fractionation model for the evolution of the Berger pegmatite.