FLUOR-ELBAITE FROM THE LEDUC PEGMATITE IN QUÉBEC, CANADA
Tourmaline crystals occur throughout the pegmatite and can range in size from <1 cm to ~75 cm in length. At least two categories of morphologically distinct tourmalines are observed: (1) narrow sprays of sub-parallel crystals emanating from a common base; and (2) massive, isolated euhedral crystals with terminal ends. In single specimens, of both types, extensive colour-zoning is observed, with the typical base-to-termination pattern being dark blue-to-black grading to regions of pinkish-red and greenish-blue. A mantle of black material is commonly observed, encasing the entire crystal. Chemical analyses show fluor-elbaite cores and schorl exteriors. The fluor-elbaite has average compositions of [X(Na0.6Ca0.2Vac0.2) Y(Li1.0Al1.5Mn0.1Fe0.4) ZAl6 (BO3)3 (Si6O18) V(OH) W((OH)0.3F0.7)] (bluish-green material), and [X(Na0.5Ca0.2Vac0.3) Y(Li1.0Al1.7Mn0.2Fe0.1) ZAl6 (BO3)3 (Si6O18) V(OH) W((OH)0.2F0.8)] (pinkish-red material).
The schorl/elbaite contact is texturally and compositionally discontinuous, showing the occurrence of resorption with the influx of a late-stage, Fe/Mn-rich fluid. The composition of Leduc tourmalines varies between fluor-elbaite and schorl near-end-member compositions along YAl3+ + YLi+ + WF- ↔ Y2Fe2+ + W(OH)-. A complex textural relationship between the schorl, F-elbaite cores, and co-precipitating feldspars (at the sub mm scale) is also observed. Investigation of these textures allows aspects of the geochemical conditions present during (and after) pegmatite emplacement to be better constrained, and Leduc to be placed in the broader context of regional geology.