Crystal chemistry of tourmalines from the Erongo Mountains, Namibia, studied by Raman spectroscopy

Abstract

Black tourmalines (general formula XY 3 Z 6 T 6 O 18 (BO 3 ) 3 V 3 W) of various morphologies from miarolitic cavities in pegmatites of the Erongo granite are analysed by Raman spectroscopy and in part by electron microprobe, in order to identify the tourmaline species and to study the compositional zoning within a specimen. The samples studied are large euhedral single crystals, intergrowths of small crystals, longitudinally zoned specimens possessing a single-crystal main body and monopolar overgrowth, and a transversely zoned specimen consisting of small crystals covering the single-crystal main body. The results show that the most abundant tourmaline species are schorl deficient in W-site OH ( W OH) and Y Al-rich schorl that additionally contains minor amounts of Y Mg and/or Y Li. In addition, X Na-rich foitite with a composition close to the midpoint of the schorl–foitite join was found in the narrow border area separating the single-crystal part and the small-crystal aggregate in tourmaline with longitudinal zoning. Near-end-member foitite was only found as the last stage of crystallization on the top edge of small secondary-grown crystals in the tourmaline sample with transverse zoning. This study also demonstrates the potential of Raman spectroscopy as a non-destructive easy-to-handle method to track major chemical changes in tourmalines, which might potentially be used to document the evolution of the tourmaline host environment.