Abstract

Yellow to yellow-green tourmaline was analyzed from granitic pegmatites in Zambia, Madagascar, Russia, Mozambique, the United States, Nepal, and the Democratic Republic of the Congo. Yellow tourmaline is predominantly Mn-rich (> 3 wt.% MnO) elbaite and “fluor-elbaite”. Though rare, yellow rossmanite and “fluor-rossmanite” from Mozambique also occur. Overall, the MnO content ranges from 3.09 to 8.90 wt.%. The FeO contents are less than 1 wt.%, and most are very low (< 0.09 wt.% FeO). Relative to typical pale-colored tourmaline, all have elevated TiO2 (average 0.28 wt.%). The absence of other chromophoric elements suggests that the yellow to yellow-green coloration is due to a Mn2+–Ti4+ intervalence charge-transfer interaction, which is consistent with earlier spectroscopic studies. Increasing Fe content progressively darkens yellow tourmaline toward hues of green, blue and black; increasing Ti results in a brownish color. Yellow tourmaline is a rare late-stage magmatic to miarolitic-cavity-stage mineral that requires very specific conditions to form. Its presence implies that the original pegmatite-forming melt must be relatively low in Fe and enriched in Mn and B. During an early stage of crystallization, Fe must be removed, but abundant B, Mn and traces of Ti must still be available during the late stages of pegmatite formation to crystallize yellow elbaite, “fluor-elbaite”, rossmanite or “fluor-rossmanite”.

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