Abstract
Copper- and Mn-bearing elbaitic tourmaline (“Paraíba tourmaline”) sometimes contains significant amounts of Pb and Bi. Their position in the tourmaline crystal structure was studied with correlation analysis and bond valence calculations. Correlations between the F content and the X-site charge allow predicting the X-site occupancy. Three sets of tourmaline analyses were studied: (1) Pb-rich tourmalines from the Minh Tien pegmatite, Vietnam; (2) Cu-, Pb- and Bi-bearing tourmalines from the Mulungu mine, Brazil; (3) Cu- and Bi-bearing tourmalines from the Alto dos Quintos mine, Brazil. Two correlations were plotted: (1) the charge by considering only Na1+, Ca2+ and K1+; (2) the charge by adding Pb2+ and Bi3+ to the X-site charge. When plotting correlations for the Minh Tien tourmalines, the correlation significantly improves by adding Pb2+ to the X site. For the Alto dos Quintos tourmalines, only a slight increase of the correlation coefficient is observed, while such a correlation for tourmalines from Mulungu interestingly shows a slight decrease of the correlation coefficient. Bond valence calculations revealed that Bi3+ and Pb2+ can indeed occupy the X site via BiLi(NaAl)−1, PbLi(NaCu)−1 and possibly PbCu(NaAl)−1 substitutions as seen in the investigated tourmaline samples. At the Y site, Pb4+ can be substituted via PbLi(AlCu)−1, and PbVO(AlVOH)−1, while Bi5+ does not have any stable arrangement in Cu-bearing fluor-elbaite. The occurrence of Pb4+ at the Y site could be one explanation for the results of the correlations of the Mulungu tourmalines. Another explanation could be that during the tourmaline crystallization some additional Bi and Pb came into the pegmatitic system and hence disturbed the correlation between the average X-site charge and the F content. Further plots of such correlations in “Paraíba tourmaline” samples might also help to distinguish between the worldwide localities of these rare and sought-after tourmalines.
Highlights
The first crystal structure of a tourmaline sample (Li-bearing olenite; Cu below the detection limit) with significant amounts of Pb (1640 ppm) and Bi (143 ppm) was described from a pegmatitic “mushroom” tourmaline from Myanmar by [24]. They were the first authors who assigned these small amounts of Pb and Bi to the X site, just because no phases other than tourmaline have been observed on their back-scattered electrons (BSE) pictures
Content in Pb-rich tourmaline samples from the Minh Tien pegmatite in Vietnam
Only limited data about the contents of Bi and Pb in “Paraíba tourmaline” (Cu- and Mn-bearing tourmaline) are available, we plotted the data of two localities in the Rio Grande do Norte State, Brazil
Summary
The first crystal structure of a tourmaline sample (Li-bearing olenite; Cu below the detection limit) with significant amounts of Pb (1640 ppm) and Bi (143 ppm) was described from a pegmatitic “mushroom” tourmaline from Myanmar by [24]. F content and the X-site charge of tourmaline samples from one granitic pegmatite was described for the first time by [36] These authors plotted the average X-site charge The average charge of the observed X-site occupants (Na, Ca and K) of different tourmalines (oxy-schorl, fluor-elbaite, elbaite) from this pegmatite were plotted against the F content (R2 ≈ 1; [37]).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
