Crystal chemistry and trace-element behaviour in tourmalines from Minas Gerais, Brazil

A series of 17 museum tourmaline samples from several pegmatites of Minas Gerais, Brazil, were investigated by several techniques including single-crystal X-ray structure refinements, electron microprobe, and laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOF-MS). Chemically, most samples are Na-dominant on the X site and therefore belong to the alkali group. They correspond to fluor-elbaites or elbaites, where similar proportions of Al and Li share the Y positions. Elbaites are located close to the elbaitic end-member, with Fe2+ strongly depleted and the schorlitic component negligible, while fluor-elbaites tend to align along the fluor-elbaite to fluor-schorl solid solution. One sample, with vacancies dominant on the X site, is identified as a rossmanite close to the foitite compositional field. Besides the classical schorlitic substitution, several other mechanisms were observed, evolving towards rossmanite (XNa++0.5YLi+=X□+ 0.5 YAl3+), towards liddicoatite (XNa++0.5YAl3+=XCa2++0.5YLi+), or towards oxy-foitite (0.5 XNa++YFe2++0.5W(OH)-=0.5X□+YAl3++0.5WO2−). Single-crystal data indicate a unit-cell parameters varying between 15.8187(3) and 15.9309(3) Å and c parameters ranging from 7.0924(2) to 7.1279(2) Å. These values are similar to those obtained for Brazilian tourmalines of the elbaite–schorl solid solution. A detailed cation distribution has also been established, indicating that the B site is fully occupied by boron, that the T site is mainly occupied by Si and sometimes by minor amounts of B, and that the Z site is mainly occupied by Al with sometimes minor amounts of Fe3+. The X site contains vacancies, Na, K, and Ca, and the Y site is occupied by Li, Al, and Fe2+ and minor amounts of Mn2+, Ti, Mg, and Zn. In two samples, a significant disorder between Li and Ca on the X and Y sites has been observed. The position of our samples in a diagram correlating the fluorine and the Fe2+ contents of tourmalines indicates that they are below the trend admitted for primary pegmatitic tourmalines. This feature can be explained by their occurrence in miarolitic cavities, where fluid circulations may affect the tourmaline composition. Finally, the trace-element contents, including rare earth elements (REEs), are discussed in detail and appear to be influenced by both the geochemical pegmatitic context and crystal–chemical constraints.