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The occurrence of tetrahedrally coordinated Al and B in tourmaline: An ¹¹B and ²⁷Al MAS NMR study

¹ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
² Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

Correspondence: ^* E-mail: frank_hawthorne{at}umanitoba.ca

Considerable uncertainty has surrounded the occurrence of tetrahedrally coordinated Al and B at the T site in tourmaline. Although previously detected in several tourmaline specimens, the frequency of these substitutions in nature, as well as the extent to which they occur in the tourmaline structure, is not known. Using ¹¹B and ²⁷Al MAS NMR spectroscopy, we have investigated the presence of B and Al at the T site in 50 inclusion-free tourmaline specimens of low transition-metal content and different species (elbaite, "fluorelbaite," liddicoatite, dravite, uvite, olenite, and magnesiofoitite) from different localities worldwide. Chemical shifts of ^[4]B and ^[3]B in ¹¹B spectra, and ^[4]Al and ^[6]Al in ²⁷Al spectra, are well resolved, allowing detection of even small amounts of T-site constituents. In the observed spectra, ^[4]B and ^[3]B peaks are located at 0 and 18–20 ppm, respectively, with the greatest intensity corresponding to ^[3]B (=3 apfu). In ²⁷Al spectra, ^[4]Al and ^[6]Al bands are located at 68–72 and 0 ppm, respectively, with the greater intensity corresponding to ^[6]Al. However, inadequate separation of ^YAl and ^ZAl precludes resolution of these two bands. Simulation of ¹¹B MAS NMR spectra shows that tetrahedrally and trigonally coordinated B can be readily distinguished at 14.1 T and that a ^[4]B content of 0.0–0.5 apfu is common in tourmaline containing low amounts of paramagnetic species. ²⁷Al MAS NMR spectra show that Al is also a common constituent of the T site in tourmaline. Determination of ^[4]Al content by peak-area integration commonly shows values of 0.0–0.5 apfu. Furthermore, the chemical shift of the ²⁷Al tetrahedral peak is sensitive to local order at the adjacent Y and Z octahedra, where ^[4]Al-^YMg₃ and ^[4]Al-^Y(Al,Li)₃ arrangements result in peaks located at ~65 and ~75 ppm, respectively. Both ¹¹B MAS NMR and ²⁷Al MAS NMR spectra show peak broadening as a function of transition-metal content (i.e., Mn²⁺ + Fe²⁺ = 0.01–0.30 apfu) in the host tourmaline. In ¹¹B spectra, broadening and loss of intensity of the ^[3]B signal ultimately obscures the signal corresponding to ^[4]B, increasing the limit of detection of ^[4]B in tourmaline. Our results clearly show that all combinations of Si, Al, and B: T = (Al, Si)₆, T = (B, Si)₆, T = (Al, B, Si)₆, and T = Si₆ apfu, are common in natural tourmalines.

Key Words: Tourmaline • ¹¹B and ²⁷Al MAS NMR • tetrahedrally coordinated B and Al