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

Abstract

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 11 B and 27 Al 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, "fluor-elbaite," liddicoatite, dravite, uvite, olenite, and magnesiofoitite) from different localities worldwide. Chemical shifts of [4] B and [3] B in 11 B spectra, and [4] Al and [6] Al in 27 Al 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 27 Al 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 Y Al and Z Al precludes resolution of these two bands. Simulation of 11 B 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. 27 Al 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 27 Al tetrahedral peak is sensitive to local order at the adjacent Y and Z octahedra, where [4] Al-Y Mg 3 and [4] Al-Y (Al,Li) 3 arrangements result in peaks located at ~65 and ~75 ppm, respectively. Both 11 B MAS NMR and 27 Al MAS NMR spectra show peak broadening as a function of transition-metal content (i.e., Mn 2+ + Fe 2+ = 0.01-0.30 apfu) in the host tourmaline. In 11 B 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 = Si 6 apfu, are common in natural tourmalines.

Si-deficient foitite with [4]Al and [4]B from the ‘Ługi-1’ borehole, southwestern Poland

“…Ertl and Hughes (2002) even schorls, and with a high olenite component can contain significant [4] B. This was corroborated by MAS NMR investigations of magnesiofoitite (Lussier et al 2009), which did not show [4] B. However, Marschall et al (2004), based on structural investigations of a metamorphic dravite from Syros, Greece, concluded that 'substantial contents of Mg at the Y site or at the Z site do not exclude the possibility of the occurrence of [4] B'.…”

“…Consequently, a structural study with the use of the Rietveld method could not be done. Magic-anglespinning nuclear magnetic resonance (MAS-NMR) spectroscopy, commonly applied for detecting [4] B and [4] Al in tourmaline (Tagg et al 1999;Marler and Ertl 2002;Lussier et al 2009), also cannot be applied due to the very high presence of paramagnetic Fe ions. Lussier et al (2009) did show that a high content of paramagnetic ions results in a dramatic decrease of intensity, and the recorded spectra are illegible.…”

“…Magic-anglespinning nuclear magnetic resonance (MAS-NMR) spectroscopy, commonly applied for detecting [4] B and [4] Al in tourmaline (Tagg et al 1999;Marler and Ertl 2002;Lussier et al 2009), also cannot be applied due to the very high presence of paramagnetic Fe ions. Lussier et al (2009) did show that a high content of paramagnetic ions results in a dramatic decrease of intensity, and the recorded spectra are illegible. Ertl et al (2008), studying Al-tourmaline with [4] B, corrected a co-variation between the cell volume of [4] B-bearing tourmalines and [4] B contents in their structures, which was previously proposed by Wodara and Schreyer (2001) and , allowing for estimation of the [4] B content from X-ray powder diffraction data.…”

Distribution of Mn in pink elbaitic tourmaline from Mogok, Myanmar

“…Since a substantial amount of B-at the T-site ( [4] B) was determined through the structural refinement and chemical analysis of olenite (Ertl et al, 1997;Hughes et al, 2000), 11 B magic-angle-spinning nuclear magnetic resonance spectroscopy has been applied frequently to estimate the fraction of [4] B at the T-site in the 'Mashuroom' tourmaline sample from Myanmar (e.g., Ertl et al, 2007;Lussier et al, 2008aLussier et al, , 2008bLussier et al, , 2009Lussier et al, , 2011. Therefore, it may be interesting to discuss the site population of the T-site by using the information on occupation parameters, unit formulae calculated from chemical analysis, and mean bond length.…”

The crystal-chemical autopsy of octahedral sites in Na-dominant tourmalines: octahedral metrics model unconstrained by the Y,Z-site disorder assignment

“…The tetrahedron can also be occupied by B (e.g. Ertl et al 2002a, 2006, 2007, 2015Hughes et al 2004;Lussier et al 2008Lussier et al , 2009Kutzschbach et al 2016Kutzschbach et al , 2017.…”