MAS NMR measurements and ab initio calculations of the 29Si chemical shifts in dumortierite and holtite

Three samples from the dumortierite group of minerals were examined with magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR): a dumortierite [c. (Al,⃞)Al 6 (BO 3 ) Si 3 O 13 (O,OH) 2 ] consisting of dark blue euhedral crystals from Madagascar (D34); a fine-grained pale blue dumortierite from Island Copper mine, British Columbia, Canada (D12); and a creamy white holtite [c. (Ta,Nb,⃞,Al)Al 6 (BO 3 )(Si,Sb,As) 3 O 12 (O,OH,⃞) 3 ] from Szklary, Lower Silesia, Poland (WPH). Restricted Hartree-Fock ab initio electronic structure calculations were performed on model clusters with the goal of matching local environments of Si atoms to peaks in the 29 Si MAS NMR spectra. The spectrum of D34 showed five resolved peaks at -95.2, -92.6, -91.3, -89.1, and -86.5 ppm with deconvoluted peak area contributions of 57, 19, 7, 10, and 7%. Electronic structure calculations, cross-polarization MAS NMR measurements and relative intensities support assigning the peaks at -95.2 and -92.6 ppm to Si2 and Si1 sites, respectively, adjacent to fully occupied Al1 sites (i.e., Q 4 Si sites), and assigning the three remaining peaks to Si sites adjacent to vacant Al1 sites (i.e., Q 3 Si sites). Due to the complexity of the dumortierite structure, clusters composed of at least the first four shells of nearest neighbor atoms to the target Si atom are necessary to model Q 4 sites. The spectrum of D12 showed two main peaks at -93 and -95 ppm, with minor peaks below -90 ppm and above -100 ppm. The spectrum of WPH showed one broad peak at -93 ppm, likely containing both Si1 and Si2 signals, and two minor peaks below -90 ppm. Single-crystal X-ray diffraction and structure refinement on D34 shows orthorhombic symmetry, Pnma, Z = 4, a = 4.6882(1), b = 11.7924(2), c = 20.1856(3) Å, and V = 1115.97(4) Å 3 with R 1 = 0.0124. Three distinct sub-sites of the face-sharing octahedral chain site Al1 were distinguished corresponding to sites with one vacancy above, with one vacancy below, and between two occupied sites; the vacancy-adjacent sites have the cation displaced to increase the Al 3+ -Al 3+ distance. Each sub-site is approximately ¼ occupied, suggesting that Al 3+ cations in individual face-sharing octahedral chains are ordered as ⃞-Al-Al-Al, although cations from chain to chain are disordered, preserving Pnma symmetry. Powder X-ray diffraction measurements were performed on both D34 and D12. The unit cell of D12 was found to be a = 4.7001(7), b = 11.785(2), c = 20.277(3) Å.

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