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

R. James Evans; Colin A. Fyfe; Lee A. Groat; Anita E. Lam

doi:10.2138/am.2012.3880

Abstract

Three samples from the dumortierite group of minerals were examined with magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR): a dumortierite [c. (Al,⃞)Al₆(BO₃) Si₃O₁₃(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)Al₆(BO₃)(Si,Sb,As)₃O₁₂(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 ²⁹Si 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., Q⁴ Si sites), and assigning the three remaining peaks to Si sites adjacent to vacant Al1 sites (i.e., Q³ 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 Q⁴ 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) Å³ with R₁ = 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 Al³⁺-Al³⁺ distance. Each sub-site is approximately ¼ occupied, suggesting that Al³⁺ 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.

MAS NMR measurements and ab initio calculations of the²⁹Si chemical shifts in dumortierite and holtite

Abstract

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