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

作者：Evans R James; Fyfe Colin A; Groat Lee A^*; Lam Anita E

来源：American Mineralogist, 2012, 97(2-3): 329-340.

DOI：10.2138/am.2012.3880

• Dumortierite
• holtite
• nuclear magnetic resonance
• borosilicates
• ab initio
• single-crystal X-ray diffraction
• powder X-ray diffraction
• electron microprobe

Three samples from the dumortierite group of minerals were examined with magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR): a dumortierite [c. (Al,square)Al-6(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,square,Al)Al-6(BO3)(Si,Sb,As)(3)O-12(O,OH,square)(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 Si-29 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 All sites (i.e., Q(4) Si sites), and assigning the three remaining peaks to Si sites adjacent to vacant All 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 1)12 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) angstrom, and V= 1115.97(4) angstrom(3) with R-1 = 0.0124. Three distinct sub-sites of the face-sharing octahedral chain site All 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 1/4 occupied, suggesting that Al3+ cations in individual face-sharing octahedral chains are ordered as square-Al-Al-Al, although cations from chain to chain are disordered, preserving Prima 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), 13 = 11.785(2), c = 20.277(3) angstrom.

• 出版日期2012-3