Oscillatory zoned liddicoatite from Anjanabonoina, central Madagascar. I. Crystal chemistry and structure by SREF and 11B and 27Al MAS NMR spectroscopy

The crystal structures of 23 samples extracted from a large slice oriented along (001) of a single crystal of liddicoatite from the Anjanabonoina granitic pegmatite in Madagascar (showing pronounced, visually discontinuous oscillatory zones and anomalous biaxiality) were refined to R 1 indices of 1.5–2.9% ( R 1 > = 1.78%). Cell parameters are in the range a ≈ 15.82–15.87, c ≈ 7.10–7.12 A. Spindle-stage measurement of 2 V gave values of 0.0° for a fragment from the (001) zone and 8(3) and 18.9(5)° for fragments from the pyramidal zone of the crystal. However, single-crystal X-ray intensity data show no deviation from 3 m Laue symmetry, indicating that there is no information in the diffraction data on any deviation from R 3 m symmetry. The T –O> distances are in the range 1.616–1.619 A, with a grand mean value of 1.6175(7) A. The occupancy of the T site was expressed as x Si + (1− x )B, and x was treated as variable in the refinement procedure. The effect of using different scattering factors ( i.e ., ionized versus neutral) on the refined site-occupancies was investigated in detail. The grand mean refined [4] B content of the T site varies from −0.04 apfu for ionized scattering-curves for O and Si to 0.25 apfu for neutral scattering-curves for O and Si, illustrating the effect of the use of different scattering curves on refined [4] B site-populations. Examination of T –O> distances as a function of aggregate cation radius for tourmalines containing [4] B and [4] Al shows a large amount of scatter, emphasizing the need for more accurate data. The limits of detection for 11 B and 27 Al in tourmaline by Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy were investigated by simulation. For minimum ( apfu ) and maximum (0.12 apfu ) contents of (paramagnetic) transition metals, the limits of detection of [4] B are~0.02 and 0.08 apfu , and of [4] Al are~0.01 and 0.01 apfu , respectively. 11 B and 27 Al MAS NMR spectroscopy gave no evidence of the presence of tetrahedrally coordinated B or Al at the T site above these detection limits in any sample. This result is in accord with our refinement results using an ionized scattering-curve for O and a neutral scattering-curve for Si, suggesting that use of these curves is giving more accurate results than refinement with neutral scattering factors. The Z –O> distances are in the range 1.904–1.907 A, with a grand mean value of 1.9047(8) A, and structure refinement indicates site-scattering values in accord with complete occupancy of the Z site by Al. Hence throughout this complexly zoned crystal, Si = 6.00 apfu and Z Al = 6.00 apfu .