The potentialities of powder-diffraction from neutron scattering in the crystal-chemistry of amphiboles

Neutron powder diffraction is nowadays a widely used technique in solid-state chemistry and physics. The low attenuation of neutrons and strong scattering power even at high “Q”, allow the collection of diffraction data from a wide range of reciprocal space, also under non-ambient conditions. The coverage of a wide range of scattering vectors, resulting from the very short neutron scattering lengths [about 10-15m vs 10-10m for X-rays] determines an insignificant variations of scattering amplitude with scattering vector Q (i.e. Bragg angle). The capability of neutrons to discriminate between iso-electronic or quasi-iso-electronic ions in crystal structures has been exploited in an increasing number of studies of solid-solutions in rock-forming minerals. In addition, the long scattering length of hydrogen (or deuterium) makes neutron diffraction the method of choice to locate proton sites and to refine their thermal displacement parameters. Rietveld structure refinements of neutron diffraction data leads to an easier separation of the information on thermal motion from that of site occupancy and a more accurate location of atoms in the crystal cell[1]. On the other hand, neutron powder diffraction techniques require a relatively large amount of material (hundreds of mg). We recently utilised the potentiality of neutron powder diffraction on amphiboles by studying a. ANaB(NaMg)CMg5Si8O22(OH,D)2 amphibole, hydrothermally synthesized at 850 °C and 0.3 GPa[2]. Neutron Time-of-Flight powder-diffraction data were collected at the ROTAX ToF diffractometer of the pulsed spallation source ISIS, U.K. The instrument uses a “white” beam with neutron wavelengths between 0.7-5.1 Å. The primary flight path is 14.0 m. The amphibole powder sample was packed into an 8 mm diameter V-can. Diffraction patterns were collected at 297 and 8 K using three stationary detector banks covering the 2θ ranges of 12-45°, 57-87°, and 100-143°, respectively. The ToF technique actually allows each detector to collect the entire diffraction pattern, hence redundancy (in T and θ) ensures a high precision of the measurements. Rietveld structure refinements of the patterns from all three detector banks were carried out simultaneously using GSAS[3]; the starting model was that obtained at room T by Cámara et al. (2003)[4]. Scale factor, background, cell parameters, and peak-profile (with a double exponential pseudo-Voigt function) were refined first. Then the atom parameters (positions, occupancies, and thermal displacements) were refined. All atoms were refined isotropically; thermal parameters were refined by grouping them on the basis of their environment and constraining the same shift within each group. The C- and T-sites were considered completely occupied by Mg and Si, respectively. In contrast, Na and Mg at the B sites were refined starting with values derived from the EPMA (and IR data). ANa and BNa were constrained to be equal during the refinement. The populations of hydrogen and deuterium at the H1 and H2 sites were also refined. At the end of the refinement, the shifts in all parameters were less than their standard deviations. The space group of the amphibole is P21/m at both temperatures, as confirmed by the presence of b-type reflections (h + k = 2n + 1). The unit-cell volumes at room T and at 8 K are 896.78(2) and 890.80(2)Å3, respectively, with a relative reduction of less than 1%. Accurate structural positions for the hydrogen atoms were obtained from the diffraction data. The O5A-O6A-O5A and O5B-O6B-O5B angles, diagnostic of the A- and B-chains kinking along the c-axis, are 190.0° and 159.2° at 293 K and 193.8° and 156.8° at 8 K, respectively. The orientation and magnitude of the thermo-elastic strain ellipsoid was calculated. A comparison between the low-temperature data reported here and the high-temperature data for a similar amphibole composition, reported by Cámara et al. (2003) up to 643 K, is discussed. The excellent agreement between the structural model refined at room-conditions by neutron powder diffraction and single-crystal X-ray diffraction (Cámara et al. 2003), respectively, confirms the reliability of the neutron powder diffraction even with complex structures with low symmetry. [1] Rinaldi, R. (2002) European Journal of Mineralogy, 14,195-202. [2] Iezzi, G., Gatta, D.G., Kockelmann, W.A., Della Ventura, G. Rinaldi, R., Schäfer, W., Piccinini, M., Galliard, F. (2005). American Mineralogist, 90, 695-700. [3]Larson, A.C., Von Dreele, R.B. (2001) GSAS: General Structure Analysis System. Document LAUR 86-748, Los Alamos National Laboratory, NM, USA. [4] Cámara, F., Oberti, R., Iezzi, G., Della Ventura, G. (2003) Physics and Chemistry of Minerals, 30, 570-581.