Browsing by Author "Vignola, P"

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    Single-crystal neutron diffraction and Mössbauer spectroscopic study of hureaulite, (Mn,Fe)5(PO4)2 (HPO4)2 (H2O)4
    (Schweizerbart Science Publishers, 2016-03-23) Gatta, DG; Redhammer, GJ; Vignola, P; Meven, M; McIntyre, GJ
    The crystal chemistry of hureaulite from the Jocão (Cigana) pegmatite, Conselheiro Pena, Doce Valley, Minas Gerais (Brazil), was investigated by electron microprobe analysis in wavelength-dispersive mode, single-crystal Laue (at 293 K) and monochromatic neutron diffraction (at 2.3 K), and57 Fe-Mössbauer spectroscopy [ M(1), M(2), M(3) Mn2+ 3.61Fe2+ 1.21Ca 0.11 Mg0.03) σ = 4.96( P(2) PO4)2 (H1.04 P(1) PO4)2 (H2O)3.92, Z = 4, a = 17.603(6), b = 9.087(2), c = 9.404(4) Å, β = 96.66(4)°, and V = 1494.1(9) Å 3 at 293 K, space group C2/c]. The neutron refinements confirm the general structure model previously reported, showing that five independent H sites (with full site occupancy) occur in the hureaulite structure, one of them as member of the unique hydroxyl group (i.e., O(1)- H(1)) and the other four belonging to two independent H2 O molecules (i.e., H(2)-O(9)-H(3) and H(4)-O(10)-H(5)). The hydroxyl group is the vertex of one of the two independent P-tetrahedra (i.e., HOPO3), whereas the two H2O molecules are the vertices of the (Mn, Fe)-octahedra. No zeolitic H2 O occurs in hureaulite structure. The complex hydrogen bonding scheme in hureaulite is now well defined, with five hydrogen bonds energetically favorable. The element distribution among the octahedral sites, deduced on the basis of the neutron structure refinements, shows that the highest fraction of Fe populates the M(2) site ( 36%), whereas lower fractions are observed at M(1) and M(3) (i.e., 13–15 and 11–12%, respectively). The 57 Fe Mössbauer spectrum of hureaulite confirms the absence of ferric iron in the sample. The spectrum shows four well-separated absorption lines which are best fitted with three doublets. The well-resolved doublet (with the largest quadrupole splitting) is here assigned to the M(3) site, in line with the pronounced preference of Mn2+ for the M(3) site as shown by the neutron structure refinements. The two innermost doublets have almost identical isomer shifts, though with slightly different quadrupole splitting values; the smaller quadrupole splitting value is assigned to the M(2) site, which is expected to have a slightly lower polyhedral distortion than the M(1) site, according to the experimental findings based on the neutron refinements. © Schweizerbart Science Publishers
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    Single-crystal neutron diffraction and Raman spectroscopic study of hydroxylherderite, CaBePO4(OH,F)
    (Cambridge University Press, 2014-07-05) Gatta, GD; Jacobsen, SD; Vignola, P; McIntyre, GJ; Guastella, G; Abate, LF
    The crystal structure, H bonding and chemical composition of hydroxylherderite from the Bennett pegmatite, Buckfield, Oxford County, Maine, USA were investigated by single-crystal X-ray diffraction and neutron Laue diffraction, electron microprobe analysis in wavelength-dispersive mode, inductively coupled plasma-atomic emission spectrometry and polarized Raman spectroscopy [ Ca (Na0.01Ca1.01)Σ1.02 Be (Be0.98Li0.01)Σ0.99 P (Si0.03P0.98)Σ1.01O4(OH0.67F0.33)Σ1, Z = 4, a = 9.7856(5), b = 7.6607(5), c = 4.8025(3) Å, b = 90.02(3)°, V= 360.02(4) Å3, space group P21/a]. The neutron-structure refinement converged with fully anisotropic displacement parameters to give a final agreement index R 1 = 0.0363 for 85 refined parameters and 1614 unique reflections with F o >4σ(F o). The structure refinement was used to determine the H position and geometry of H bonding in the structure. One H site was found on the O5 anion with an O–H interatomic distance, corrected for “riding motion”, of 0.996(2) Å. The H bond of hydroxylherderite is bifurcated with O2 and O4 acceptors forming H bonds with O5···O2 = 3.163(1) Å, H···O2 = 2.544(2) Å and O5–H···O2 = 121.8(1)°; O5···O4 = 3.081(1) Å, H···O4 = 2.428(2) Å and O5–H···O4 = 124.4(1)°. The highly non-linear O–H···O hydrogen bonds in hydroxylherderite and in the isotypic datolite [ideally CaBSiO4(OH)] are constrained by the tetrahedral network topology. Two main O–H stretching modes were observed in the Raman spectra at 3565 and 3620 cm–1, which are attributed to the bifurcated H bond. Two additional weak bands at 3575 and 3610 cm–1 are attributed to Si–P disorder in the tetrahedral sites. Results of this study will contribute to the correlation of H-bonding geometry and O–H stretching frequencies of highly non-linear H bonds.© The Mineralogical Society of Great Britain & Ireland.