Browsing by Author "Jacobsen, SD"

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    Minerals in cement chemistry: a single-crystal neutron diffraction and Raman spectroscopic study of thaumasite
    (De Gruyter, 2015-04-02) Gatta, GD; McIntyre, GJ; Swanson, JG; Jacobsen, SD
    Thaumasite, Ca3Si(OH)6(CO3)(SO4)⋅12H2O, is recognized as a secondary-alteration mineral and indicator of sulfate attack in Portland cement in contact with sulfate-rich groundwater, especially in cold regions. The hydrogen positions in thaumasite have been determined from single-crystal neutron diffraction structure refinements at 300 and 22 K. No phase transitions occur within the temperature range investigated. The structure of thaumasite is largely held together by hydrogen bonding. The major structural units [CO3 groups, SO4 tetrahedra, Si(OH)6 octahedra, and Ca(OH)4(H2O)4 polyhedra] are interconnected via 10 distinct hydrogen bonds. Analysis of the difference-Fourier maps of the nuclear density reveals the positions of all 10 hydrogen atoms in the structure, and the hydrogen bonding becomes shorter (stronger) upon decreasing temperature to 22 K. The SO4 tetrahedron expands upon decreasing temperature (i.e., negative thermal expansion at the molecular level), driven by shortening of the hydrogen bonding between [Ca3Si(OH)6(H2O)12]4+ columns. Polarized Raman spectra of thaumasite show that the ν1 symmetric stretching modes of Si(OH)6, SO4, and CO3 occur at 658, 983, and 1066 cm-1, respectively. In addition, the out-of-plane bending mode (ν2) and asymmetric stretching mode (ν3) of the carbonate group are tentatively assigned to bands at 887 and 1400 cm-1, respectively. Bands at 418 and 455 cm-1 (and possibly at 477 cm-1) are attributed to the symmetric bending modes (ν2) of the sulfate group, and we observe a possible asymmetric stretching mode (ν3) of SO4 at 1090-1100 cm-1. Splitting of some sulfate and carbonate vibrational modes may occur due to hydrogen bonding on all the oxygen sites. At 1685-1710 cm-1 we observe the H2O bending modes (H-O-H), and from 2900-3600 cm-1 there are 13 distinct bands associated with bending overtones and the O-H stretching vibrations corresponding to H-positions determined in the neutron diffraction study. The effect of the low-temperature stability of thaumasite on the pronounced “thaumasite sulfate attack” of Portland cements observed in cold regions is discussed. © 2015 by Walter de Gruyter Berlin/Boston
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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.