Browsing by Author "Gatta, GD"

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    A crystal chemical investigation of armenite, BaCa2Al6Si9O30·H2O: the behavior of extra framework Ca cations and H2O molecules in microporous silicates
    (European Geosciences Union, 2012-04-22) Geiger, CA; Gatta, GD; Xue, X; McIntyre, GJ
    The crystal chemistry of armenite, ideally BaCa2Al6Si9O30·H2O, from Wasenalp, Valais, Switzerland was studied. Armenite typically forms in relatively low-temperature hydrothermal veins and fissures and has small pores containing Ca cations and H2O molecules as extra-framework species. Single-crystal neutron and X-ray diffraction measurements were made on armenite from Wasenalp for the first time. IR powder spectroscopic measurements were made from room temperature (RT) down to 10 K. 1H and 29Si NMR measurements were made at RT. Attention was given to investigating the behavior of the extra-framework species and hydrogen bonding. The neutron results also give the first static description of the protons, allowing bond distances and angles relating to the H2O molecules and H-bonds to de determined. The diffraction results indicate complete Al-Si order in the framework and four crystallographically independent Ca and H2O molecule sites. Both sites appear to have partial occupancies such that locally a Ca atom can have only a single H2O molecule bonded to it through an ion-dipole interaction. The Ca cation is further bonded to six O atoms of the framework forming a quasi cluster. The IR spectrum of armenite is characterized in the OH-stretching region at RT by two broad bands at roughly 3470 and 3419 cm−1 and by a single H2O bending mode at 1654 cm−1 and four intense OH bands at 10 K. The 1H MAS spectra contain a single main resonance near 5.3 ppm and a smaller one near 2.7 ppm. The extra-framework “Ca-oxygen-anion-H2O-molecule quasi-clusters” and the nature of H-bonding in the microporous zeolites scolecite, wairakite and epistilbite were also analyzed. The average OH stretching wavenumbers shown by the IR spectra of armenite and scolecite are, for example, not far removed from that observed in liquid H2O, but greater than that of ice. What remains poorly understood in microporous silicates is how the ion-dipole interaction in quasi clusters affects H-bonding strength between the H2O molecules and the aluminosilicate framework. © Author(s) 2012
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    Crystal chemistry of boron-containing minerals by single-crystal neutron diffraction
    (Australian Institute of Nuclear Science and Engineering, 2012-11-15) Gatta, GD; McIntyre, GJ
    Many natural and highly stable minerals contain a significantly high amount of boron which makes their synthetic counterparts of interest as potential neutron absorbers or hosts for storage of nuclear waste. Examples include londonite, with ideal formula (Cs,K)Al4Be4(B,Be)12O28, and hambergite, with formula Be2BO3(OH)0.96F0.04. Single-crystal neutron diffraction is well suited to determining the B/Be distribution and the presence or location of H atoms in these minerals but the high neutron attenuation of B requires very short neutron wavelengths and/or crystal volumes that are considerably smaller than those traditionally desired for neutron diffraction. Here we describe the principal novel neutron diffraction techniques that we used to determine the complete crystal structures of londonite and hambergite. For londonite, neutron Laue diffraction on OrientExpress at the Institut Laue-Langevin (ILL), Grenoble, was an efficient way to select a good quality small crystal for further study on the monochromatic diffractometer D9, also at the ILL, at the short wavelength of 0.7058 A from the hot neutron source. For hambergite, neutron Laue diffraction using KOALA at the OPAL reactor at ANSTO, yielded excellent diffraction data within 36 hours from two small crystals, 2 mm3 in volume.
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    First accurate location of two proton sites in tourmaline: a single-crystal neutron diffraction study of oxy-dravite
    (Mineralogical Society, 2014-06-03) Gatta, GD; Bosi, F; McIntyre, GJ; Skogby, H
    A single-crystal neutron diffraction study of oxy-dravite from Osarara (Narok district, Kenya) was performed. Intensity data were collected in Laue geometry at 10 K and anisotropic-structure refinement was undertaken. For the first time, two independent H sites were refined unambiguously for a mineral belonging to the tourmaline supergroup and located at 0.26, 0.13, 0.38 (labelled as H3, site occupancy ~98%) and at 0, 0, 0.9 (labelled as H1, site occupancy ~25%). The H-bonding scheme can thus be defined as follows: (1) the O at the O3 site acts as a ‘donor’ and the O at the O5 site as ‘acceptor’, the refined O3–H3 bond distance is 0.972(2) Å (and 0.9946 Å corrected for “riding motion”), H3⋯O5 = 2.263(2) Å, O3⋯O5 = 3.179(1) Å and O3–H3⋯O5 = 156.6(1)°; (2) the oxygen at the O1 site acts as a ‘donor’ and the O atoms at O4 and O5 as ‘acceptors’, the refined O1–H1 bond distance is 0.958(8) Å (and 0.9833 Å corrected for “riding motion”), H1⋯O4 = 2.858(6) Å, O1⋯O4 = 3.378(1) Å and O1–H1⋯O4 = 115.12(1)°, whereas H1⋯O5 = 2.886(6) Å, O1⋯O5 = 3.444(1) Å and O1–H1⋯O5 = 118.23(1)°. A further test refinement was performed with the H1 site out of the three-fold axis (at 0.02, 0.01, 0.90); this leads to O1–H1 = 0.995(8) Å (and 1.0112 Å corrected for “riding motion”), H1⋯O4 = 2.747(6) Å and O1–H1⋯O4 = 121.7(4)°, whereas H1⋯O5 = 2.654(9) Å and O1–H1⋯O5 = 136.5(6)°. Bond-valence analysis shows that the H-bonding strength involving O3 is stronger than that involving O1: ~0.11 and <0.05 valence units, respectively. The refined angle between the O3–H3 vector and [0001] is 3.40(9)°. Such a small angle is in line with a pleochroic scheme for the OH-stretching absorption bands measured by infrared spectroscopy. © The Mineralogical Society of Great Britain & Ireland.
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    Hydrogen-bond and cation partitioning in muscovite: a single-crystal neutron-diffraction study at 295 and 20 K
    (Mineralogical Society of America, 2011-01-01) Gatta, GD; McIntyre, GJ; Sassi, R; Rotiroti, N; Pavese, A
    The crystal chemistry of a pegmatitic Fe-bearing muscovite (with FeO similar to 5.1 wt%) from Val di Crana, Arvogno (Verbania, Nothern Italy) has been investigated by means of wavelength-dispersive X-ray spectroscopy and constant-wavelength [lambda = 0.9462(2) angstrom] single-crystal neutron diffraction at 295 and 20 K (2 theta(max) = 90 degrees). The structure refinement at 295 K shows no significant deviation from the full occupancy of the K site (using the scattering length of potassium alone), and a disordered Si/Al-distribution in the two independent tetrahedral sites (i.e., T1 and T2) and Fe/Al-distribution in the M2 octahedral site. The difference Fourier map of the nuclear density shows that only one independent H site occurs in the muscovite structure, at both room and low temperature. No evidence of disorder in two sub-sites, as observed in a previous study, was found. The thermal displacement of the H site is here described anisotropically. A trifurcated hydrogen bonding scheme was found: the O6-H bond distance corrected for "riding motion" is similar to 0.984 angstrom at 295 K, and three significantly weak hydrogen bonds to the O atoms O2, O4, and O6 occur, with H center dot center dot center dot O2 = 2.635(5), H center dot center dot center dot O4 = 2.657(4), and H center dot center dot center dot O5 = 2.647(4) angstrom and O6-H center dot center dot center dot O angles all similar in value (similar to 138 degrees). The structure refinements show that the structure configuration of muscovite is maintained at least down to 20 K: the cation disordering in the tetrahedral and octahedral site and the H-bonding scheme are preserved. The structural evolution induced by decreasing temperature is governed by the contraction along a direction perpendicular to (001), mainly due to the compression of the "inner" K-O bonds, which leads to an increase of the tetrahedral rotation angle (alpha) of the six-membered ring. A further effect in response to lowering the temperature is a significant reduction of the magnitude of the thermal displacement parameters. © 2011, Mineralogical Society of America
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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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    A neutron/x-ray diffraction, IR, and 1H/29Si NMR Spectroscopic investigation of armenite: behavior of extra framework Ca cations and H2O molecules in microporous silicates
    (European Mineralogical Union, 2012-09-02) Geiger, CA; Gatta, GD; Xue, X; McIntyre, GJ
    The crystal chemistry of armenite, ideally BaCa2Al6Si9O30·2H2O, from Wasenalp, Valais, Switzerland was stud ied. Armenite typically forms in relatively low-temperature hydrothermal veins and fissures and has small pores containing Ca cations and H2O molecules as extra-framework species. Single-crystal neutron and X-ray diffraction measurements were made on armenite from the above locality for the first time. IR powder spectroscopic measure ments were made from room temperature (RT) down to 10 K. 1H and 29Si NMR measurements were made at RT. Attention was given to investigating the behavior of the extra-framework species and hydrogen bonding. The diffraction results show new features not observed before in published diffraction studies on armenite crystals from other localities. The neutron results also give the first static description of the protons, allowing bond distances and angles relating to the H2O molecules and H-bonds to be determined. The diffraction results indicate Al/Si order in the framework. Four crystallographically independent Ca and H2O molecule sites were refined, whereby both sites appear to have partial occupancies such that locally a Ca atom can have only a single H2O molecule bonded to it through an ion-dipole interaction. The Ca cation is further bonded to six O atoms of the framework forming a quasi cluster around it. The IR spectrum of armenite is characterized in the OH-stretching region at RT by two broad bands at roughly 3470 and 3410 cm−1 and by a single H2O bending mode at 1654 cm−1 . At 10 K four intense OH bands are located at 3479, 3454, 3401 and 3384 cm−1 and two H2O bending modes at 1650 and 1606 cm−1. The 29Si MAS NMR spectra show four resonances at -81.9, -83.2, -94.9 and -101.8 ppm that are assigned to crystallographically different Si sites in an ordered structure, although their relative intensities deviate somewhat from those predicted for complete Al/Si order. The 1H MAS spectra contain a single main resonance near 5.3 ppm and a smaller one near 2.7 ppm, which can be assigned to H2O molecules bonded to Ca and a second H2O type lo cated in a partially occupied site, respectively. Bonding for the extra-framework “Ca-oxygen-anion-H2O-molecule quasi-clusters” and also the nature of H-bonding in the microporous zeolites scolecite, wairakite and epistilbite are analyzed. The average OH stretching wavenumbers shown by the IR spectra of armenite and scolecite are, for example, not far removed from that observed in liquid H2O, but greater than that of ice. What remains poorly understood in microporous silicates is how the ion-dipole interaction in quasi clusters affects H-bonding strength between the H2O molecules and the aluminosilicate framework. © Author(s) 2012
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    A neutron/x-ray diffraction, IR, and 1H/29Si NMR spectroscopic investigation of armenite: behavior of extra framework Ca cations and H2O molecules in microporous silicates
    (Oldenbourg Wissenschaftsverlag, 2012-04-18) Geiger, CA; Gatta, GD; Xue, X; McIntyre, GJ
    The crystal chemistry of armenite, ideally BaCa2Al6Si9O30 2H2O, from Wasenalp, Valais, Switzer-land was studied. Armenite typically forms in relatively low-temperature hydrothermal veins and fissures and has small pores containing Ca cations and H2O molecules as extra-framework species. Single-crystal neutron and X-ray diffraction measurements were made on armenite from the above locality for the first time. IR powder spectroscopic measurements were made from room temperature (RT) down to 10 K.1H and29Si NMR measurements were made at RT. Attention was given to investigating the behavior of the extra-framework species and hydrogen bonding. The diffraction results show new features not observed before in published diffraction studies on armenite crystals from other localities. The neutron results also give the first static description of the protons, allowing bond distances and angles relating to the H2O molecules and H-bonds to be determined. The diffraction results indicate Al/Si order in the framework. Four crystallographically in-dependent Ca and H2O molecule sites were refined, whereby both sites appear to have partial occupancies such that locally a Ca atom can have only a single H2Omolecule bonded to it through an ion-dipole interaction. The Ca cation is further bonded to six O atoms of the framework forming a quasi cluster around it. The IR spectrum of armenite is characterized in the OH-stretching region at RT by two broad bands at roughly 3470 and3410 cm 1and by a single H2O bending mode at1654 cm 1. At 10 K four intense OH bands are located at3479, 3454, 3401 and 3384 cm 1and two H2O bending modes at 1650 and 1606 cm 1. The29Si MAS NMR spectra show four resonances at 81.9, 83.2, 94.9 and 101.8 ppm that are assigned to crystallographically different Si sites in an ordered structure, although their relative intensities deviate somewhat from those predicted for complete Al/Si order. The1H MAS spectra contain a single main resonance near 5.3 ppm and a smaller one near2.7 ppm, which can be assigned to H2O molecules bondedto Ca and a second H2O type located in a partially occupied site, respectively. Bonding for the extra-framework“Ca-oxygen-anion-H2O-molecule quasi-clusters” and als the nature of H-bonding in the microporous zeolites scolecite, wairakite and epistilbite are analyzed. The average OH stretching wavenumbers shown by the IR spectra of armenite and scolecite are, for example, not far removed from that observed in liquid H2O, but greater than that of ice. What remains poorly understood in microporous silicates is how the ion-dipole interaction in quasi clusters affects H-bonding strength between the H2O molecules and the aluminosilicate framework. © Oldenbourg Wissenschaftsverlag
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    Order of [6]Ti4+ in a Ti-rich calcium amphibole from Kaersut, Greenland: a combined x-ray and neutron diffraction study
    (Springer Nature, 2016-10-01) Gatta, GD; McIntyre, GJ; Oberti, R; Hawthorne, FC
    In order to characterize the role of Ti in the crystal structure of calcium amphiboles with high or even dominant oxo-component, the crystal structure of a Ti-rich calcium amphibole from a gabbro at Kaersut, Greenland, has been refined with single-crystal MoKα X-ray intensity data to an R 1(F) index of ~0.025, and with single-crystal Laue neutron intensity data to an R 1(F) index of ~0.053. The crystal used for X-ray structure refinement was characterized by electron- and ion-microprobe analysis. The site populations of the C-group cations Mg, Fe and Ti were calculated from the refined site-scattering values for the M(1), M(2) and M(3) sites derived by both X-ray and neutron diffraction. Ti is distributed among all the three sixfold coordinated M sites, with a strong preference for the M(1) and M(3) sites, where its main role is maintaining electroneutrality at the deprotonated O(3) site. The pattern of distortion of the M(1), M(2) and M(3) octahedra differs from that in F-free deprotonated or partly deprotonated amphiboles, where Ti4+ does not occur at the M(3) site. The neutron structure refinement provides also a clear picture of the environment of the proton, anisotropic displacement behaviour and potential hydrogen-bonding arrangements. A trifurcated hydrogen-bonding configuration has been identified, with two O(6) and one O(7) oxygen atoms as acceptors of weak hydrogen-bonds. © 2016, Springer-Verlag Berlin Heidelberg
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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.
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    A single-crystal neutron diffraction study of hambergite, Be2BO3(OH,F)
    (De Gruyter, 2012-12-01) Gatta, GD; McIntyre, GJ; Bromiley, G; Guastoni, A; Nestola, F
    The crystal chemistry and crystal structure of hambergite from the Anjanabonoina mine, Madagascar [Be2BO3(OH)(0.96)F-0.04, Z = 8, a = 9.762(2), b = 12.201(2), c = 4.430(1) angstrom, V = 527.6(2) angstrom(3), space group Pbca], were reinvestigated by means of electron microprobe analysis in wavelength-dispersive mode, secondary-ion mass spectrometry, single-crystal X-ray and neutron Laue diffraction. Chemical analyses show only a small amount of F (0.7-0.8 wt%, approximately 0.04 atoms per formula unit) substituting OH and no other substituent at a significant level. An anisotropic neutron structural refinement has been performed with final agreement index R-1 = 0.0504 for 76 refined parameters and 1430 unique reflections with F-o>4 sigma(F-o). The geometry of the hydroxyl group and hydrogen bonding in hambergite is now well defined: (1) only one independent H site was located and the O4-H distance, corrected for "riding motion," is similar to 0.9929 angstrom; (2) only one hydrogen bond appears to be energetically favorable, with a symmetry-related O4 as acceptor and with O4 center dot center dot center dot O4 = 2.904(1) angstrom, H center dot center dot center dot O4 = 1.983(1) angstrom, and O4-H center dot center dot center dot O4 = 157.5(1)degrees. In other words, O4 sites act both as donor and as acceptor of the hydrogen bond, with a zigzag chain of H-bonds along [001]. The hydrogen-bonding scheme in hambergite found in this study is consistent with the pleochroic scheme of the infrared spectra previously reported, with two intensive modes ascribable to stretching vibrations of the hydroxyl group, at 3415 and 3520 cm(-1), respectively. The two modes suggest at least two distinct hydrogen-bonding environments, ascribable to the presence of oxygen and fluorine at the acceptor site. © 2012, Mineralogical Society of America
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    Static positional disorder in ulvöspinel: a single-crystal neutron diffraction study
    (GeoScience World, 2014-02-12) Gatta, GD; Bosi, F; McIntyre, GJ; Hålenius, U
    A single-crystal neutron diffraction study of a synthetic ulvöspinel sample of composition Fe3+0.40Fe2+1.80Ti0.80O4 was performed to investigate the static positional disorder at the octahedrally coordinated M site. Anisotropic structural refinement was performed in the space group Fd3̄m against neutron Laue diffraction data collected at 298 K from two millimetric-sized crystals. Initial structure refinements were conducted with Fe and Ti sharing the M site (at 1/2, 1/2, 1/2), and their partial site occupancy was refined. The tetrahedrally coordinated T site (at 1/8, 1/8, 1/8) was modeled as fully occupied by Fe. For both crystals, the final R1 index was about 3% for 9 refined parameters and 129 unitetrahedrallyque reflections, with no significant residuals. As the atomic displacement factors obtained were anomalously high, according to the previous experimental findings, Fobs- and (Fobs–Fcal)-difference Fourier maps of the nuclear density were generated. Fourier maps showed a significant minimum located out-of-center of the M site, and indicating a displacement of the Ti4+ from the center of the octahedron. A further test refinement was successfully conducted with two mutually exclusive sites: MTi out-of-center (at 0.49, 0.49, 0.49) and MFe on the center (at 1/2, 1/2, 1/2). The resulting displacement of Ti from the octahedral center appears to be shorter than 0.15 Å. Using bond-valence theory, the out-of-center distortion of MTi4+ is interpreted as a result of intrinsic distortions in the ulvöspinel structure. The potential implication of the octahedral distortion on the behavior of ulvöspinel at non-ambient conditions is discussed.© 2014, Mineralogical Society of America.