MA23B-01 INVITED
Applications of Neutron Powder Diffraction to Mineralogy
Ian P. Swainson Canadian Neutron Beam Centre National Research Council of Canada Stn 18 Chalk River Laboratories Chalk River, ON K0J 1J0 Canada Applications of Neutron Powder Diffraction to Mineralogy The strengths of neutron diffraction when compared to X-ray diffraction are (i) the lack of a form factor for nuclear scattering, so that the decay in the intensity of powder lines with sin θ/λ is solely due to thermal motion, which is particularly useful for pair distribution function studies; (ii) the lack of dependence of the amplitude of atomic scattering with atomic number, which gives rise to strong scattering contrast for some neighbouring elements, particularly in the transition metal series, and also to strong sensitivity to light elements; (iii) the penetrating nature of the radiation, greater than even that of high-energy X-rays; and (iv) the ability to study the spin lattice of magnetically ordered compounds. I will give some examples of the use of these properties on mineralogical and related systems. Neutrons are frequently used to study hydrous salts and minerals as well as ice and clathrates. While deuterated synthetic analogs are often required for compounds with many hydrogen atoms, for materials that have a low degree of hydrogenation this problem can be surmountable. Co-refinement with X-ray data can be helpful. For some compounds the absolute properties (e.g., bulk modulus) are significantly different between D- and H-analogs, while the phase diagrams are broadly similar; in others, phases can be pushed out of existence or have significantly modified phase boundaries. An area that has been of recent mineralogical concern is the symmetrization of hydrogen bonds under pressure for which neutrons can provide a direct measure. Similarly, due to scattering contrast, neutrons have proved to be useful for studying Al-Si and Mn-Mg-Fe partitioning in mineral systems. Due to the penetrating power, simple rocks can sometimes be studied using multiphase refinements and still pull out structural information about single phases. Another area that has received relatively little study is the study of magnetic structures, and the temperature, pressure and composition dependence of those ordered states. There have been recent developments in the codes that deal with magnetic symmetry, such that the symmetry of complex structures such as incommensurate magnets are now frequently describable. The codes are now sufficiently developed that non- experts in symmetry can use them in refinements. I will give some examples where magnetic studies have been performed on mineral structures, and the kind of information that can be retrieved.
MA23B-02 INVITED
Geoscience Applications of the Neutron Diffractometer HIPPO
In this presentation we describe the capabilities for and give examples of geoscience experiments performed
with the neutron time-of-flight diffractometer HIPPO (High Pressure - Preferred Orientation) at the Los Alamos
Neutron Science Center (LANSCE). The large detector coverage of HIPPO allows to measure the orientation
distribution function (ODF) of textured samples from sample rotation around one axis, allowing to measure the
ODF quickly (beam times of 5 to 90 minutes for most samples) at ambient condition. It also allow to measure
the texture at non ambient conditions, allowing to study for instance variant selection in phase transformation
textures. Furthermore, the large detector coverage allows to acquire data of sufficient statistics relatively quickly
(count times of 5 to 60 minutes per sample), allowing for rapid collection of datasets for crystal structure
refinement at ambient conditions. This has been also used for instance to follow the lattice parameter change
in sandstone after a rapid change of the sample temperature to investigate the reason for an observed de-
coupling of the change in elastic modules and temperature. Finally, the large volume of the sample chamber
(~1 m3) allows implementation of various large-scale sample environments, such as a 500 ton toroidal
anvil cell, various gas/liquid pressure cells, and a 100 kN load frame. Planned instrument upgrades for the
next few years will be also presented.
http://lansce.lanl.gov/lujan/instruments/hippo/
MA23B-03 INVITED
High Pressure Structural Modifications of Gas Hydrates
Clathrate hydrates (or gas hydrates) are a class of water inclusion compounds formed when water molecules hydrogen bond to form a crystalline lattice of cages that are stabilized by guest atoms or molecules. Generally, the structure of clathrate hydrates fall into three categories, two cubic forms known as sI and sII and a hexagonal form known as sH. At low pressures guest atoms and molecules such as noble gasses and methane, nitrogen, oxygen, and carbon dioxide form the cubic form while larger molecules such as cyclo- octane form sH, this in the presence of a small help gas molecule. Recently, small molecules such as methane have been shown to form the sH form at elevated pressure, in those forms small clusters of guests play the role of large molecules in stabilizing the large cage. Here we report the low and high pressure structural modifications of krypton, xenon and argon clathrate hydrates. Studies have been conducted both in situ at high pressure, and in the pressure quench recovered forms using high energy (100 keV) synchrotron x- ray scattering (HEXRD), neutron scattering and nuclear magnetic resonance techniques. The modifications of structure that result from application of pressure will be the focus of this presentation. Initial neutron scattering data collected from the new SNAP high pressure diffractometer at the Spallation Neutron Source will also be presented.
MA23B-04
The Formation of 1.13 nm Tobermorite: New Insights by In-situ Neutron Diffraction
The mineral phase 1.13 nm tobermorite (tobermorite), belonging to the group of Calcium-Silicate-Hydrate (CSH)-phases, is synthesised by the hydrothermal treatment of ground quartz sand, quicklime and water to produce steam cured building materials. Tobermorite is the predominant CSH-phase in Aerated Autoclaved Concrete (AAC) and significantly controls physicochemical properties of the building material. At common conditions (190°C/Psat) tobermorite is metastable, hence, kinetic data is essential to control the reaction. Neutron diffraction is an excellent method to collect data of the reaction process in-situ. A hydrothermal autoclave for neutron diffraction (HAND) was recently designed to record the dynamic process during the hydrothermal reaction. New experiments were carried out in November 2008 at the D20 powder diffractometer of Institute Laue-Langevin (ILL) in Grenoble. The technical specification of this instrument enabled us to collect data within the range of 8 and 153.6 ° 2Θ with a time resolution of 1 diffractogram per minute. The chosen wavelength of 2.4 Å allows the analysis of d-spacing up to 11.3 Å, where the basal reflection of the evolving tobermorite is expected. To see the influence of different independent parameters, experiments were carried out in the temperature range of 170 up to 210 °C in the systems CaO-SiO2-D2O and CaO-SiO2-Al2O3- D2O using two different grain sizes of quartz. Lime-silica based green bodies instead of suspensions were prepared for the measurements, to stay as close as possible to the industrial production process of AAC. The first mineral phases formed are poorly crystallized with a varying Ca/Si ratio between 1.1 and 1.3. Subsequently tobermorite is formed by the reaction of this precursor phases with quartz. The mechanism of the reaction can be described by the conversion of quartz and displays a non-isokinetic behaviour.
MA23B-05
Combined Neutron and X-Ray Diffraction Studies on H/D Isotope Effects in Brucite.
The mineral brucite, Mg(OH)2, belongs to the group of the CdI2-type minerals with a simple hexagonal layered structure. The structure is comprised of stacked sheets of edge-sharing octahedrons of magnesium hydroxide. The sheets are held together by weak intermolecular forces. The high-pressure behavior of brucite is of great geochemical and geophysical interest, because these brucite-type minerals serve as a simple analog for more complex, hydrogen-bearing oxide and silicate minerals in the deep-earth, and was already the object of several studies.[1-4] A combined neutron and synchrotron x-ray powder diffraction study of hydrogenated and deuterated brucite was conducted at ambient temperature and at pressures to 9 GPa, using a Paris-Edinburgh and 20 GPa using a diamond anvil high-pressure cell (for neutron and x-ray radiation respectively). The two materials were synthesized by the same method and companion measurements of neutron diffraction were conducted under the same conditions. Due to the weaker interlayer interaction, this direction is more compressible than the strong Mg oxide bonds within the octahedral layer, consequently the compressibility along the c axis is much higher than along the a axis. The relative unit-cell volume (V/Vo) of normal and deuterated brucite obtained from Rietveld analysis of our data were fit to the third-order Birch-Murnaghan EOS equation. Our values of the bulk modulus (K0=41.46 ± 0.52 and 39.04 ± 0.30 GPa for hydrogenated and deuterated brucite, respectively) are consistent with the literature values, and yet show a more compressible deuterated sample Our data of H-D isotope effects on the unit-cell volume of brucite at elevated pressures confirm that the reduced partition function ratio (beta-factor) of brucite increases with pressure, - while for water-, the inverse is observed. Brucite preferentially incorporates deuterium over hydrogen with increasing pressure. Therefore, the D/H fractionation factor (α) between brucite (and other hydrous minerals) and water (α = βbrucite/βwater) increases significantly with pressure, as our experimental study to 0.8 GPa demonstrated.[3] This pattern of deuterium enrichment in brucite continues at least to 10 GPa, suggesting that D/H fractionation between hydrous minerals and water under mantle conditions differ significantly from near- surface environments. References [1] Parise, J.B., et al., Am. Miner., 79(1-2), 193-196 (1994). [2] Duffy, T.S et al. Am. Mineral., 80(3-4), (1995). [3] Horita, J. et. al. Geochim. et Cosmochim. Acta, 66(21), 3769 (2002). [4] Horita, J. et. al. Am. Miner. submitted.
MA23B-06
Neutron Diffraction Study of Hydrogen in Birnessite Structures
Mn oxide minerals having the birnessite structure readily participate in cation-exchange and oxidation-reduction reactions, and because they typically occur as coatings and fine-grained aggregates with large surface areas, even small quantities can significantly affect the chemical composition and behavior of sediments and associated aqueous systems. The chemical activity exhibited by birnessites is, at least in part, due to the apparent ease by which the structure adjusts to accommodate a range of interlayer water and cation compositions. A more complete understanding of the structure is relevant to many of its proposed uses (e.g., as catalysts and cation-exchange agents) and to understanding its role in geochemical systems. X-ray diffraction studies of birnessite-like phases typically reveal disordered, split interlayer sites that are occupied by both cations and water, but do not provide information about orientations of the water molecules or about H atom positions. The purpose of this study is to use powder neutron diffraction to better understand the structure and bonding environments of water in the interlayer region of cation-exchanged synthetic birnessites. Time-of-flight powder neutron diffraction data were collected at 10K and 300k, respectively, for a series of deuterated, cation-exchanged synthetic birnessite samples at the Intense Pulsed Neutron Source using the general purpose powder diffractometer. Rietveld refinements and difference Fourier synthesis for K- and Na- birnessite samples revealed positionally disordered D sites above and below the interlayer O/cation site. The D positions vary with different types of interlayer cations, but suggest a H-bonding scheme among the interlayer water molecules and O atoms in the octahedral layers. These results are consistent with molecular dynamic structure models calculated for birnessite.
MA23B-07 INVITED
Monitoring CO2 Adsorption Into the Coal: Application of Small-ngle Neutron Scattering Techniques (SANS and USANS)
Injection of CO2 in unmineable deep coal seams is one of the options of geologic CO2 sequestration. Whereas it has been demonstrated that organic matter has high gas adsorption capacity, the mechanisms and the consequences of this adsorption in subsurface conditions are poorly understood. Small-angle scattering techniques can provide unique, pore-size-specific insight into the density of adsorbed CO2. This study reports the results of the first small-angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) studies on coal, using the Seelyville Coal from the Illinois Basin as an example. Experimental conditions employed in this work were chosen to simulate a range of coal subsurface conditions including those at 518 feet depth (P,T) = (1-50 bar, 16ºC), and the coal was saturated with subcritical CO2. Experimental results illustrate that coal microstructure is unaffected by pressurised subcritical CO2, and these findings suggest that depths of burial do not constitute a stability barrier to storage of CO2. The physical density of CO2, fluid phase adsorbed in the porous coal matrix exceeds by a factor of 3-4 the density of the bulk fluid at the same thermodynamic conditions. The applied methodology can be extended to studies of the sorption kinetics and capability of other naturally occurring porous materials of interest for carbon geological storage (saline aquifers, porous rocks, basalts, etc.) as well as investigations of supercritical fluid mixtures (e.g. CO2 and methane) in various coals.
MA23B-08
Inelastic Neutron Scattering Studies of Confined Surface Water on Oxide Nanoparticles
Water is ubiquitous on the surface of oxide nanoparticles and can exert a profound influence on the thermodynamic properties of the oxide [1-3]. The exceptionally large incoherent neutron scattering cross- section of hydrogen compared to other elements makes neutron scattering an attractive technique to study the structure and dynamics of surface water. Here we show that inelastic neutron scattering (INS) is an excellent method to investigate the structure and vibrational density of states (VDOS) of confined surface water on oxide nanoparticles. We present recent INS results for two TiO2 rutile nanoparticle samples with differing levels of hydration. The temperature dependency of the heat capacities for the two samples has been quantified from the VDOS. The results from this study are compared with previously reported data for water confined on anatase-TiO2 nanoparticles [4]. [1] Levchenko et al. (2006) Chem. Mat., 18, 6324. [2] Predota et al. (2004) J. Phys. Chem. B, 108, 12049. [3] Boerio-Goates et al. (2006) Nano Lett., 6, 750. [4] Levchenko et al. (2007) J. Phys. Chem. A, 111, 12584.