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Abstract: The distribution and substitution mechanism of Ge in the Ge-rich sphalerite from the Tres Marias Zn deposit, Mexico, was studied using a combination of techniques at micro m- to atomic scales. Trace element mapping by Laser Ablation Inductively Coupled Mass Spectrometry shows that Ge is enriched in the same bands as Fe, and that Ge-rich sphalerite also contains measurable levels of several other minor elements, including As, Pb and Tl. Micron- to nanoscale heterogeneity in the sample, both textural and compositional, is revealed by investigation using Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) combined with Synchrotron X-ray Fluorescence mapping and High-Resolution Transmission Electron Microscopy imaging of FIB-prepared samples. Results show that Ge is preferentially incorporated within Fe-rich sphalerite with textural complexity finer than that of the microbeam used for the X-ray Absorption Near Edge Structure (XANES) measurements. Such heterogeneity, expressed as intergrowths between 3C sphalerite and 2H wurtzite on [11¯0] zones, could be the result of either a primary growth process, or alternatively, polystage crystallization, in which early Fe-Ge-rich sphalerite is partially replaced by Fe-Ge-poor wurtzite. FIB-SEM imaging shows evidence for replacement supporting the latter. Transformation of sphalerite into wurtzite is promoted by (111)* twinning or lattice-scale defects, leading to a heterogeneous ZnS sample, in which the dominant component, sphalerite, can host up to ~20% wurtzite. Ge K-edge XANES spectra for this sphalerite are identical to those of the germanite and argyrodite standards and the synthetic chalcogenide glasses GeS2 and GeSe2, indicating the Ge formally exists in the tetravalent form in this sphalerite. Fe K-edge XANES spectra for the same sample indicate that Fe is present mainly as Fe2+, and Cu K-edge XANES spectra are characteristic for Cu+. Since there is no evidence for coupled substitution involving a monovalent element, we propose that Ge4+ substitutes for (Zn2+, Fe2+) with vacancies in the structure to compensate for charge balance. This study shows the utility of synchrotron radiation combined with electron beam micro-analysis in investigating low-level concentrations of minor metals in common sulfides.

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Abstract: The mechanism of the Brust-Schiffrin gold nanoparticle synthesis has been investigated through the use of ion transfer voltammetry at the water|1,2-dichloroethane (DCE) solution interface, combined with X-ray absorption fine structure (XAFS) of the reaction between [AuCl4]- and thiol (RSH) in homogeneous toluene (TL) solution. Ion transfer calculations indi- cate the formation of [AuCl2]- at RSH:Au ratios from 0.2 – 2 with a time-dependent variation observed over several days. At RSH:Au ratios above 2 and after time periods greater than 24 hours, the formation of Au(I)SR is also observed. The relative concentrations of reaction products observed at the liquid/liquid interface are in excellent agreement with those observed by XAFS for the corresponding reaction in a single homogeneous phase. BH4- ion transfer reactions between water and DCE indicate that the reduction of [AuCl4]- and/or [AuCl2]- to Au nanoparticles by BH4- proceeds in the bulk organic phase. On the other hand, BH4- was unable to reduce the insoluble [Au(I)SR]n species to Au nanoparticles. The number and size of the nanoparticles formed was dependent on the concentration ratio of RSH:Au, as well as the experimental duration because of the competing formation of the [Au(I)SR]n precipitate. Higher concentrations of nanoparticles, with diameters of 1.0 – 1.5 nm, were formed at RSH:Au ratios from 1 to 2.

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Abstract: The phyllomanganate birnessite is the main Mn-bearing phase in oxic marine sediments where it exerts a primary control on the concentration of micronutrient trace metals in seawater. However, during sediment diagenesis and under mild hydrothermal conditions birnessite transforms into the tectomanganate todorokite. We have recently shown that the transformation of birnessite to todorokite proceeds via a four-stage nucleation and growth mechanism, beginning with todorokite nucleation, then crystal growth from solution to form todorokite primary particles, followed by their self-assembly and oriented growth via oriented attachment to form crystalline todorokite laths, culminating in traditional crystal ripening (Atkins et al., 2014, GCA 144, 109-125). Here we determine the fate and mobility of Ni sorbed by birnessite during this transformation process. Specifically, in our recent work we predict that the presence of Ni within the phyllomanganate matrix will disrupt the formation of todorokite primary particles. As such, contrary to current understanding, we suggest that Ni sorbed by birnessite will slow the transformation of birnessite to todorokite and/or be released to marine porewaters during sediment diagenesis. Here we transform a synthetic, poorly crystalline, Ni-sorbed (~1 wt% Ni) hexagonal birnessite, analogous to marine birnessite, into todorokite under a mild reflux procedure, developed to mimic marine diagenesis and mild hydrothermal conditions. We characterize our birnessite and reflux products as a time series, including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy. In addition we determine Ni speciation and mineral phase associations in a suite of natural marine ferromanganese precipitates, containing intermixed phyllomanganate and todorokite. Our work shows for the first time that Ni significantly slows the transformation of birnessite to todorokite and reduces the crystallinity of the neo-formed todorokite phase, but does not alter the mechanism and pathway of todorokite formation, compared to a Ni-free system. Furthermore, in systems tending towards todorokite as the final diagenetic product, we see that up to 50 % of the Ni originally sequestered by birnessite is released to solution during the transformation. Our work indicates that the transformation of birnessite to todorokite in oxic marine sediments likely provides a significant source of Ni to marine sedimentary porewaters and potentially a hitherto unrecognized benthic flux of Ni to seawater.

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Abstract: This thesis consists of three main investigations . The first of these is a study of the magnetic properties of Fe nanoparticles embedded in an Al matrix, with different volume fraction. Both Fe nanoparticles, with a diameter of ̴ 2 nm, and the Al matrix were deposited from the gas phase. Th e atomic Fe moment of the Fe nanoparticles in Al is much less than the bulk Fe value because of considerable alloying at the Fe nanoparticle and Al matrix interface. Two important parameters , the exchange field (H ex ) and random anisotropy field (H r ) , were investigated using the Random Anisotropy Model (RAM). Fitting the data to this model reveals that with increasing volume filling f raction (VF F ) of Fe nanoparticles in Al , both H ex and H r show an increase, with H r showing a more significant increase t han for H ex . The second main investigation in this thesis is a study of structure and magnetism in Co nanoparticles embedded in antiferromagnetic Cr. Co K edge and Cr K edge extended x - ray absorption fine structure (EXAFS) experiments were performed in ord er to investigate atomic structure in the Cr - embedded Co nanoparticles and Cr matrix respectively, whereas magnetism was investigated using a vibrating sample magnetometer (VSM). The atomic structure of the Co nanoparticles is same as the host Cr matrix (bcc) , although with a degree of disorder , rather than the bulk Co hcp structure. The net Co moment per atom in the Co/Cr nanocomposite films is significantly lower than bulk Co value, and decreases as the proportion of Co nanoparticles in the film is decr eased; for the sample with the most dilute concentration of Co nanoparticles (4.9% by volume), the net Co moment was 0.18 μ B /atom. Both the struc tural and magnetic results show that there is a degree of alloying at the nanoparticle/matrix interface, leadin g to a core/shell structure in the embedded nanoparticles consisting of an antiferromagnetic CoCr alloy shell surrounding a reduced ferromagnetic Co core. The final part of this work is an investigation into the structure and magnetism of Fe nanoparticles embedded in a Cu 1 - x Al x alloy matrix, where the structure of the matrix could be controlled by control over its composition. Cu K edge EXAFS measurements show that there is a slight stretch in the Cu - Cu interatomic distances in the alloy matrix , while the face centred structure in the Cu 1 - x Al x matrix is maintained , as the Al - content is increased . Fe K edge EXAFS measurement s reveal that for low Al - content in the Cu 1 - x Al x matrix , Fe nanoparticles have both fcc and bcc structures, but for higher Al - content the structure of Fe nanoparticles is consistent with bcc. The magnetism measurement s, obtained from VSM and SQUID magnetometer s , show that the Fe atomic moment increases sharply due to the increasing proportion of bcc Fe nanoparticles. However for Al - conte nt higher than 0.13, the net atomic moment value of Fe decreases slightly , which is consistent with a high degree of alloying between Fe and Al atoms.

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Abstract: The solubility of uranium and thorium has been measured under the conditions anticipated in a cementitious, geological disposal facility for low and intermediate level radioactive waste. Similar solubilities were obtained for thorium in all media, comprising NaOH, Ca(OH)2 and water equilibrated with a cement designed as repository backfill (NRVB, Nirex Reference Vault Backfill). In contrast, the solubility of U(VI) was one order of magnitude higher in NaOH than in the remaining solutions. The presence of cellulose degradation products (CDP) results in a comparable solubility increase for both elements. Extended X-ray Absorption Fine Structure (EXAFS) data suggest that the solubility-limiting phase for uranium corresponds to a becquerelite-type solid whereas thermodynamic modelling predicts a poorly crystalline, hydrated calcium uranate phase. The solubility-limiting phase for thorium was ThO2 of intermediate crystallinity. No breakthrough of either uranium or thorium was observed in diffusion experiments involving NRVB after three years. Nevertheless, backscattering electron microscopy and microfocus X-ray fluorescence confirmed that uranium had penetrated about 40 μm into the cement, implying active diffusion governed by slow dissolution-precipitation kinetics. Precise identification of the uranium solid proved difficult, displaying characteristics of both calcium uranate and becquerelite.