Browsing by Author "Ilavsky, J"

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    Characterization of porosity in sulfide ore minerals: a USANS/SANS study
    (GeoScience World, 2014-11-18) Xia, F; Zhao, J; Etschmann, BE; Brugger, J; Garvey, CJ; Rehm, C; Lemmel, H; Ilavsky, J; Han, YS; Pring, A
    Porosity plays a key role in the formation and alteration of sulfide ore minerals, yet our knowledge of the nature and formation of the residual pores is very limited. Herein, we report the application of ultra-small-angle neutron scattering and small-angle neutron scattering (USANS/SANS) to assess the porosity in five natural sulfide minerals (violarite, marcasite, pyrite, chalcopyrite, and bornite) possibly formed by hydrothermal mineral replacement reactions and two synthetic sulfide minerals (violarite and marcasite) prepared experimentally by mimicking natural hydrothermal conditions. USANS/SANS data showed very different pore size distributions for these minerals. Natural violarite and marcasite tend to possess less pores in the small size range (<100 nm) compared with their synthetic counterparts. This phenomenon is consistent with a higher degree of pore healing or diagenetic compaction experienced by the natural violarite and marcasite. Surprisingly, nanometer-sized (<20 nm) pores were revealed for a natural pyrite cube from La Rioga, Spain, and the sample has a pore volume fraction of ~7.7%. Both chalcopyrite and bornite from the massive sulfide assemblage of the Olympic Dam deposit in Roxby Downs, South Australia, were found to be porous with a similar pore volume fraction (~15%), but chalcopyrite tends to have a higher proportion of nanometer-size pores centered at ~4 nm while bornite tends to have a broader pore size distribution. The specific surface area is generally low for these minerals ranging from 0.94 to 6.28 m2/g, and the surfaces are generally rough as surface fractal behavior was observed for all these minerals. This investigation has demonstrated that USANS/SANS is a very useful tool for analyzing porosity in ore minerals. We believe that with this quantified porosity information a deeper understanding of the complex fluid flow behavior within the porous minerals can be expected. © 2014, Mineralogical Society of America.
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    Chemomechanical influences during replacement of limestones by siderite
    (Goldschmidt, 2022-07-15) Weber, J; Starchenko, V; Zhang, R; Ilavsky, J; Debeer-Schmidt, L; Mata, JP; Littrell, K; He, L; Chen, WR; Allard, LF; Stack, AG; Anovitz, L
    A fundamental and predictive understanding of mineral-fluid interactions is important for a wide range of energy topics including carbon sequestration, nuclear waste management and legacy contamination clean up. The properties of aqueous solution are altered by confinement, which can be present within natural geomaterials, e.g., in grain boundaries and nanopores. Mineral replacement reactions have been reported to proceed via grain boundaries possibly due to higher diffusion rates than in solids. In addition to confinement effects, chemomechanical effects such as crystallization pressure induced fracturing can also alter mineral-fluid interactions. To test the effects of porosity and grain boundaries on replacement in single component and impurity-containing systems we experimentally investigated the model system of limestone replacement by siderite by batch reactor experiments at 200°C from 2 to 120 days with FeCl2. Variation in initial microstructure and solid impurities were used to identify reaction controls. Changes in porosity were spatially resolved analyzed using inverse scattering techniques ((ultra) small angle neutron/X-ray scattering), and these were combined with imaging by scanning (SEM) and transmission electron microscopy (TEM). In high-porosity limestones replacement is rapid (complete replacement within 2 days), and transport controlled, whereas in low-porosity limestones elevated porosity throughout the whole rock volume was observed that was independent of the reaction rim. Image analysis showed widening of selected grain boundaries with increasing reaction time. This led to increased grain boundary width distributions that were observed as higher porosity by scattering methods. SEM imaging showed that nucleation of siderite crystals either at dolomite impurities within the limestone or other defects lead to exertion of crystallization pressure, widening grain boundaries, which led to formation of preferential transport pathways that limited replacement of solid impurity-containing limestone rocks. This highlights how chemomechanical effects can alter reaction pathways.
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    Understanding solvothermal crystallization of mesoporous anatase beads by in situ synchrotron PXRD and SAXS
    (American Chemical Society, 2014-07-07) Xia, F; Chen, DH; Scarlett, NVY; Madsen, IC; Lau, D; Leoni, M; Ilavsky, J; Brand, HEA; Caruso, RA
    Submicrometer-sized mesoporous anatase (TiO2) beads have shown high efficiency as electrodes for dye-sensitized solar cells and are recoverable photocatalysts for the degradation of organic pollutants. The detailed mechanism for crystallization of the amorphous TiO2/hexadecylamine (HDA) hybrid beads occurring during the solvothermal process needs to be understood so that reaction parameters can be rationally refined for optimizing the synthesis. In this work, the solvothermal crystallization was monitored by in situ synchrotron powder X-ray diffraction (PXRD) and synchrotron small-angle X-ray scattering (SAXS) techniques. In situ PXRD provided crystallization curves, as well as the time evolution of anatase crystallite mean size and size distribution, and in situ SAXS provided complementary information regarding the evolution of the internal bead structure and the formation of pores during the course of the solvothermal process. By exploring the effects of temperature (140-180 °C), bead diameter (300 and 1150 nm), bead internal structure, and solvent composition (ethanol and ammonia concentrations) on this process, the crystallization was observed to progress 3-dimensionally throughout the entire bead due to solvent entrance after an initial fast partial dissolution of HDA from the nonporous precursor bead. On the basis of the kinetic and size evolution results, a 4-step crystallization process was proposed: (1) an induction period for precursor partial dissolution and anatase nucleation; (2) continued precursor dissolution accompanied by anatase nucleation and crystal growth; (3) continued precursor dissolution accompanied by only anatase crystal growth; and (4) complete crystallization with no significant Ostwald ripening. © 2014 American Chemical Society.