Browsing by Author "Wykes, JL"

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    Anatomy of a complex mineral replacement reaction: Role of aqueous redox, mineral nucleation, and ion transport properties revealed by an in-situ study of the replacement of chalcopyrite by copper sulfides
    (Elsevier, 2021-10-20) Chaudhari, A; Webster, NAS; Xia, F; Frierdich, AJ; Ram, R; Etschmann, BE; Liu, WH; Wykes, JL; Brand, HEA; Brugger, J
    The fluid-driven transformation of chalcopyrite (CuFeS2) into Cu-rich sulfides (e.g., digenite, Cu1.8S; covellite, CuS; and chalcocite, Cu2S) is a complex mineral replacement reaction where the reaction pathway is controlled by the interplay between evolving mineral make-up, texture/porosity, and solution chemistry. This trans-formation was investigated in CuCl2 +H2SO4 solutions under mild hydrothermal conditions (180 to 300 ◦C); the reaction kinetics, nature of minerals formed, and oxidation states of aqueous Fe and Cu were followed in-situ in real-time using synchrotron powder X-ray diffraction (PXRD) and X-ray absorption spectroscopy (XAS). These results are corroborated by an analysis of the textures of reaction products from comparative ex-situ quench experiments. The in-situ and ex-situ experiments revealed that: (i) aqueous Cu2+quickly reduced to Cu+ during chalcopyrite replacement in all experiments, and Fe dissolved as Fe2+; (ii) covellite was the first mineral to form, followed by digenite-high with delayed nucleation; and (iii) a non-quenchable hydrated Fe sulfate mineral (szomolnokite, FeSO4.H2O) formed at 240 ◦C at relatively low concentrations of added CuCl2, which supressed the formation of digenite-high. The quantitative mineral phase evolution retrieved using in-situ PXRD was modelled using a novel dual power law (dual Avrami approach). Avrami exponents revealed that chalcopyrite replacement proceeded initially via a 3-dimensional growth mechanism, followed by diffusion-controlled growth. This is consistent with initial formation of a porous covellite rim around chalcopyrite, confirmed by the observation of the ex-situ reaction products, followed by a second reaction stage where the transport properties of aqueous Fe (released from the chalcopyrite) and aqueous Cu (added from the initial solution) to and from the reaction front become the rate-limiting step; and these two kinetic stages exist even where covellite was the only replacement product. The activation energies calculated for these two kinetic stages were 42.9 ±7.4 kJ mol −1 and 39.3 ± 13.1 kJ mol−1, respectively. We conclude that (i) the replacement of chalcopyrite by covellite and digenite proceeds via an interface coupled dissolution and reprecipitation mechanism; (ii) availabilities of aqueous Cu+ and of Fe2+ play a critical role in covellite nucleation and on the sequence of mineral precipitation during chalcopyrite replacement; the Cu+ to Cu2+ ratio is controlled by the kinetics of Cu2+ to Cu+ reduction, which increases with increasing temperature, and by the transport of Cu2+ through the daughter mineral to the reaction front, while Fe2+ availability is limited at high temperature by the formation of insoluble ferrous sulfate; and (iii) this reaction evolves from a bulk fluid-chemistry controlled reaction (initial formation of covellite) to an interface-controlled reaction (digenite-high or further transformation to covellite). The current findings highlight the complex feedback between Cu2+/Cu+ aqueous redox, mineral nucleation, and ion transport properties during replacement reactions, and the applicability of combined in-situ PXRD and XAS techniques in deciphering complex fluid-driven mineral replacement reactions. © 2021 Elsevier B.V
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    Ancient micrometeorites suggestive of an oxygen-rich Archaean upper atmosphere
    (Springer Nature, 2016-05-11) Tompkins, AG; Bowlt, L; Genge, M; Wilson, SA; Brand, HEA; Wykes, JL
    It is widely accepted that Earth’s early atmosphere contained less than 0.001 per cent of the present-day atmospheric oxygen (O2) level, until the Great Oxidation Event resulted in a major rise in O2 concentration about 2.4 billion years ago1. There are multiple lines of evidence for low O2 concentrations on early Earth, but all previous observations relate to the composition of the lower atmosphere2 in the Archaean era; to date no method has been developed to sample the Archaean upper atmosphere. We have extracted fossil micrometeorites from limestone sedimentary rock that had accumulated slowly 2.7 billion years ago before being preserved in Australia’s Pilbara region. We propose that these micrometeorites formed when sand-sized particles entered Earth’s atmosphere and melted at altitudes of about 75 to 90 kilometres (given an atmospheric density similar to that of today3). Here we show that the FeNi metal in the resulting cosmic spherules was oxidized while molten, and quench-crystallized to form spheres of interlocking dendritic crystals primarily of magnetite (Fe3O4), with wüstite (FeO)+metal preserved in a few particles. Our model of atmospheric micrometeorite oxidation suggests that Archaean upper-atmosphere oxygen concentrations may have been close to those of the present-day Earth, and that the ratio of oxygen to carbon monoxide was sufficiently high to prevent noticeable inhibition of oxidation by carbon monoxide. The anomalous sulfur isotope (Δ33S) signature of pyrite (FeS2) in seafloor sediments from this period, which requires an anoxic surface environment4, implies that there may have been minimal mixing between the upper and lower atmosphere during the Archaean. © 2016 Macmillan Publishers Limited
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    Medium Energy Spectroscopy (MEX) - sample environments and supporting infrastructure
    (Australian Nuclear Science and Technology Organisation, 2021-11-26) Lamb, K; Glover, CJ; James, S; Finch, E; Wykes, JL
    The Medium Energy Spectroscopy (MEX) beamline aims to facilitate a wide variety of ex- and in-situ experimental work from a variety of research areas. As such, we will provide a number of sample environments as standard set-up, in addition to ancillary equipment that can be used with custom or BYO sample environments. Sample environments will likely include; room temperature cell, electrochemical flow cell, micro-fluidic cell, flammable gas cell, furnace with gas environments,and a battery testing cell. In addition, supporting infrastructure and ancillary equipment will likely include; flammable and toxic gas handling (flow and pressure control), gas and vapor ventilation, electrochemical testing station (Autolab or similar), fluid (gas or vapour) syringe pumps with pressure monitoring. Most, if not all, of the sample environments and supporting infrastructure will be controlled with the beamline systems, enabling integration and triggering for maximum achievable automation of experiments.© 2021 The Authors