Browsing by Author "Gates, W"

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    Hydrohalite formation in frozen clay brines
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Gates, W; Bordallo, HN; Ferhervari, A; Klapproth, A; Acikel, A; Bouazza, A; Aldridge, LP; Iles, GN; Mole, RA
    Hydrated forms of cryosalts in frozen brines play important roles in the polar landscape and troposphere of Earth [1], and their melting [2] is implicated in recurring slope lineae (RSL) in Antarctica’s McMurdo Dry Valley [3] and equator-facing, mid-latitude (42ºN-52ºS) slopes of Mars [4]. Observation of the widespread occurrence of clay minerals and salts on the Martian surface [5] indicates that saline groundwater [6] may still be present on Mars. The surface of Mars ranges in temperature from 293 K on the equator at noon to 120 K at the poles and mobility of sub-surface water ice will depend on the local temperature and the mobility of confined water in the crustal clays. We applied quasielastic neutron scattering using the backscattering spectrometer EMU (Australian Nuclear Science and Technology Organisation) at 1 μeE resolution, to the system: sodium montmorillonite – 5M NaCl (Na-Mt-NaCl and calcium montmorillonite – 5M CaCl2 (Ca-Mt-CaCl2); to establish boundary conditions influencing the dynamics of confined water. Results from elastic fixed window (EFW) data indicate a substantial increase in the mean square displacement of hydrogen (H) in the brine conditions at all temperatures above 100K, indicating enhanced mobility of water in the presence of brines. A phase transition was observed in Na-Mt-NaCl at 255K (on heating) indicating the presence of the cryosalt hydrohalite (NaCl·2H2O), but no phase transition was observed in Ca-Mt-CaCl2. In addition, quasielastic neutron scattering (QENS) spectra highlighted that water in the Ca-Mt-CaCl2 system was strongly confined at room temperature. Recently [6] hydrohalite was observed to form in frozen gels of Na-Mt brines, but not in Ca-Mt brines. They considered that textural differences in the two forms allowed the gel pores of the Na-Mt to retain liquid saline pore water to well below the freezing point of pure water. Based on our analysis, water is restricted to rotational mobility in the Na-Mt-NaCl below 255K, but presents more translational mobility above 255K. These findings largely support those of Yesilbas [7] in the importance of pore structure in controlling cryosalt formation, and further implicate their role in associated phenomena such as RSL.
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    Water dynamics in minerals on the surface of Mars
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Iles, GN; Gates, W; Mole, RA; FehervariI, A
    Water was discovered 1.5 km below the surface of Mars in 2018 and some liquid water may occur transiently on the Martian surface in the spectrally dominant phyllosilicate group, smectite. In the same year, NASA confirmed that water ice is present in silicates on the surface of the Moon in the polar regions. This discovery has prompted the return of people to the Moon, whereby the Artemis program planned for 2024 will see the next man and the first woman land at the South Pole. The extraction of trapped and frozen water from minerals on other planetary bodies such as the Moon and Mars is a technical challenge if humanity is to implement an innovative and sustainable program of exploration enabling human expansion across the solar system. We have investigated the hydration properties of clays and minerals found on Mars using time-of-flight neutron spectroscopy. [1] From Quasi-Elastic Neutron Scattering data we determined water diffusion coefficients for input into our model to identify possible sites where the water resides in Na-smectites. Additional characterisation of montmorillonite has been conducted at the Australian Synchrotron facility using far-infrared radiation to obtain proportions of bound and unbound water in Na- and Ca-smectite. We observed the cation rattle at low energy (~45 cm-1) as a distinct signal from that of the bulk-like water and cationic bound water, where the latter is ‘trapped’ within the clay layers. [2,3] Understanding of water hydration processes in these abundant soils and minerals will be of use not just on other planetary bodies but also in extreme environments such as Antarctica. On Earth, knowledge of water dynamics at clay mineral surfaces can be utilised to improve performance and durability of lining materials for barriers used in environmental protection.