Date Published: February 01, 2017
Publisher: International Union of Crystallography
Author(s): A. Dominic Fortes, Kevin S. Knight, Ian G. Wood.
We have employed neutron and X-ray powder diffraction and density functional theory calculations to determine the structure and thermoelastic properties of a new hydrate in the MgSO4–H2O binary system, magnesium sulfate enneahydrate. We show that this 9-hydrate could occur naturally in certain hypersaline lakes on Earth and indicate where it may be formed as a more persistent mineral elsewhere in the solar system.
Magnesium sulfate is a quite common constituent of water on Earth, although the number of locations where it is sufficiently concentrated to produce MgSO4 hydrate (or cryohydrate) minerals is not large, the stability of the higher hydrates being dependent on low ambient temperatures and high humidity. The highest hydrate, MgSO4·11H2O, occurs naturally as the mineral meridianiite in a variety of glacial and periglacial environments (Sakurai et al., 2009 ▸; Genceli et al., 2009 ▸) and in a limited number of MgSO4-rich hypersaline lakes during the winter months; example localities where the mineral has been identified include the Basque Lakes, Clinton Lake and kłlil’xw (aka Spotted Lake), all in British Columbia, Canada (e.g. Peterson et al., 2007 ▸; Cannon, 2012 ▸). Whilst MgSO4-rich saline waters are comparatively rare on Earth due to the influence of continental weathering, such liquids are expected to be common on other rocky planets where the weathering of basaltic materials dominates (King et al., 2004 ▸). On Mars, abundant magnesium(II) and iron(III) sulfates are known to occur, including minerals such as kieserite and jarosite (e.g. Clark et al., 1976 ▸; Toulmin et al., 1977 ▸; Wänke et al., 2001 ▸; Foley et al., 2003 ▸; McSween, 2004 ▸; Chipera & Vaniman, 2007 ▸) and it is hypothesized that meridianiite may occur in a permafrost-like deposit, forming a substantial reservoir of bound water in the near-surface regolith (Feldman, Mellon et al., 2004 ▸; Feldman, Prettyman et al., 2004 ▸; Peterson & Wang, 2006 ▸). Similarly, water–rock interactions during the accretion and differentiation of icy planetary bodies in the outer solar system may have resulted in large brine reservoirs crystallizing substantial quantities of MgSO4 and Na2SO4 hydrates (Kargel, 1991 ▸). These are apparent in near-IR spectra of their surfaces (Orlando et al., 2005 ▸; Dalton, 2007 ▸; Dalton et al., 2005 ▸; Shirley et al., 2010 ▸), although it remains unclear the extent to which some of the hydrated salts on Europa’s surface are due to endogenic versus exogenic processes, such as radiolysis of MgCl2 combined with sulfur implantation from neighbouring Io (Brown & Hand, 2013 ▸).
We have determined the crystal structure of MgSO4·9H2O, including all hydrogen positions using a combination of X-ray and neutron powder diffraction. Additionally, we have determined the thermal expansion and incompressibility tensors of MgSO4·9H2O over the range 10–260 K at ambient pressure and 0–1 GPa at 240 K. Our observations have been instrumental in understanding the decomposition of meridianiite at high pressure and provide useful information on structural systematics and the transformation of metastable cryohydrates at low temperature. Further work is required to verify and further characterize an apparent elastic softening observed under compression at 240 K, but which is not reproduced in athermal ab initio calculations.
References cited in the supporting information include: Alexandrov et al. (1963 ▸), Arbeck et al. (2010 ▸), Cook-Hallett et al. (2015 ▸), Fortes, Wood et al. (2003 ▸), Geruo et al. (2014 ▸), Gowen et al. (2011 ▸), Hill (1952 ▸), Nimmo (2004 ▸), Nimmo & Matsuyama (2007 ▸), Nimmo & Schenk (2006 ▸), Reuss (1929 ▸), Tsuji & Teanby (2016 ▸), Voigt (1910 ▸), Wahr et al. (2009 ▸).