hydrohalite: properties of a major hydrate in the H2O-NaCl binary

Details hydrohalite: properties of a major hydrate in the H2O-NaCl binary hydrohalite: properties of a major hydrate in the H2O-NaCl binary

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmmr23a1677d&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #MR23A-1677

Physics

[0750] Cryosphere / Sea Ice, [3924] Mineral Physics / High-Pressure Behavior, [3934] Mineral Physics / Optical, Infrared, And Raman Spectroscopy, [3954] Mineral Physics / X-Ray, Neutron, And Electron Spectroscopy And Diffraction

Scientific paper

Hydrohalite NaCl.2H2O is widespread at the surface of the Earth in cold environments. It crystallizes in sea ice, and strongly affects radiative transfer in sea ice. Hydrohalite is also thermodynamically stable at the surface and in the icy shell of the galilean satellite Europa. It may represent 7 to 15 vol% of the salts of Europa’s crust, although not detectable. Hydrohalite plays also a key role in the cryogenic formation of the hypersaline brines occuring in crystalline basement rocks. Finally there is a more applied concern with hydrohalite that likely forms on heavily salted roads during cold winter and provides a slippery, glassy surface that looks like ice, but is resistant to further salting. Although hydrohalite appears almost ubiquitous in cold planetary environments, there is only limited structural and thermodynamic data available for hydrohalite. At low temperature, the system NaCl-H2O crystallises into hydrohalite and ice Ih. Hydrohalite is monoclinic with a space group P21/c. The symmetric stretching vibrations of hydroxyl bonds and the bending modes of H2O have been early investigated by infrared, Raman and NMR spectroscopy in the 70’. They all agree on two independent crystallographically water molecules and four distinct OH groups in the structure, leading to two bending modes and four stretching modes, respectively. However, there is no spectroscopic data available for the lattice vibrations in hydrohalite although they would be necessary for the calculation of the thermodynamic properties of hydrohalite. In the present study, the full Raman spectrum of hydrohalite has been calculated as a function of pressure. It has also been measured at high pressure in association with ice VI or ice VII, despite a very narrow field of observation of 0.1 GPa at 2.1 GPa. The Raman spectrum and the X-ray diffraction pattern of hydrohalite have also been measured as a function of temperature between 77 and 280 K, in order to calculate its thermodynamic properties. Microscopic and macroscopic properties of hydrohalite calculated from this unique data set (ab initio calculations, Raman spectroscopy and X-ray diffraction) will be presented and the thermodynamic stability of hydrohalite subsequently discussed in the aforementioned cold planetary environments.

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