Confined water in rock salt

Moisture on grain boundaries of salt crystals is the principal expedient in halokinesis, the rheology of rock salt, yet its quantitative determination has proven to be elusive. Here we elaborate on results from a first-of-its-kind incoherent inelastic neutron scattering (INNS) of bedded salts to distinguish three water environments: intergranular water molecules confined to grain boundaries, water trapped as brine in fluid inclusions, and structural water in hydrous minerals (Ding et al., 2015). Sixteen spectral lines can be distinguished unambiguously in the 0-1100 cm-1 multiphonon and librational domain, yielding an unprecedented high resolution for a natural material. The spectral response to temperature illustrates the bimodality of the technique enabling the intergranular water component to be distinguished from that of the brine, shedding light on a nearly 30-year old problem in characterizing different forms of water in rock salt. This pioneering study shows that INNS provides insight into the cause and effect of moisture migration and its coupling to thermomechanical properties in salt formation. Our results are pertinent to subsurface energy exploration and storage, including nuclear waste storage in salts.