Water in microporous materials such as the sub-nanometer channels in the structures of some minerals presents an extreme case of confinement in environments that are not much larger than the water molecule itself. While such mineralogical confinement may be fairly rare (e.g. bassanite, beryl, cordierite, dioptase, gypsum, hemimorphite, mordenite, and chrysotile), as pressures increase closing grain boundaries become very similar environments. The nature of water in these environments, however, varies tremendously with the specific mineral. For example, a “quantum tunneling state” occurs in water molecules confined in 5 Å channels in beryl, characterized by extended proton and electron delocalization [1]. <i>The average kinetic energy of the water protons, a measure of their quantum-mechanical zero-point motion, is ~30% less than in bulk liquid or solid water, in complete disagreement with accepted models based on the energies of its vibrational modes.</i> Such behavior is not observed in nearly isostructural cordierite. In hemimorphite, water molecules form a planar hydrogen bond network with hydroxyl groups that undergo a continuous orientational phase transition. The incoherent dynamic structure factor reveals two thermally activated relaxation processes, a faster one on the subpicosecond time scale and a slower one on the 10-100 ps time scale, between 70-130 K. The slow process is an in-plane reorientation involving breaking hydrogen bonds analogous to rotational diffusion of water molecules in the bulk liquid. The fast process is a localized motion of the water molecule that has no apparent analogs within known bulk or confined water phases. Thus, once water in the subsurface becomes confined to nanoscale grain boundaries it is unlikely that its properties match those of the bulk material, thus affection the stability of the surrounding mineral phases.

Water under ultra-confinement: Implications for mineral stability

256th National Meeting (2018)

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