Structure and diffusive properties of fluid-filled grain boundaries: An in-situ study using infrared (micro) spectroscopy

Abstract

The rate of numerous diagenetic, deformation and metamorphic processes in crustal rock systems ultimately depends on the structure and diffusive properties of water-bearing grain boundaries. We present the first in-situ spectroscopic study of the nature of aqueous films present in mineral interfaces undergoing stress-induced dissolution or ‘pressure solution’. Using infrared micro-spectroscopy, we show that during active pressure dissolution of the (111) plane of halite (NaCl) against a CaF2 plate, the confined intercrystalline liquid has an average thickness of 85–185 nm and occupies a rough, non-equilibrium grain boundary structure. The hydrogen bonding of the water within the grain boundary film is modified due to increased polymerization towards ice-like (hard) water. Simultaneous pressure solution rate measurements show that diffusion within the grain boundary fluid is about one order of magnitude slower than in bulk solution. During active pressure solution of the (100) plane of halite, a similar non-equilibrium grain boundary structure develops but the average thickness of the confined water film is only ∼40 nm and no evidence is found for modification of the hydrogen bonding of the water. The difference in hydrogen bonding between the two orientations studied is attributed to differences in surface charge distribution on the (111) and neutral (100) interfaces in NaCl. The difference in mean film thickness is attributed to differences in crystallographically controlled roughness of the dissolving NaCl Surface. If similar grain boundary thickness and diffusivity effects occur in other rock forming minerals, pressure solution in such systems will tend to be interface reaction controlled and may be capable of producing significant seismic anisotropy.