Ubiquity of the Micrometer-Thick Interface along a Quartz-Water Boundary

Water-rock interactions determine how the geochemical cycles revolve from the Earth's surface to the deep interior (large T-P intervals). The underlying mechanisms interweave the fluxes of matter, time, and reactivity between fluid phases and solids. The deformation processes of crustal rocks are also known to be significantly affected by the presence or absence of water, typically with the hydrolytic weakening of quartz, olivine, and other silicate minerals. In fact, fluid-rock interactions mechanistically unfold along their interfaces, developing over a certain thickness within the two phases. Diffraction-limited mid-infrared microspectroscopy was employed to monitor the thermodynamic characteristics of liquid water along a quartz boundary. The hyperspectral Fourier transform infrared data set displayed a very strong distance-dependent signature for water over a 1 +/- 0.5 mu m thickness, while quartz appears unmodified, which is consistent with recent studies. This unexpected thick interface is tested against the geometry of the inclusion, the chemistry of the occluded liquid (especially pH), and the thermal conditions ranging from room temperature to 155 degrees C. Throughout this range of physicochemical conditions, the micrometer-thick interface is characterized by a ubiquitous, significant shift in the Gibbs free energy of water inside the interfacial layer. This conclusion suggests that the interface-imprinting phenomenon driving this microthick layer has thermodynamic roots that give rise to specific properties along the quartz-water interface. This finding questions the systematic use of the bulk phase data sets to evaluate how water-rock interactions progress in porous media.