Adsorbed water clusters in garnet cracks detected by impedance and Raman spectroscopies at the supercooled water phase transition

Abstract

The combination of electric impedance and Raman spectroscopies at the analysis of the supercooled water phase transition enables the detection of adsorbed water clusters in garnet minerals of leucogranite samples. This transition is revealed by a change in the parameters measured by both techniques at low temperatures, Tc ∼ 220 K, and it is attributed to adsorbed water clusters in the walls of garnet cracks. The study of this transition gives important insights into thewater-rock interaction, one of the most important points in the study of rock alteration. The dielectric spectra were fitted to the Havriliak-Negami model of dielectric relaxation and enabled the estimation of the activation energy of the dielectric relaxation process as Ea ∼ 76 3 kJmol−1, which is higher than the energy attributed to water molecule reorientation in bulk ice. We determined that this energy should be related with the interaction forces between the adsorbed water clusters and the crack walls that hinder the reorientation of those molecules. The logarithmic frequency dependence of the critical transition temperature was also verified. Raman spectra allowed the identification of the water cluster vibration band, ∼3680 cm−1, in the garnet minerals. The band disappears for temperatures around 423 K where the joint action of the laser beam and the temperature evaporates the adsorbed water clusters. This excludes the possibility that the observed supercooled phase transition could be related with structural water in chlorite minerals. The samples had low apparent porosity ∼1.29%, specific surface area, and adsorption average crack width (using the Brunauer- Emmett-Teller method) of ∼0.18 m2 g−1 and ∼7.92 nm, respectively.