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

摘要

The combination of electric impedance and Raman spectroscopiesat the analysis of the supercooled water phase transitionenables the detection of adsorbed water clusters in garnet mineralsof leucogranite samples. This transition is revealed by achange in the parameters measured by both techniques at lowtemperatures, Tc ∼ 220 K, and it is attributed to adsorbed waterclusters in the walls of garnet cracks. The study of this transitiongives important insights into thewater-rock interaction, one of themost important points in the study of rock alteration. The dielectricspectra were fitted to the Havriliak-Negami model of dielectricrelaxation and enabled the estimation of the activation energyof the dielectric relaxation process as Ea ∼ 76 3 kJmol−1,which is higher than the energy attributed to water moleculereorientation in bulk ice. We determined that this energy shouldbe related with the interaction forces between the adsorbed waterclusters and the crack walls that hinder the reorientation of thosemolecules. The logarithmic frequency dependence of the criticaltransition temperature was also verified. Raman spectra allowedthe identification of the water cluster vibration band, ∼3680 cm−1,in the garnet minerals. The band disappears for temperaturesaround 423 K where the joint action of the laser beam and thetemperature evaporates the adsorbed water clusters. This excludesthe possibility that the observed supercooled phase transitioncould be related with structural water in chlorite minerals.The samples had low apparent porosity ∼1.29%, specific surfacearea, and adsorption average crack width (using the Brunauer-Emmett-Teller method) of ∼0.18 m2 g−1 and ∼7.92 nm,respectively.