Low-temperature dielectric measurements of confined water in porous granites

Abstract

The dielectric behavior of rocks affected by the known phase transition of supercooled water is the main problem we analyzed. Three different granitic rocks were used to perform dielectric measurements in the frequency range from 100 Hz to 1 MHz and temperatures 100-350 K. Thin cylindrical samples were prepared, and circular electrodes were established using silver conductive paint. A clear change in the dielectric measurements appears at T ∼ 220 K for one of the samples. This coincides with the known phase transition of supercooled water. Tightly bounded water confined in the pores of the rock do not crystallize at 273 K, but form a metastable liquid down to 200-220 K maintaining water polarization. Below this temperature, water molecules solidify and polarizability decreases. The rock presenting the most sizable change has a very low specific surface area of ∼0.09 m 2 g −1 , has connected porosity of ∼1.10%, and has the smallest degree of alteration. In addition, geochemical analyzes reveal a low percentage of hydration water in its structure confirming the role of pore water in this change. A comparison between water-saturated, oven-dried, and vacuum-dried samples was done. Finally, a logarithmic dependency of the critical temperature for the supercooled water phase transition with the measuring frequency was found.

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

“…Nevertheless, in Silva et al (2014), it was not possible to identify which were the minerals that retained the adsorbed water clusters and consequently where the water-rock interaction forces should be stronger. Such forces would probably be a result of hydrogen bond formation between absorbed water clusters and the rock minerals (Osipov, 2012).…”

“…The analysis revealed that those samples had a low percentage of hydration water in their structure, and for that reason it was argued that adsorbed water, at the rock pores, was resistant to drying and should play the dominant role in the relaxation (Silva et al, 2014). Actually, impedance spectroscopy (IS) at the supercooled water phase transition is sensitive to very small amounts of water (Gomes et al, 2006).…”

“…In this context, recently a dielectric relaxation was found at temperatures near the supercooled water phase transition in a granodiorite (Silva et al, 2014), persistent even after a careful drying procedure, but the minerals at which the water was adsorbed could not be identified. The analysis revealed that those samples had a low percentage of hydration water in their structure, and for that reason it was argued that adsorbed water, at the rock pores, was resistant to drying and should play the dominant role in the relaxation (Silva et al, 2014).…”

“…This technique is widely used on material characterization (Graça et al, 2008), but few studies are done on rocks (Silva et al, 2014). Moreover, other dielectric relaxations developed at low temperatures that are of interest for fundamental and Manuscript received by the Editor 4 October 2014; revised manuscript received 4 March 2015; published online 26 May 2015.…”

“…E-mail: ana .vinagre@gmail.com; pmoita@uevora.pt. applied geophysics (Silva et al, 2014) because they can provide valuable information on water-rock interaction at low temperatures. This is of special significance in permafrost-bearing rocks because permafrost damages rock walls causing rock deformation (Krautblatter et al, 2013).…”