# Laboratory Measurements of Zeta Potential in Fractured
## 3 Results and Discussion
### Effect of Rock Type and Mineralogy
On the other hand, the zeta potentials in the gneiss sample saturated with low-salinity electrolytes (7.35 \(\times\) 10\({}^{-3}\) M) were found to be more positive compared with both sandstone and carbonate samples, with the exception of \(-\)5 mV recorded with crushed Iceland spar saturated with 0.01 M NaCl mixed with 1 mM CaCl\({}_{2}\) solution [51], and \(-\)4 mV measured in reservoir carbonate sample BA of unknown mineralogy saturated with artificial 0.01 M electrolyte containing Ca\({}^{2+}\) and Mg\({}^{2+}\)[50]. The results with Iceland Spar were obtained at an unconfirmed chemical equilibrium with solutions and showed strong dependence on the concentration of added CaCl\({}_{2}\), so that the measured zeta potential varied between \(-\)3 mV with 5 mM of CaCl\({}_{2}\) and \(-\)14 mV with 0.1 mM of CaCl\({}_{2}\), thus, limiting the applicability of direct comparison between our results and those reported by [PERSON] et al. [51]. In contrast, the results reported by [PERSON] et al. [50] were obtained under equilibrium conditions, but the solution used by the authors contained a considerable amount of Mg\({}^{2+}\) (1 mM), Ca\({}^{2+}\) (0.3 mM) and no SO\({}_{4}^{2-}\) in their 0.01 M solution. It is generally agreed that divalent ions act as potential determining ions (PDIs) for clays (e.g., [53]), so that divalent cations make the zeta potential more positive while divalent anions tend to make the zeta potential more negative. Hence, in absence of SO\({}_{4}\)\({}^{2-}\) in the tested low-salinity solution, the presence of Ca\({}^{2+}\) and Mg\({}^{2+}\) made the zeta potential more positive compared with pure NaCl and seawater in sandstones, other carbonates or gneiss. There have been numerous reported measurements of the zeta potential in carbonate samples (e.g., refer to Figure 3 in [PERSON] et al. [23]) and the values in these studies ranged between \(-\)20 mV and +20 mV for low-salinity (ionic strength below 0.01 M) solutions with pH of 7. However, as discussed by [PERSON] et al. [34], none of these measurements were conducted under conditions of thermo-chemical equilibrium between the tested solutions and rock minerals, hence, these data cannot be directly compared with our results. Moreover, considering the mineralogy of our gneiss sample, we have found a limited number of published studies reporting measurements of the zeta potential of plagioclase [54] including albite [55], muscovite [56] and biotite [57]. None of these studies conducted their electrophoretic mobility measurements under equilibrium conditions, hence, the tested minerals could dissolve during the experiments with single salt aqueous solutions. This situation would be similar to that reported by [PERSON] et al. [39] and [PERSON] et al. [34] for carbonate samples, or the one reported by [PERSON] and [PERSON] [46] for granite, all of which observed varying zeta potential during their experimentsuntil it stabilized at the final equilibrium value after hundreds of hours of equilibration. Therefore, the measured zeta potentials of plagioclase, albite, biotite and muscovite systems could not be compared with our data and between each other in any other way, but qualitatively. Furthermore, it is worth noting that the scarce data available for these minerals cannot be utilized for interpreting the SP signal measured in fractured gneiss aquifers, which are expected to be fully equilibrated with aqueous solutions, thus emphasizing the importance of our laboratory results. The reported laboratory data on mica and feldspar minerals generally suggest that the zeta potentials remain negative across the pH range of interest (6-8) but become progressively smaller in magnitude following the order: \(\left|\zeta_{plagioclase}\right|<\left|\zeta_{altbite}\right|\cong\left|\zeta_{ biotite}\right|<\left|\zeta_{muscovite}\right|\) with the values reported for plagioclase ranging between \(-10\) mV and \(+15\) mV obtained with \(10^{-2}\) M di-sodium tetraborate solution [54].