# Laboratory Measurements of Zeta Potential in Fractured
## 3 Results and Discussion  
### Effect of Rock Type and Mineralogy  
We begin with comparing the zeta potentials measured in the fractured gneiss sample against published data on pure sandstone (>99% quartz), clayey sandstone (2-5% clay content), sand packs (>99% quartz), carbonate (>95% calcite) and granite samples saturated with aqueous solutions of similar ionic strength and composition, and under equilibrium conditions (i.e., rock samples were thoroughly equilibrated with aqueous solutions). The results presented in Figure 2 indicate that the zeta potentials measured with gneiss saturated with high-salinity electrolytes were generally more negative compared with values obtained  
Figure 4: Impact of the confining pressure on the zeta potentials measured on Lewisian gneiss sample saturated with aqueous solutions of 0.007 M (**a**) and 0.7 M (**b**) ionic strength. The dotted horizontal lines demonstrate that the zeta potentials were identical (within the experimental uncertainty) across the salinity range. Figure 3: Impact of ionic strength on the zeta potential measured on Lewisian gneiss sample saturated with (**a**) AGW and ASW, and (**b**) with NaCl solutions. For comparison, empirical linear regressions for clean sandstones (>95% quartz; [31]) and carbonates saturated with equilibrated NaCl solutions [34] are shown in green and black dashed lines, respectively. Filled symbols correspond to the confining pressure of 7 MPa. Green circles represent zeta potentials measured in Berea sandstone saturated with NaCl solutions and averaged over seven samples (right vertical axis); the black circles correspond to results obtained with Berea sandstones saturated with CaCl\({}_{2}\) solutions and averaged over six samples [45]. in carbonates saturated with aqueous solutions of ionic strengths within the range of those tested in this study, and were more positive across the salinity range compared with the data from sand packs, as well as from clean and clayey sandstones. All of our results in Figure 2 are color-coded with red symbols denoting the ionic strength of 0.7 M and the blue symbols corresponding the ionic strength of 7.35 \(\times\) 10\({}^{-3}\) M.  
The individual data points included in the grey and white shaded areas are summarized in Table 2, for which zeta potentials measured in intact and crushed sandstone, carbonate and granite samples have been reported. To the best of our knowledge, there have been no published data on zeta potentials measured with basement rocks (e.g., granite or gneiss) saturated with high ionic strength electrolytes, hence, the corresponding column is populated with N/A (not available). The majority of the data detailed in Table 2 indicate that the zeta potentials of gneiss saturated with high-salinity solutions (0.7 M) are negative and larger in magnitude compared with carbonates, but smaller in magnitude compared with sandstones. However, there are two exceptions, with the zeta potential of \(-\)10 mV measured in Doddington sandstone saturated with 0.5 M NaCl solution, and the zeta potential of \(-\)9 mV measured in Portland limestone saturated with equilibrated 0.5 M NaCl solution, both of which are comparable with the values obtained for the gneiss sample saturated with ASW. We attribute these anomalous reported zeta potentials to a higher clay content in the Doddington sample and quartz inclusion in the Portland limestone, which made these samples' response to 0.5 M NaCl solutions to be dissimilar to that of pure silica or calcite rocks, but comparable with the signal measured with gneiss.