Quantifying the influence of the type and arrangement of conductive phases on the electrical properties of rocks using impedance spectroscopy&#160;
Authors: Mansouri Hadiseh , Toy Virginia , Klimm Kevin , Tholen Sören , Hawemann Friedrich 
Published: March 2024

We have employed impedance spectroscopy to investigate the impact of chemical composition and microstructure on the electrical properties of geological samples. Our study focused on a metapelite sample containing graphite, extracted from a depth of 530 meters in borehole DT-1B as part of the DIVE (Drilling the Ivrea-Verbano zonE) project in Ornavasso, Italy. Additionally, we examined the electrical properties of synthetic mineral assemblages. These were created by combining quartz powders with variable amounts of graphite (1% and 5% by weight), muscovite (5% by weight), and biotite (5% by weight). The goal was to identify which conductive or semiconductive phases predominantly influenced the electrical behavior of the metapelite. Measurements were conducted using a Solartron-1260 Impedance/Gain-Phase Analyzer within a piston cylinder apparatus. The experiments were carried out at a pressure of 500 MPa, temperatures ranging from 22 to 1000 &#176;C, nominally dry conditions, and across a frequency range from 0.1 Hz to 200 kHz.All samples exhibited high electrical resistance (R > 106 &#8486;.m), low electrical conductivity (< 10-6 S/m) and behaved as capacitors, with a phase angle magnitude exceeding 70&#176; for most frequency ranges at temperatures below 200 &#176;C. A representative impedance spectrum (Nyquist plot) illustrates this behavior through a partial semicircular arc originating from the origin. An inverse relationship between electrical conductivity and temperature was observed in almost all samples when temperatures increased from 300 to 500 &#176;C. This phenomenon is attributed to the presence of open grain boundaries in the samples, leading to electrical charge scattering. Notable variations in electrical behavior were observed at temperatures exceeding 600 &#176;C, including a linear increase in electrical conductivity, changes in Nyquist plots such as a reduction in prominence of the &#8216;grain interior arc&#8217; and an increase in significance of the &#8216;grain boundary arc&#8217;, a decrease in sample capacitance, and a significant decline in the phase angle's frequency dependency. Microstructural analysis reveals that these changes were associated with dehydration melting of mica in mica-bearing samples and the growth and interconnection of graphite grains in graphite-bearing samples. Variations in activation enthalpy with temperature suggested that impurity conduction and small polaron hopping played a crucial role at lower temperatures, while the diffusion of H and alkali ions (in mica-bearing samples) or carbon (in graphite-bearing samples) along grain boundaries became significant at higher temperatures.The natural metapelite sample exhibited electrical conductivities similar to the quartz + 5% graphite sample at high temperatures, reaching 10-1.5 S/m at 1000&#176;C. This is comparable to the conductivity levels typically measured by magneto-telluric (MT) surveys in Earth's crust.