Non-Equilibrium Effect of Spectral Induced Polarization in Electrically Conductive Minerals: Pyrite versus Pyrrhotite models

Detection of electron-conducting minerals with induced polarization methods is extensively explored for both environmental and mineral exploration applications. Spectral-induced polarization (SIP) is a technique used to determine soil and mineral properties by investigating the complex electrical conductivity over a range of frequencies. Numerous studies have shown a relationship between SIP measurements and conducting mineral grain size, the volume of particles, and pore fluid conductivity. However, the role of fluid chemistry on induced polarization, being controlled by the electrochemistry of the mineral surface requires further investigation. The non-equilibrium effect which can be defined by the elapsed time from initial fluid saturation to reaching equilibrium of the SIP signal can be regarded as a significant parameter related to fluid chemistry. Pyrite and pyrrhotite are abundant iron-sulfide minerals found in the Earth's crust. We conducted laboratory experiments on pyrite and pyrrhotite with the same grain size, volume concentrations, and fluid conductivity. This study observed that while the non-equilibrium effect is not seen on the SIP signature of pyrite, pyrrhotite exhibits a strong time-dependent SIP signature associated with non-equilibrium conditions. Over 50 hours of consecutive measurements on the pyrrhotite sample demonstrated that the phase peak decreased from about 22 mrad to approximately 13 mrad at a constant temperature. Furthermore, while the pyrrhotite SIP model shows that phase shift values are increasing at low frequencies, the values dropped at high frequencies. Therefore, the initial shape of the phase shift spectrum model is quite different from the equilibrium spectrum model. It is considered that this non-equilibrium effect can be related to not only fluid chemistry in the sample but also the electrochemistry of the mineral interface.