In geology, pressure solution is a long-standing problem that has engendered continuing arguments with respect to its mechanism, thermodynamic basis, and geological significance. In the past few decades, pressure solution has drawn renewed attention for its important role in the diagenesis of sedimentary rocks and its relation to rock deformation. Pressure solution has been observed as a fluid migration mechanism, a hydrocarbon accumulation mechanism, and a deformation mechanism under diagenetic conditions.

Professor LI Heping from Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) and his collaborators studied the electrochemical response of pyrite in a FeCl3 solution under differential stress to understand the dissolution kinetics and electrochemical mechanism. The team used polarization curves and electrical impedance spectroscopy measurements examined pyrite pressure dissolution.

For electrically conductive minerals (such as pyrite) in the shallow crust, the researchers found that the minerals’ pressure dissolution process was a galvanic corrosion process when an electrolyte was present. The experimental data revealed that the pyrite potential difference and the stress action had negative a linear relationship. Different transfer coefficients and numbers of electrons transferred disclosed that pyrite had different electrochemical dissolution mechanisms in 0.0010, 0.010, and 0.10 mol/L FeCl3 solutions. Furthermore, increasing the concentration of the FeCl3 solution enhanced pyrite electrochemical dissolution. Besides, the results showed that stress action decreased the charge transfer resistance and the passive resistance. Additionally, the stress action increased the species diffusion capacitance by significantly enhancing the ionic transport and diffusion processes.

The researchers believe that their findings will provide the experimental basis for a pressure solution mechanism of pyrite in the shallow crust and will provide the mechanism by which geo-stress affects metal and acidic pollution arising from metal sulfide mine drainage.

The above research is financially supported by the National Natural Science Foundation of China (No.40803017), and has published in Applied Geohemistry (Qingyou Liu, Yanqing Zhang, Heping Li. Pressure solution of electrically conductive minerals in shallow crust-galvanic processes: A case study from pyrite under differential stress. Applied Geochemistry, 2013, 29: 144-150.).

 

Bode plots and phase angles (a), Nyquist impedance spectra (b) and equivalent circuit (c) for pyrite under differential stresses. （Image by IGCAS）

(By LIU Qingyou)