Dissolution and oxidation of mudstone under stress

Abstract

Weathering of soft mudstone in nature, characterized by the presence of an oxidized zone and an underlying dissolved zone, was simulated in the laboratory in 15 °C water with a pH of 3–7 and dissolved oxygen contents of 4–16 mg/L under stresses up to 0.49 MPa for as long as 70 days. When the water is acid, hydrogen ions diffuse from the rock–water interface through the rock and attack acid-labile calcite in mudstone preferentially and easily at a well-defined dissolution front, leaving a dissolved zone behind the front. Density decreases, porosity increases, ultrasonic P-wave velocity decreases, and shear strength decreases in the dissolved zone. The dissolution front migrates toward unreacted rock, and the migration rate is controlled by the diffusion of hydrogen ions in the reacted dissolved zone, so the rate is influenced by the stress exerted on the rocks; the effective diffusivity becomes smaller when the rock is stressed. Oxygen also diffuses through the rock to a well-defined oxidation front and changes the rock color from greenish grey to pale brown by oxidation. Slight deterioration occurs with the color change and change in density, P-wave velocity, and porosity. Depletion of pyrite by oxidation and the complete color change take a long time in comparison with the slight color change. Chlorite seems to transform into smectite in acid and highly oxidizing conditions. Key words : mudstone, dissolution, oxidation, weathering.