text: Oxidizing conditions are common in some environments of deposition and diagenesis of sedimentary rocks. The fugacity of oxygen at the MH buffer (magnetite-hematite) is only about 10 at 25 °C, but it is about 0.2 atmospheres in the Earth's atmosphere, so some sedimentary environments are far more oxidizing than those in magmas. Other sedimentary environments, such as the environments for formation of black shale, are relatively reducing.