Variation of the mass susceptibility of sandstone after high-temperature heating

Burnt rocks usually exhibit magnetic enhancement, which can be used to detect the spontaneous combustion zone of a coal fire or locate the resources in burnt rocks. To elucidate the mechanisms underlying magnetic enhancement, sandstone is heated to high temperatures between 100 °C and 1200 °C. The mass susceptibility of the sandstone is measured after cooling, and the magnetic characteristics are explained by the changes in color and porosity, thermogravimetric and differential thermal analysis experiments, and the observation of thin sections of rocks and SEM images. The findings are as follows: (1) The magnetic susceptibility of sandstone increases rapidly by two orders of magnitude as the temperature rises above 800 °C. (2) The oxidation of iron ions and the formation of magnetite enhance the magnetic susceptibility of sandstone. The formation of magnetite at 800 °C significantly enhances the magnetic susceptibility. (3) The highest temperature experienced by a burnt rock can be evaluated by analyzing the different responses of the susceptibility of sandstone to temperature. Furthermore, it is possible to use the magnetic anomaly of the burnt rock to find the burnt rock and develop and utilize the beneficial resource in the burnt rock.