Effect of high temperatures on the magnetic susceptibility of loess

Coal fires refer to fires generated by the self-combustion of coal seams, which have a maximum temperature of 1000 °C. Such fires can significantly alter the properties of surrounding rock and soil. However, in Northern Shaanxi, China, coal resources are abundant, and the buried depth of coal seams is shallow; therefore, coal fires often occur. Because Northern Shaanxi is located in China's Loess Plateau, there is a large amount of loess accumulation on the surface. For this reason, it is imperative to study the effects of high temperatures on loess. In this paper, the loess samples are heat treated at different temperatures (200-1000 °C), and their surface redness (a*) and mass magnetic susceptibility (χ) are measured to simulate the high-temperature effects of coal fires on the loess in northern Shaanxi, and also study its chromaticity changes and abnormal magnetic properties. The results show that (1) the loess χ value is affected by the heat-treatment temperature; at 600 °C, the χ value decreases, increasing rapidly above after 850 °C. Further, the magnetic susceptibility changes significantly. (2) The thermal-induced transformation of ferromagnetic minerals and the buried depth of loess are the main reasons for these changes in magnetic susceptibility and red chroma of loess.