Magnetism of Al‐substituted magnetite reduced from Al‐hematite

Aluminum‐substituted magnetite (Al‐magnetite) reduced from Al‐substituted hematite or goethite (Al‐hematite or Al‐goethite) is an environmentally important constituent of magnetically enhanced soils. In order to characterize the magnetic properties of Al‐magnetite, two series of Al‐magnetite samples were synthesized through reduction of Al‐hematite by a mixed gas (80% CO2 and 20% CO) at 395°C for 72 h in a quartz tube furnace. Al‐magnetite samples inherited the morphology of their parent Al‐hematite samples, but only those transformed from Al‐hematite synthesized at low temperature possessed surficial micropores, which originated from the release of structural water during heating. Surface micropores could thus serve as a practical fingerprint of fire or other high‐temperature mineralogical alteration processes in natural environments, e.g., shear friction in seismic zones. In addition, Al substitution greatly affects the magnetic properties of Al‐magnetite. For example, coercivity (Bc) increases with increasing Al content and then decreases slightly, while the saturation magnetization (Ms), Curie temperature (Tc), and Verwey transition temperature (Tv) all decrease with increasing Al content due to crystal defect formation and dilution of magnetic ions caused by Al incorporation. Moreover, different trends in the correlation between Tc and Bc can be used to discriminate titanomagnetite from Al‐magnetite, which is likely to be important in environmental and paleomagnetic studies, particularly in soil.