Magnetism of Al‐substituted magnetite reduced from Al‐hematite

Abstract

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.

An Integrated Study of the Eolian Dust in Pelagic Sediments From the North Pacific Ocean Based on Environmental Magnetism, Transmission Electron Microscopy, and Diffuse Reflectance Spectroscopy

“…Previous studies have demonstrated that high‐Ti magnetite is the dominant iron oxide in tephra from Kamchatka (Braitseva et al, ; Ponomareva et al, ), the Aleutian Arc (Myers et al, ; Nye & Turner, ; Romick et al, ), and in Okhotsk Sea sediments (Wang et al, ). In addition, cation substitution in the magnetite crystal lattice may increase coercivity significantly (Akimoto, ; Jiang et al, ; Liu et al, ). Abundant high‐coercivity volcanogenic magnetite may disturb the original coercivity distribution, so that HIRM may not indicate the concentration of antiferromagnetic minerals in volcanic ash layers.…”

A Mineral Magnetic Approach to Determine Paleo‐Firing Temperatures in the Neolithic Settlement Site of Mursalevo‐Deveboaz (SW Bulgaria)

“…As revealed by the diagnostic magnetic experiments (Figures and and Table ), the main iron oxide phases identified in the burnt daubs from Mursalevo‐Deveboaz are magnetite/maghemite and hematite, found in a varying proportion in differently colored samples. A strongly magnetic phase dominating the magnetic susceptibility, magnetization (Figure ), and isothermal remanence (Figure ) may be linked either to the presence of magnetite with certain foreign‐ion substitutions (Jiang et al, ) or to maghemite with aluminum substitutions (Dunlop & Ӧzdemir, ; Özdemir & Banerjee, ) which is thermally stable against conversion to hematite. Considering the results from the thermomagnetic analysis and the hysteresis parameters (Figure and Table ), it could be supposed that the magnetically softer phase in the purple‐colored burnt daub is most probably magnetite of stable single‐domain (SD) to pseudo single domain state, taking into account the relatively narrow drop in χ close to the T c , in combination with high saturation magnetization ( M s ) values and the highest coercivities ( B c and B cr ) as compared to the other samples (Table ).…”

Control of Earth-like magnetic fields on the transformation of ferrihydrite to hematite and goethite

“…If these samples were preserved in short-lived periods with reducing conditions, they may be reduced to magnetite but inherit the same shape. For example, in the presence of iron-reducing bacteria and organic matter, hematite in well-drained soils can be transformed to magnetite through fermentation 53 54 or be reduced to magnetite during burning and maintain its morphology, as discussed by Jiang et al 55 . This kind of abiotic chain-like magnetite may confuse magnetofossil identification, and would lead to erroneous paleoenvironmental interpretation 56 .…”