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Characteristics and genesis of maghemite in Chinese loess and paleosols: Mechanism for magnetic susceptibility enhancement in paleosols |
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| Authors: | Tianhu Chen Huifang Xu Qiaoqin Xie Jun Chen Junfeng Ji Huayu Lu |
| Institution: | aSchool of Natural Resources and Environment, Hefei University of Technology, Hefei, Anhui 230009, China bDepartment of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA cDepartment of Earth Sciences, Nanjing University, Nanjing 210093, China dState Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, Shaanxi 710075, China |
| Abstract: | Morphological characteristics and microstructures of magnetic minerals extracted from Chinese loess and paleosols were investigated using powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Our results indicate that maghemite in loess–paleosol sequences was transformed from magnetite through oxidation of magnetite. Maghemite transformed from eolian magnetite during chemical weathering has low-angle grain boundaries among maghemite nano-crystals. Some nano-crystalline maghemites with nanoporous texture resulted from microbe-induced precipitation of magnetite or transformation of poorly crystalline ferric Fe (oxy)hydroxides in presence of Fe-reducing bacteria. Aggregates of euhedral maghemite nano-crystals were transformed from magnetite magnetosomes. Both microbe-induced nanoporous magnetite and microbe-produced magnetite magnetosomes are directly related to microbial activities and pedogenesis of the paleosols. It is proposed that the formation of nano-crystalline maghemite with superparamagnetic property in paleosol results in the enhancement of magnetic susceptibility, although the total amount (weight percent) of magnetic minerals in both paleosol and loess units is similar. Our results also show that nano-crystalline and nanoporous magnetite grains prefer to transform into maghemite in semi-arid soil environments instead of hematite, although hematite is a thermodynamically stable phase. This result also indicates that a decrease in crystal size will increase stability of maghemite. It is also inferred that surface energy of maghemite is lower than that of hematite. |
| Keywords: | loess paleosol maghemite biogenic magnetite magnetic susceptibility Transmission Electron Microscopy (TEM) |
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