Products of Magnetite Oxidation in Chinese Loess-Paleosol Sequences: Role of Crystal Size on Phase Transformation of Magnetite

The “stable” phase of polymorphs also depend upon their crystal sizes and morphology. Our study from loess-palesol sequences in Chinese Loess Plateau confirms that size-dependent structural stability occurs in natural Fe2O3 system. Morphological characteristics, microstructures and mineral composition of the magnetic minerals extracted from Chinese loess and paleosols were investigated using powder X-ray diffraction (XRD), field emission scanning electron microscope and high-resolution transmission electron microscope (HRTEM). Geneses of magnetic grains were identified according to their morphology, compositions and textures. Mineral phases of magnetic grains were identified according to XRD pattern and lattice fringe from HRTEM analyses. Our results indicate that micron eolian magnetite was partly oxidized into hematite before or/and after deposit, and sub-micrometer and nanometer size magnetites were completely oxidized into maghemite during pedogenic process, which indicates that products of magnetite oxidation in Chinese loess-paleosol was constrained by magnetite grain sizes. In addition, maghemite transformed from eolian submicron and nanometer magnetite experienced different pathways and display different textures of the oxidized products. Maghemite transformed from eolian magnetite during chemical weathering display multidomain of nano-crystals. However, maghemite transformed from nanometer magnetite preserves morphology of precursor magnetite, which indicates a topological phase transformation. The results show that magnetite nano-crystals prefer to transform into maghemite in the loess environment instead of hematite, although hematite is the thermodynamically stable phase. This result indicates that decreasing in crystal size will increase stability of maghemite and enhance phase transformation from magnetite to maghemite.