CHARACTERISTICS AND GENESIS OF MAGHEMITE IN LOESS-PALEOSOL SEQUENCES OF CHINA

Chinese loess is regarded as one of the best continental archives of paleo-climatic and palaeo-environmental changes of the late Cenozoic era. Even though many publications argue that ultra-fine maghemite is formed during pedogenesis and the most important ferromagnetic mineral according to the studies from magnetism and Mossbauer spectroscopy. However, little information is available concerning direct observation of the nanometer scale maghemite crystals, which limits us to understand the genesis of maghemite and mechanism of susceptibility enhancement in the paleosol layers. Our results indicate that maghemite in loess-paleosol layers were transformed from oxidation of sub-micrometer scale magnetite, and the nanometer scale magnetite was mainly resulted from microbial activities. Very coarse magnetite grains (more than 2 micrometer) were partly oxidized into hematite. Important phenomena have been found that maghemite formed from oxidation of magnetite preserves the magnetite morphology. However, maghemite formed from oxidation of sub-micrometer magnetite grains displays a texture with multi-nanocrystals. The oxidation of sub-micrometer size magnetite into maghemite nanocrystal is responsible for paleosol susceptibility enhancement, because the nanostructured iron oxide minerals have abnormal high susceptibility. This investigation also suggests that nanometer maghemite is more stable than hematite because the maghemite has lower surface energy than that of hematite, according to the results of McHale for surface energies and thermodynamic phase stability in nanocrystalline alumina. The work was supported by the Outstanding Overseas Chinese Scholars Fund of Chinese Academy of Sciences (2003-1-7), National Science Foundation of China (40331001)