DISSOLUTION BEHAVIOR OF IRON OXIDE MINERALS IN THE CITRATE-BICARBONATE-DITHIONITE (CBD) EXTRACTION

In Chinese loess-paleosol sequences, the sensitivity and high-resolution of the magnetic susceptibility has been used as a proxy for paleoclimate changes. Although the mechanism of the magnetic susceptibility remains poorly understood, the enhancement of the magnetic susceptibility in the paleosol is mainly caused by superparemagntic/ultrafine-fine magnetic minerals. To investigate which magnetic minerals contribute to the environmental signal in the samples, physical and chemical techniques have been introduced in environmental magnetism studies. The citrate ¨Cbicarbonate-dithionite extraction method is considered as an important technique to determine the amounts of fine-grained pedogenic ferromagnetic minerals with respect to coarse-grained inherited magnetic minerals, in order to distinguish lithogenic mineral (magnetite) from pedogenic magnetic mineral (maghemite). Unfortunately, interpretation of CBD extraction is not straightforward. To investigate the effect of extraction temperature, particle size and iron oxide species on the dissolution behavior of the oxide minerals, natural pure phase minerals (magnetite, hematite and maghemite respectively) were used for CBD extraction experiments at the different temperatures (25°æ, 70°æ respectively), at the similar particle size (>90¦Ìm , 90¦Ìm~75¦Ìm, 75¦Ìm~50¦Ìm, <50¦Ìm, respectively). At the same time, natural and synthetic magnetite and maghemite with different crystals sizes were also used for the extraction experiments (<0.1¦Ìm, 0.1~0.2¦Ìm, 0.3~1.0¦Ìm, respectively). Our results show that a high temperature enhances the dissolution rates for all samples, fine grain size minerals dissolve faster than coarse grain minerals. The dissolution behavior is similar for sub-micron size magnetite and maghemite minerals. Thus the parameters from CBD extraction experiments have a poor influence on the dissolution behavior of magnetite, hematite and goethite. In practice, the CBD extraction results cannot be well compared for natural samples. Therefore, CBD extraction cannot accurately distinguish coarse-grained magnetite and fine-grained magnetite, or distinguish pedogenic ferromagnetic minerals(<1¦Ìm) and lithogenic mineral (>1¦Ìm) in the loess-paleosol layers.