Hematite is an important antiferromagnetic mineral in nature, with great paleomagnetic and paleoenvironmental significances. Because the soil solution usually contains lots of foreign ions and molecules, then these foreign ions can come into the crystal structure of hematite during the process of pedogenesis. Therefore, hematite forming during pedogenesis is not pure,but with some ion-substitution,typically with aluminum( Al) substitution.In this study, several series of Al-substituted hematite were synthesized in the lab to investigate the influence of different factors on the Al substitution through changing the synthetic conditions. In order to test the purity of these samples,X-ray diffraction ( XRD ) measurement was carried out. Moreover, the atomic absorption spectroscopy was also conducted to obtain the final Al content in hematite. Our data shows that the final Al content of hematite is linearly correlated with the original Al content in solution, which indicates the initial Al content in the environment is the most primary factor. Moreover, the final Al content is higher for samples forming in the solution with pH near to 7 than that of samples growing in strong acidic or alkaline solution. Temperature is also one of factors affecting the final Al content in hematite. It's displayed that for samples formed above 100℃ ,A1 content is rather low.Based on previous results and our investigations,three conclusions can be obtained: (1 )The final Al content in hematite is controlled by lots of factors, where the initial Al content in the environment is a primary factor, which plays a role of source. In addition, the pH and temperature are also two indispensable factors. Al can easily come into the crystal structure in weakly acidic or alkaline environment at room temperature. (2)The relative Al content in natural hematite can be acquired based on the XRD, magnetic, the visible diffuse reflectance spectrum measurements and so on. Then the relative Al content in hematite can be used to indicate the enrichment degree of Al in soil. ( 3 ) Although there are lots of factors affecting the Al content in hematite, a large difference exists for different soils, such as loess-paleosol and red soil in southern China. Therefore, Al substitution in hematite can be used as the proxy to identify hematite from different sources.
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