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.rnIn 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.rnBased 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.%赤铁矿在自然界中广泛存在,具有重要的古地磁学和环境磁学意义.土壤中含有大量的杂质离子和分子,在成土作用过程中,这些杂质离子(主要是Al)会进入到赤铁矿的晶体结构中,形成含铝赤铁矿(Al-Hm).为了确定不同的形成环境因素对Al-Hm中所含Al成分的影响,本文通过实验室合成了一系列的Al-Hm,并对各组样品的最终Al含量进行了测试.结果显示,最终样品的Al含量与初始溶液中的Al含量呈线性关系,前者随着后者的增加而增加,这说明环境中的初始Al含量是影响赤铁矿中Al替代量的一个最主要因素,它起到了源的作用.另外在弱酸或者弱碱性的常温环境中利于Al进入晶格结构.赤铁矿中Al的替代量可用来指示土壤中相对的富铝化强度.尽管影响赤铁矿中Al替代量的因素很多,但是不同环境中其影响因素存在很大差异,如黄土-古土壤和南方红土,所以赤铁矿中的Al替代量可作为区分不同来源赤铁矿的有利参数,具有重要的环境气候意义.
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