中文摘要：

根据前人关于自然燃烧作用生成磁赤铁矿的两阶段模式,模拟自然燃烧作用形成磁赤铁矿的过程和条件。实验分2阶段进行,第一阶段氢气氛围300℃煅烧针铁矿2 h,煅烧产物为纳米磁铁矿;第二阶段70℃空气条件下氧化煅烧成因的纳米磁铁矿70 d。对实验两阶段样品的矿物学和磁学特性进行系统测定,结果表明,本研究的煅烧条件可获得接近理想成分的多孔纳米磁铁矿,晶粒及聚集体的粒径分别在30 nm和57 nm左右;在70℃空气氛围下磁铁矿快速向磁赤铁矿转化,70 d的实验时间里2价铁/全铁比值（Fe2＋/TFe）由初始31.4%降至5.4%;纳米磁铁矿向磁赤铁矿转化伴随着矿物结晶颗粒的减少和样品总体积的增大,磁铁矿结晶学粒径缩小约17%~19%;磁化率和频率磁化率随氧化时间逐渐降低,前者主要受制于矿物物相变化,而后者与矿物粒径变化相关。模拟结果表明,第一阶段的关键是具有生成晶粒尺寸为亚微米-纳米量级的磁铁矿煅烧条件,第二阶段的关键为具备一个合适的温度条件,以能够快速、高效氧化磁铁矿为磁赤铁矿。

英文摘要：

This study designed a two-staged simulating experiment to investigate the role of natural combustion in the formation of maghemite in soil. In the first stage, goethite was calcined at 300 ℃ in hydrogen atmosphere for 2 hours. In the second stage, the calcined product was oxidized at 70 ℃ in the air for 70 days. Mineralogical and magnetic properties of the samples were determined during and after the experiments. Results show that goethite was transformed into pure, porous and nano-sized magnetite under the given calcination conditions. The sizes of magnetite grains and aggregates are about 37 nm and 57 nm, respectively. The nanomagnetite quickly transformed to maghemite. After being oxidized for 70 days, the sample changed the ratio of ferrous Fe to total Fe（Fe2＋/TFe） from the initial 31.4% to 5.4%. The process of magnetite-maghemite transformation was accompanied by the decrease of the mineral crystal size（ca. 17% ~ 19%） and the increase of mass volume. Both the magnetic susceptibility and frequency magnetic susceptibility kept decreasing during the second stage. The decrease of magnetic susceptibility was possibly related to mineral phase transformation, while the decrease of frequency magnetic susceptibility was related to the change of mineral particle size. This study provides experimental supports to the two-staged model of maghemite formation under natural combustion. The study further shows that controlling calcination conditions to generate nano-sized magnetite is the key step in the first stage, while that for the second stage is a suitable transformation temperature.