湿润亚热带花岗岩风化壳发育红壤中不同类型磁赤铁矿及其成因

本研究对福建省武夷山市发育于花岗岩风化壳上的亚热带红壤剖面进行了系统的热磁和常温磁学测量，剖析该剖面磁学性质与成土因素（气候和母质）的关系，探讨2种类型磁赤铁矿的成因。研究结果表明：该剖面具有较高的亚铁磁性矿物含量，磁颗粒以多畴（MD）颗粒为主。土壤发育过程中，成土作用使磁颗粒变细、单畴（SD）颗粒增加、亚铁磁性矿物含量降低使得磁化率减小。红壤剖面中至少存在2种不同类型的磁赤铁矿：磁赤铁矿A，在300~450℃即转化为赤铁矿，不能测得居里点；磁赤铁矿B，在650℃以上才较为完全地转化为赤铁矿，可测得645℃的居里点。磁赤铁矿A是水铁矿转化为赤铁矿的中间产物，该化学反应过程在氧化环境下广泛存在，是被普遍认识的一类磁赤铁矿。土壤淀积层、淀积-母质过渡层、母质层的主导磁性矿物磁赤铁矿B由含铁硅酸盐矿物经风化成土作用演化而成，而非原生磁铁矿低温氧化；磁赤铁矿B形成与气候条件（较高的气温）和母岩/母质物质组成有关，两者缺一不可。

In order to explore the magnetic characteristics of the red soil section in the humid subtropical region and the relationship between its magnetic properties and soil forming factors(climate and parent material), this study selected a Wuyishan red soil section(WYS section) derived from granite weathering crust. The section was located in Wuyishan City, northern Fujian Province, with geographic coordinates of 27°37'55.67″N and 117°59'40.59″E, it belonged to a humid climate zone of mid-subtropical monsoon. The multi-year average temperature in this area was 18.3 ℃, and the multi-year average precipitation was 1926.8 mm. The section was 4.5 m thick, and could be divided into 4 layers from top to bottom:Horizon A(leached horizon, 0~20 cm), horizon B(illuvial horizon, 20~230 cm), horizon BC(B~C transition layer, 230~330 cm), horizon C(parent material horizon). After removing the surface floating soil, 46 soil samples were collected at intervals of 10 cm; In addition, one semi-weathering parent material sample and one weakly weathering parent rock sample were collected in the outcrop area within 2 meters of the section. From the comparison of the degree of weathering:Soil samples(horizon A, horizon B, horizon BC and horizon C)>semi-weathering parent material samples>weakly weathering parent rock samples. This study carried out detailed room temperature magnetic parameters measurement on all samples, conducted thermomagnetic analysis on representative samples, and performed sample magnetic separation at equal intervals of 20 cm. System room temperature magnetic parameters were also performed on the remaining samples after magnetic separation. The following results were obtained:The overall magnetic susceptibility of the WYS section was relatively high, and the content of magnetic minerals was relatively high. The magnetic particles in the horizon BC and horizon C of the section were mainly multi-domain particles. With the decreased of depth, the magnetic particles became finer and single-domain particles increase. There were almost no superparamagnetic particles. The M-T curve and κ-T curve(in air environment and argon environment) of representative samples of horizon A, horizon B, horizon BC and horizon C of this study showed a rapid decrease of magnetization/susceptibility in 300~450 ℃, this was the signal that maghemite was converted to hematite, indicating that the sample contained thermally unstable maghemite A. The heating curves of the M-T curves of representative samples of the horizon B, horizon BC, and horizon C showed an obvious Curie point of 645 ℃, and most of the magnetization was lost after heating, indicating that these samples contained thermally unstable maghemite B. There might be a small amount of maghemite C in the sample at 2.8 m. The research has the following conclusions:There are two different types of maghemite in this red soil section:Maghemite A, which is converted into hematite at 300~450 ℃, and the Curie point cannot be measured; Maghemite B, which is only quite completely converted into hematite above 650 ℃, and the Curie point is 645 ℃. Maghemite A in WYS section is an intermediate product of the conversion of ferrihydrite to hematite. This chemical reaction process is widespread in an oxidizing soil environment. The dominant magnetic mineral maghemite B in the soil horizon B, horizon BC, and horizon C is evolved from iron-containing silicate minerals through weathering and pedogenesis, rather than low-temperature oxidation of lithogenic magnetite; Its formation is related to climatic conditions and the composition of the parent rock/parent material, both of which are indispensable.