机械研磨作用对磁铁矿低温氧化及磁化率的影响

磁铁矿是岩石、土壤和沉积物中少量而又普遍存在的副矿物,是环境磁记录的主要载体.磁铁矿在表生地质过程中受到机械、生物和化学风化作用使其结构、粒径、氧化程度、磁学性质发生一系列变化,这些变化对环境磁学以及表生地质过程的研究具有重要意义.从矿物学角度研究了磁铁矿粒径、相转变、氧化程度和磁化率随着研磨时间的变化规律.结果表明,平均粒径为4 μm的磁铁矿在研磨1 h后粒径变化趋于稳定,但随着研磨时间的增加,磁化率逐步降低而氧化率逐渐增加.通过对研磨后样品进行XRD分析表明,随着研磨时间的增加,磁铁矿衍射峰强度明显减弱,衍射峰宽化,赤铁矿的衍射峰强度逐渐增强.由此可见,原生磁铁矿(对沉积物而言属于碎屑磁铁矿)低温氧化产物是赤铁矿而不是磁赤铁矿.在表生地质演化过程中颗粒的机械碰撞不仅使磁铁矿颗粒变细,而且可能影响到磁铁矿常温氧化、磁学性质和晶体结构.

EFFECTS OF MECHANICAL PULVERIZATION ON THE LOW-TEMPERATURE OXIDIZATION AND MAGNETIC SUSCEPTIBILITY OF MAGNETITE

Magnetite is a minor and ubiquitous subsidiary mineral in rock, soil and sediment, which is the principal contributor to environmental magnetism record. When magnetite experienced mechanical, biological and chemical weathering, a series of changes would take place in its structure, grain size, oxidation degree and magnetic properties in the supergene geological process, which is of important significance in the study of environmental magnetism and supergene geological processes. From the viewpoint of mineralogy, we pulverized magnetite by means of a planet ball mill and studied the trends of changes in grain size, transformation, oxidation degree, magnetic susceptibility under the ball milling. The results indicated there was little change in grain size (~ 4μm) after the magnetite was pulverized for 1 h, magnetic susceptibility decreased while the oxidation degree increased gradually with milling time. The XRD results showed the intensity of magnetite diffraction peak tended to weaken and broaden, meanwhile the intensity of hematite diffraction peak increased gradually with milling time. Therefore, it was found that the protogenesis magnetite was transformed into hematite instead of maghemite under the low temperature oxidation conditions. The grains collided with each other and mechanically pulverized, not only making the magnetite refined, but also affecting the normal temperature oxidation and magnetic properties of magnetite in the supergene process.