Formation mechanism of ferromagnetic minerals in loess of China: TEM investigation

摘要

The results of TEM investigation indicate that magnetite and maghemitc arc the major ferromagnetic minerals in loess-paleosol sequences. Primary magnetite has the similar morphology and surface characteristics as colian detrital particles. The magnetite can be classified into two categories, high-titanium and low-titanium, which may be the indicators of magmatic rocks and metamorph if rocks, respectively. TEM investigation at nanometer scale shows that primary detrital magnetite of micron scale hadbeen partially weathered to maghemite of 5-20 nanometer during the pedogenic process, which maintain the pseudomorphism of the aeolian dehris. Some chlorite particles were also weathered to nanometer scale magnetite or maghemite in the pedogenic process. So weathering of the two minerals leads to formation of superparamagnetism, which may be the important mechanism of magnetic-susceptibility increase in paleosols. The magnetite or maghemite resulting from the weathering of chlorite contains a small amount of P and S, which is the signal of microbe-mineral interaction, and indicates that microbes may play a certain role in chlorite weathering and formation of superparamagnetic particles.