Maghemite (γ-Fe2O3) is a very common mineral at the earth’s surface and also an important material for making music and video tapes. Maghemite is usually synthesized from magnetite under oxidizing conditions after a few hours or a few days below a temperature of 300°C. The magnetic property of thermal instability and the chemical action after heating is an important character for maghemite. That is, it will become hematite in certain proportion after being heated above 250°C. Maghemite is therefore actually unable to have its Curie temperature measured. But late using synthetic sample, maghemite was further found partially thermal stable with a measurable Curie temperature ~645°C. During our thermally magnetic experiments for a set of synthetic magnetite, we found that extra fined grain size (pseudo single domain (PSD) and small multi-domain (MD), mainly 1-10 μm) magnetite was formed to a completely thermally stable maghemite. This maghemite can also be produced by heating the same powder up to 700°C in an oven and keeping this temperature for 10 min, then cooling it down. When the generated maghemite by these two ways is heated from room temperature to 700°C, it shows almost fully reversible, or thermally stable. We used X-ray powder diffraction and Mssbauer spectroscopy to confirm the identity of this maghemite and compared its magnetic hysteresis, high temperature magnetization, low temperature thermal demagnetization, and low temperature susceptibility with those of the original preheated magnetite. Such quickly oxidized maghemite by heating to high temperature implies some types of maghemite formed in certain natural condition can carry a thermal remnant magnetization (TRM). Four types of maghemite were characterized and discussed according to their thermal stability. Among them, partially stable and fully thermally stable maghemite after heating should possess capability of carrying TRM. There is possibly a compensation of synthetizing maghemite between heating temperature and heating duration. The
The magnetic susceptibility of loess from the Ily Basin,northwestern China shows maximum values in S0 paleosols but minimum values in other paleosols,the mechanism of which has been well debated.In this work,systematic magnetic measurements were made on a representative section from Neleke county.The results show that the loess horizons(L1,L2 and L3) have multi-domain magnetite grains of aeolian origin,S0 is characterized by production of pedogenetic ultrafine-grained ferrimagnetic minerals,and the other paleosols(S1,S2,and S3) are characterized by the formation of nonferrimagnetic minerals associated with waterlogging.The correlation between the low concentration of ferrimagnets,high paramagnetic content,high magnetic coercivity remanence,fine ferrimagnetic grain size and intensified pedogenesis suggest two competing processes of pedogenetic enhancement and pedogenetic depletion in the lower paleosols.Pedogenetic depletion dominates and is responsible for the low susceptibility.Changes in magnetic grain size distribution occur during pedogenetic depletion.The susceptibility variations are of multiple origins in the loess of the Ily Basin.Pedogenetic enhancement,pedogenetic depletion,and allochthonous input of magnetic minerals should all be taken into account to explain the variations of magnetic parameters.
This paper reports a loess-paleosol sequence located in the Danube River basin,Serbia,which formed at least since the latest part of the early Pleistocene,and before the paleomagnetic B/M boundary.Various magnetic parameters of the Serbian V-L1-V-S4 loess-paleosol sequence have been measured and analyzed in the Titel Loess Plateau.These parameters show a very similar magnetic behavior compared with that of the Chinese loess.There is a general positive relationship between magnetic susceptibility() and pedogenesis.The main contributors to are the magnetic grains of SP(superparamagnetic) and SD(single domain) magnetic domains,while MD(multi domain) contributes only a small percentage.The difference in between loess and paleosol mainly is caused by pedogenesis.The very fine magnetic minerals increase gradually with increasing soil development(from loess to soil),and they lead to higher.The thermomagnetic curves show thatmagnetic minerals in the loess layers are magnetite and maghemite,both providing a major contribution to.In contrast the paleosol layers mainly are composed of magnetite,with almost no or a very small amount of maghemite,as implied by a reversible thermomagnetic behavior.This indicates that pedogenic conditions during V-S3 and V-S4 strong soil development have resulted in maghemite that is no longer stable,and has been resolved or converted to other stable phase minerals.This likely indicates that soil moisture during V-S3 and V-S4 development exceeded a critical condition of maghemite stability.
LIU XiuMingLIU ZhiLü BinMARKOVI S BCHEN JiaShengGUO HuiMA MingMingZHAO GuoYongFENG Hua
