In order to make up for the deficiency of XRD analysis, people have done a lot of Mossbauer spectroscopic studies on fine and ultrafine iron oxide minerals in soil, weathering crust and surface sediments. For example, Child( 1979) has studied 8 kinds of red soil and 8 kinds of yellow soil or brown soil, but no crystalline iron oxide minerals can be seen by XRD, and hematite is found in red soil by Mossbauer spectrum. Xu Li et al. (199 1) also found that there were only hematite diffraction peaks in XRD analysis, but maghemite and hematite were very obvious in Mossbauer spectrum. However, it is a great pity that their research failed to obtain maghemite samples for XRD research, and failed to conduct transmission electron microscopy research.
The experiment of Mossbauer spectrum in this paper was completed in the Mossbauer effect laboratory of physics department of Beijing Normal University. With the help and advice of Professor Li Zhe, he has a deep understanding of ssbauer's spectral research methods. M-500 Mossbauer spectrometer (combined with Canbera35- 1024 multichannel analyzer) was used to test the Mossbauer spectra of four red weathering crust sections at room temperature. The radioactive source is ~ 50 MCI 57 Co (PD), the detector is a xenon (methane) proportional counter, and the spectrometer is calibrated with iron foil (α-Fe).
Table 3-3 gives the room temperature Mossbauer parameters of red weathering crust samples fitted by computer least square method (MOSFUN program), and Figure 3-7 gives the room temperature Mossbauer spectra of representative samples. For comparison, the Mossbauer parameters of known minerals α-Fe2O3 and γ-Fe2O3 are also measured in this experiment, which are listed in Table 3-3. Except for sample 93ZC-2 (paddy soil), the room temperature of other samples is m? Mossbauer spectra contain two groups of magnetic hexagonal spectra, among which the isomeric shifts of the first group of magnetic hexagonal spectra are = 0.35 ~ 0.38 mm/s, QS =-0. 17 ~-0.22 mm/s, and H = 504 ~ 5 17 koe. According to the m of known minerals? According to ssbauer parameters and TEM observation, it is formed by hematite, and the decrease of internal magnetic field intensity (H) is mainly caused by isomorphic replacement of aluminum [α-(Fe, Al) 2O 3] in hematite widely existing in red weathering crust. What is the m of the second magnetic hexagram spectrum? The parameters of ssbauer are IS = 0.32 ~ 0.37mm/s, QS =-0.06 ~-0.09 mm/s, and H = 472 ~ 4,965,438+0 keV, which is formed by maghemite (γ-Fe2O3) in red weathering crust. Although no scholars have studied the magnetic field intensity of the internal magnetic field after replacing iron ions with aluminum in maghemite, it is certain that the magnetic field intensity of the internal magnetic field will decrease after replacing magnetic ions with nonmagnetic aluminum ions in maghemite. M of maghemite in the topsoil of red weathering crust? The area percentage of Mossbauer spectrum peak is 6% ~ 10%, which quantitatively reflects the content of maghemite in red weathering crust. Mossbauer spectra of all the above samples also contain quadrupole peaks, and their isomeric shifts and quadrupole splitting are IS = 0.34 ~ 0.35 mm/s and QS =-0.5 1 ~-0.54 mm/s, respectively. According to the Mossbauer parameter characteristics of goethite (Stevens et al., 1983), this double peak is formed by superparamagnetic goethite [α-(Fe, Al)OOH] and Fe3+ ions in clay minerals, and IS= 1. 10 mm/s, QS =.
Table 3-3 Mossbauer Spectral Parameters of Some Red Weathered Crust Soil Samples
Figure 3-7 Room temperature m of dolomite red weathering crust soil sample? Mossbauer spectrum