Magnetite Mossbauer

A number of ferrites have been subjected to shock modification and studied with x-ray diffraction as well as static magnetization and Mossbauer spectroscopy [87V01], Studies were carried out on cobalt, nickel, and copper ferrites as well as magnetite (iron ferrite). 

Fig. 3.16 Schematic drawing of the MIMOS II Mossbauer spectrometer. The position of the loudspeaker type velocity transducer to which both the reference and main Co/Rh sources are attached is shown. The room temperature transmission spectrum for a prototype internal reference standard shows the peaks corresponding to hematite (a-Fe203), a-Fe, and magnetite (Fe304). The internal reference standards for MIMOS II flight units are hematite, magnetite, and metallic iron. The backscatter spectrum for magnetite (from the external CCT (Compositional Calibration Target) on the rover) is also shown...

Fig. 3.22 Backscatter MIMOS II spectra collected in eight temperature intervals on the CCT target (magnetite rock) during a simulated overnight Mossbauer experiment on Mars...

During the mission, the magnetite CCT was measured in several runs to verify the functionality of MIMOS II. The well-known Mossbauer parameters of magnetite were used for velocity calibration, as shown in Fig. 3.22 for different temperatures. This kind of measurement was done in the laboratory with the flight units as a function of temperature to be used as a reference for the measurements on Mars. Figure 3.22 shows the Mossbauer spectra of the CCT at different mean temperatures. 

Fig. 8.33 Left, robotic arm with MIMOS II positioned on the rock Adirondack, as seen by the navigation camera of the rover Right. Mossbauer Spectrum (14.4 keV temperature range 220-280 K) of the rock Adirondack at Spirit landing side Gusev Crater, plains. The data were taken at the as-is dusty surface (not yet brushed). The spectrum shows an olivine-basalt composition, typical for soil and rocks in Gusev plains, consisting of the minerals olivine, pyroxene, an Fe doublet, and nonstoichiometric magnetite...

Two sextets assigned to the A and B sites, can be fitted to a broadened Mossbauer spectrum. For maghemite formed via reduction of hematite to magnetite, followed... 

The Mossbauer spectrum of paramagnetic magnetite consists of one broad line which can be resolved into a component for the cubic A sites and a quadrupole split doublet for the B sites. Between the Gurie and Verwey temperatures (see Ghap. 6), magnetite is ferrimagnetic and the Mossbauer spectrum can be fitted with two sextets (Fig. 7.7),... 

The kinetics of a similar oxidation reaction was studied by Pritchard and Dobson (236). These authors studied the oxidation between 450 and 560 K of a metallic-iron foil (0.02 mm thick electroplated with 1 mg cm 2 57Fe) by deoxygenated water. The resulting Mossbauer spectra (at room temperature) showed Fe304 to be the only detectable reaction product, and from the ratio of the Fe304 spectral area to that of metallic iron, the magnetite film thickness y can be calculated. Assuming that the rate law is of the form... 

In general, as discussed earlier, the chemical properties of small particles may be different from the properties of the corresponding bulk samples. An investigation of this effect in oxidation reactions was made by Tops0e et al. (237) in the study of the room temperature oxidation of Fe304 particles 40 nm in size. The Mossbauer spectra of several partially oxidized samples and that of magnetite are shown in Fig. 37, in which it can be seen that oxidation is reflected in the ratio S of octahedral to tetrahedral spectral areas (see Section III, A, 2). Specifically, the value of S for stoichiometric magnetite... 

Flo. 37. Mossbauer spectra of small Fe304 particles exposed to air at room temperature and after C02/C0 treatment at 700 K. (a) and (b) are room temperature Mossbauer spectra of nonsupported magnetites exposed to air for 80 days (c) is sample (b) treated at 700 K with a C02/C0 mixture. Spectrum at room temperature. Zero velocity is with respect to a 57Co in copper source. Figure according to Tops0c el al. (237). 

After performing FT synthesis on an unreduced iron oxide catalyst, Kuivila et al.12 observed 22% carbide in the bulk by Mossbauer spectroscopy, but only —3% carbide on the surface by XPS, and therefore concluded that a sub-surface carbide phase had formed beneath a magnetite surface layer. Based in part on this result, they conclude that magnetite is the active phase for FT synthesis. Reymond et a/.10 also observed substantial amounts of carbide by XRD, but little or no carbide by XPS. The observation of a 2-4 nm thick carbon layer on the carbide phase, but not on the magnetite, allows a reinterpretation of the data in these two papers. Sputtering of the surface carbon layer permits the XPS to see the underlying carbide, and therefore it is not necessary that the carbide be present beneath an oxide layer. Thus, measurement of low carbide signals by XPS cannot be interpreted to mean that carbide is absent from the catalyst surface, and therefore not an important phase in FT... 

Opacity of mixed-valence minerals. The opacities of many end-member Fe2+-Fe3+ oxide and silicate minerals result from electron hopping between neighbouring cations when they are located in infinite chains or bands of edge-shared octahedra in the crystal structures. Opaque minerals such as magnetite, ilvaite, deerite, cronstedtite, riebeckite and laihunite owe their relatively high electrical conductivities to thermally activated electron delocalization, contributing to intermediate valence states of iron cations which may be detected by Mossbauer spectroscopy. 

This method is applied for the determination of the amount of magnetite present in the natural zeolite rocks C2 and C4 (see Table 4.1). The magnetite present in this zeolite was magnetically separated from the grounded samples and identified by XRD and Mossbauer spectrometry [44],... 

The application of Mossbauer studies to magnetic materials is well illustrated by the spectra of magnetite (Fe304) shown in Fig. 2.48. Although this ferrimagnetically ordered material is an inverse spinel with nominally tetrahedral Fe + and both Fe and Fe on octahedral sites, at temperatures above 119 K (the Verwey transition temperature) only... 

Fig. 2.48. The Fe Mossbauer spectra of magnetite (Fe304) at temperatures (a) above and (b) below the Verwey transition (after Sawatzky et al., 1969).

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