Magnetite Mossbauer spectra

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),... 

Additionally, it was proposed that reaction 19 occurred only in the outermost 3-4 atomic layers of the magnetite crystallites. The MOssbauer spectrum of the catalyst in the reduced form agreed with this substitution. The spectral parameters of the tetrahedral cations were unaffected by the substitution, whereas the isomer shift and magnetic hyperfine field of the octahedral cations decreased. Also, the line width of the octahedral cations increased relative to an unsubstituted catalyst. Finally, the spectral area ratio of the iron cations in the tetrahedral to octahedral sublattices decreased. 

Figure 6. Mossbauer spectrum of an iron-rich muscovite from a western Maine metapelite (data from Guidotti et al. 1994). This samples contains 82% of its total Fe as Fe. Such a large amount of Fe might be expected because this muscovite coexists with both magnetite and hematite. Spectrum is newly collected data on the same mounts used by Guidotti et al. (1994). Data are of relatively poorer quahty because there is far less total Fe in muscovite.

Below the Verwey transition, at about 120 K, the Mossbauer spectrum of magnetite can be properly decomposed into five components (Table 20.2), one related to Fe ions on the tetrahedral sites and four corresponding to Fe and Fe " " ions on two nonequivalent octahedral sites [59,60]. This methodology was successfully used to analyze the MS at 77 K of a magnetite as a corrosion product [61,62]. 

Figure 13. Room temperature Mossbauer spectrum of magnetite subspecira of lattice sites A and B are indicated by the stick diagram...

The statistical distribution of the di- and triva-lent ions of iron on equivalent lattice sites (B) explains many unusual properties of magnetite, in particular its extremely high electrical conductivity comparable to that of metals. Investigations have shown that the conductivity is caused by electron and not by ion transport. A fast electron-transfer process (electron hopping) between ferrous and ferric ions on the octahedral B-sites takes place [27], The Mossbauer spectrum of magnetite at room temperature (cf. Figure 13) confirms this interpretation. 

Figure 17 shows the Mossbauer spectrum of a one dollar bill. For the measurement the note was folded in such a manner that approximately 10 layers of the bill were analyzed. The result of the analysis the banknote contains about 20 wt-% of magnetite and 80 wt-% of goethile as iron containing color pigments. 

Natural magnetite may also occur with a small particle morphology yielding a Mossbauer spectrum with asymmetrically shaped lines. Due to the increased overlap of the lines of both sextets in that case, it becomes difficult to determine the ratio R = S(Fe )/S(Fe ) accurately. 

Maghemite, which is a fully oxidized form of magnetite, has the structural formula (Fe +) [Fej 3ni/3]g04 where represents again the vacancies on the octahedral sites. The corresponding Mossbauer spectrum consists of a somewhat broad-lined... 

Figure 4.69 Mossbauer spectrum of sample 3.1 A red) and B dark red) sites in magnetite, as weU as goethite pink) and hydroxide green) were identified...

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...

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). 

In another study, the effect of silica incorporation into the Fe O lattice was studied (5,49,50) A 20% Fe O on silica catalyst was prepared using conventional techniques. Ir was found that while direct oxidation of the catalyst at 800 K produced the expected a -catalyst was previously reduced in CO/CO2 to produce magnetite, then subsequent oxidation resulted in the formation of y-Fe203 Figure 10 shows MOssbauer spectra of this catalyst after various thermal treatments. In these spectra, the central doublets were demonstrated to be a result of small iron oxide particles which were superparamagnetic at the conditions where the spectrum was recorded. The suppression of the y - oOg to a -Fe203 transition is characteristic of the substitution of roreign cations into the mag-... 

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