G. M. da Costa

Mössbauer characterization of iron oxides and (oxy)hydroxides: the present state of the art

Mössbauer spectroscopy is a powerful direct technique for the identification and quantification of iron oxides and (oxy)hydroxides in soils and sediments. However, further characterization with respect to structural properties such as crystallinity, Al substitution, stoichiometry, water content, etc. is rather limited. With some examples of synthetic and natural goethite and hematite sample series it is illustrated that the hyperfine parameters depend on much more structural features than the Al content and crystallinity alone. Neither the Morin transition in hematite nor the Verwey transition in magnetite is directly applicable for analytical purposes in natural samples.

Influence of nonstoichiometry and the presence of maghemite on the Mossbauer spectrum of magnetite

Several samples of large- and small-particle magnetite (Fe3O4), as well as its thermal decomposition products formed at different temperatures and atmospheres, have been studied extensively by Mössbauer spectroscopy (MS), both with and without an applied field of 6T. Synthetic mixtures of magnetite and poorly- or well-crystallized maghemite have also been studied. Large-particle magnetite (MCD > 200 nm), when heated in air for 12 hours at T < 400°C, transforms to a mixture of well-crystallized hematite and magnetite, the latter one remaining stoichiometric, according to the relative area-ratios obtained from MS. Thermal treatment at 1300°C in a controlled O2 partial pressure, produced a mixture of stoichiometric and nonstoichiometric magnetite, but the latter component seems to be composed of particles with different degrees of nonstoichiometry. The Mössbauer spectra of the decomposition products at T < 200°C in air of small-particle magnetite (MCD ~ 80 nm) could be successfully interpreted as a mixture of magnetite and maghemite, rather than nonstoichiometric magnetite. This suggestion is further supported by the experiments with the synthetic mixtures. It is clearly demonstrated that is not possible, even by applying a strong external field, to separate the contribution of the A-site of magnetite from that of maghemite.

Mössbauer studies of magnetite and Al-substituted maghemites

This paper reviews a systematic Mössbauer study of maghemite γ-Fe2O3 and Al-substituted maghemites γ-(Fe1−yAly)2O3. Three series of samples prepared from different methods and having different morphological characteristics and aluminum contents were investigated. It was found that both the cation distribution and the solubility limit depend on the preparation method, and no general conclusion in that respect could be inferred. From the temperature dependence of the hyperfine fields the exchange integrals could be calculated, and were found as: JAB= −25 K, JAA= −18 K, and JBB= −3 K. The hyperfine fields show a crossing in the vicinity of 300 K, for both substituted and unsubstituted samples, as a result of the relatively strong A–A interaction. The Curie temperatures were found to be in the range of 948–730 K, the lower value referring to the sample containing 22 mole% Al. The influence of maghemite on the Mössbauer spectra (MS) of magnetite was explored in some detail. It was demonstrated on the basis of the MS recorded for a variety of reference mixtures, that it is not possible to resolve the ferric A-site components due to maghemite and magnetite, even with the absorber subjected to a strong external field.

About the Morin transition in hematite in relation with particle size and aluminium substitution

The results of a Mössbauer study of the Morin transition behaviour in three series of hematite and Al-hematite samples are reviewed and discussed. The first two series comprise small-particle hematites and Al-hematites prepared from decomposition of lepidocrocite, whereas in the third series Al-hematites up to the µm range are obtained from co-precipitated oxinates. It is demonstrated that the Morin transition temperature follows quite well the overall properties of the samples such as particle size and Al substitution, while the transition region is rather determined by all kind of distributive effects. A model involving intermediate states is suggested for the Morin transition behaviour in non-ideal hematite.

IRON OXIDES IN A SOIL DEVELOPED FROM BASALT

A dusky red Oxisol forming on a tholeiitic basalt is found to contain varying proportion of aluminous hematite (Hm) and titanoaluminous maghemite (Mh) in the different size fractions. Maghemite is the main iron oxide in the sand and silt fractions whereas Hm is dominant in the clay fraction, together with gibbsite (Gb), kaolinite (Ka), rutile (Rt) (and probably anatase, An) and Mh. Maghemite is also the major oxide mineral in the magnetic separates of soil fractions (sand, about 65% of the relative Mössbauer spectral area; silt, 60%). Hematite (sand, 30%; silt, 15%) and ilmenite (Im) (sand, 5%; silt, 16%) are also significantly present in the magnetic extract. Accessory minerals are Rt and An. No magnetite (Mt) was detected in any soil fraction. Sand- and silt-size Mh have similar nature (a0= 0.8319 ± 0.0005 nm; about 8 mol% of Al substitution; saturation magnetization of 49 J T−1 kg−1), and certainly a common origin. Lattice parameters of clay-Mh are more difficult to deduce, as magnetic separation was ineffective in removing nonmagnetic phases. Al content in Hm varies from 14 mol% (clay and silt) to 20 mol% (sand). The proposed cation distribution on the spinel sites of the sand-size Mh is:

Rock paintings from Minas Gerais, Brasil, investigated by in-situ Mössbauer spectroscopy

\rm{[Fe_{0.92}Al_{0.08}] {Fe_{1.43}Ti_{0.18}\square_{0.39}}O_4}

CHARACTERISATION OF SOIL-OXIDE ANALOGS BY APPLIED-FIELD 57FE MOSSBAUER SPECTROSCOPY

[Fe0.92Al0.08]Fe1.43Ti0.18◻0.39O4(◻ = vacancy, [ ] = tetrahedral sites and { } = octahedral sites), with a corresponding molar mass of 208.8 g mol−1. The predicted magnetization based on this formula is σ ≅ 68 J T−1 kg−1, assuming collinear spin arrangement. The large discrepancy with the experimentally determined magnetization is discussed.

Mössbauer spectroscopic study of synthetic leucophosphite, KFe2(PO4)2(OH)·2H2O

Seven Al-containing lepidocrocite samples, γ-Fe1−xAlxOOH, prepared from FeCl2/Al(N03)3 solutions with initial Al/(Al + Fe) mole ratios Ci of 0.0025, 0.01, 0.025, 0.05, 0.075, 0.10 and 0.15 mol/mol, were examined by means of Mössbauer spectroscopy at room temperature (RT) and at various temperatures in the range of 8 to 80 K. The spectra at RT and 80°K consist of broadened quadrupole doublets and were analyzed in terms of a single doublet and of a model-independent quadrupole-splitting distribution, the latter yielding the best fit. The observed variations of the quadrupole-splitting parameters with increasing Ci are inconclusive as to whether the Al cations are substituting into the structure. The temperature at which the onset of magnetic ordering is reflected in the spectra, was measured by the thermoscan method with zero source velocity. A gradual shift from 50 K for Ci = 0.0025 mol/mol to 44 K for Ci = 0.10 mol/mol was observed for that temperature. As compared to earlier studies of Al-free γ-FeOOH samples with similar morphological characteristics, the fractional doublet area in the mixed sextet-doublet spectra at 35 K is significantly higher for the present lepidocrocites. This observation is ascribed to the substitution of Al cations into the lepidocrocite structure. A similar conclusion is inferred from the variation with Ci of the maximum-probability hyperfine field derived from the spectra recorded at 8 K and fitted with a model-independent hyperfine-field distribution. The magnetic results suggest that for the sample corresponding to Ci = 0.15 mol/mol, not all of the initially present Al has been incorporated into the structure.