H. Pollak

Documentation and evaluation of Mössbauer data for minerals

More than 2,500 Mössbauer spectroscopic studies on minerals have been published since 1960. These papers contain approximately 8,000 sets of Mössbauer mineral data on at least 400 different minerals. This information has been compiled into a database that includes isomer shifts, quadrupole splittings, and magnetic hyperfine interactions at room, liquid nitrogen, and liquid helium temperatures. The database provides a profile of the Mössbauer research performed to date on minerals, including type and locality of minerals investigated, significance of the studies and results, and location of the research facilities. The data and information are available both in printed and magnetic form.With the establishment of this resource, the Mössbauer Effect Data Center has begun a systematic evaluation of the Mössbauer mineral data. It is anticipated that this resource can be used for the identification of minerals as powder X-ray diffraction data is.

ESR and Mössbauer studies on detoxified crocidolite: Mechanism of reduced toxicity

Abstract A process for the detoxification of crocidolite fibers was previously reported in the literature. The fibers of this mineral asbestiform were treated with ferric oxide salts to form a metal-micelle polymer surface coating which prevented physiological reactions with the mineral. In the present study, detoxified crocidolite was tested for its ability to generate hydroxyl radicals in the presence of hydrogen peroxide; the intensity of the electron spin resonance signal was less than that produced by the native toxic crocidolite fibers. Similar experiments showed that the ability of the detoxified crocidolite to reduce oxygen was also decreased compared with the native crocidolite. The availability of ferrous iron present in the two crocidolite fibers to catalyze the above reactions was investigated with the chelating agent ferrozine. The results indicate that ferrozine was able to mobilize fewer ferrous ions from detoxified crocidolite compared with the native crocidolite. Moreover, Mossbauer-effect spectroscopy studies have shown that the detoxification process results in both bulk and surface changes of the crystal-chemistry of the detoxified sample. This detoxification process also introduces a surface coating comprising ferric ions which shield near-surface ferrous irons and consequently reduces the Fenton-type reactivity of the fibers. It is therefore inferred from this combination of techniques that the ability of the crocidolite fibers to generate oxygen-centered radicals is dependent on the iron redox state and its chelation to different molecules. This in turn, may have an important effect on the ability of the fibers to exert their toxicity.

Mössbauer spectroscopy and magnetic susceptibility studies of natural chromites

A series of naturally-occurring chromites from the Bushveld Complex in South Africa, have been studied by Mössbauer spectroscopy at 300, 136, 110, 81, 77 and 4.8 K. Magnetic susceptibility measurements have been taken on two representative samples between 300 and 4 K. The presence of two types of chromites, referred to as type I and type II, has been confirmed. Type I has Fe cations situated on octahedral (B) sites only, while type II has Fe on both octahedral (B) and tetrahedral (A) sites. The difference between these two types arises from the degree of oxidation of the spinel toward defect (lacunar) structures. In type II chromites, a transition from cubic to tetragonal symmetry occurs at some temperature between 300 and 110 K, and the Néel transition occurs at about 77 K.

Bulk and surface modifications in detoxified crocidolite

Crocidolite is a fibrous mineral asbestiform which is widely used in industry. The fibers of this material have a high electron donor capability. This promotes electrostatic repulsion between the fibers and they are readily dispersed into the atmosphere. Airborne fibers are eventually inhaled into the lungs where they induce carcinoma and mesothelioma. Therefore much effort has been directed towards moderating the toxicity of crocidolite and related mineral asbestiforms. One detoxifying procedure has involved coating crocidolite fibers with an iron complex. In the present study, Mossbauer-effect spectroscopy has been used to monitor any crystal and chemical modifications that have occurred after this detoxification process has been applied. An analysis of the Mossbauer data has shown that the detoxification process 1) induces a change of valence at some of the bulk ferrous ion sites and 2) produces a ferric-base surface complex. There is therefore a concomitant modification of electron donor characteristics of the fibers. Experimental evidence has been presented which suggests that the coating applied in the detoxification process is a chemical complex of the form [Fe(H2O)6]3+. The nature and location of this complex may help to inhibit surface Fenton-type reactions and may consequently moderate the toxicity of the fibers.

HAEMPEPTIDE MODELS FOR HAEMOPROTEINS. PART 3 : N-ACETYLMICROPEROXIDASE-8 :EPR, MOSSBAUER AND MAGNETIC SUSCEPTIBILITY STUDIES ON AN IRON(III) PORPHYRI N IN THERMAL EQUILIBRIUM BETWEEN S = 3/2, 5/2 AND S = 1/2 STATES

The N-terminus acetylated haemoctapeptide from cytochrome c, acetylmicroperoxidase-8 (AcMP8·OH2) displays complex magnetic behaviour, contingent on its pH-dependent axial ligand combinations (histidine, H2O; histidine, hydroxide; histidinate, hydroxide). The EPR spectra of the aqua complex reveal that a low-spin state (S = 1/2) and a quantum-mechanically admixed spin state (S = 3/2, 5/2) comprising 12 to 22% S = 3/2 character are thermally accessible at 77 K and room temperature. At pH 10.8 the hydroxo complex has similar crystal field parameters to the aqua complex in both the S = 1/2 and S = 3/2, 5/2 states. At very high pH, the histidinate–hydroxide complex is predominantly low-spin although the S = 3/2, 5/2 state, amounting to less than 20% of the sample at pH 14.3 (77 K), still exhibits significant S = 3/2 character. It is, therefore, demonstrated that a thermal spin-equilibrium between the S = 1/2 and S = 3/2, 5/2 states exists for the three principal pH-dependent forms of AcMP8 above pH 6. This was confirmed by the Mossbauer spectra of lyophilised AcMP8. The effective magnetic moment of monomeric AcMP8 in 23% alcohol–water solution, determined by NMR methods at 25 °C, showed two pH*-dependent transitions above neutral pH*. The first transition (pKa = 7.83) was assigned to ionisation of iron-bound water (µeff = 5.33µB) and formation of the hydroxo species (µeff = 4.09µB). The second transition (pKa = 10.9) was assigned to ionisation of coordinated histidine and formation of the histidinate–hydroxide complex (µeff = 3.16µB). The neutral and alkaline forms of AcMP8 are equilibrium mixtures of S = 3/2, 5/2 and S = 1/2 species and the nature of the axial ligand field determines the spin distribution at this temperature. Magnetic susceptibility data collected to 6 K using a SQUID susceptometer revealed that the spin-distribution between the S = 3/2, 5/2 and S = 1/2 states was temperature dependent for both a lyophilised [µeff = 2.87µB (6 K), µeff = 3.83µB (297 K)] and a concentrated solution sample of the haempeptide at pH 6 [µeff = 2.68µB (6 K), µeff = 2.81µB (182 K)].

Cation to anion stoichiometry of chromite: A new perspective

Ferric to ferrous ratios of a batch of chromite samples from the Bushveld Complex, South Africa, were measured to calculate the cation to oxygen stoichiometry per unit cell. Cation deficiency, due to post-magmatic oxidation of Fe2+ to Fe3+ is widespread. Oxidised chromite reduces more readily than unoxidised material and naturally occurring non-stoichiometric chromite is likely to be a more suitable feedstock for the ferro-alloys industry. This factor is often overlooked during routine mineralogical characterisations.

The influence of temperature on σ-phase formation and the resulting hardening of Fe–Cr–Mo-alloys

Hardening in Fe–Cr–Mo-alloys due to the formation of σ-phase, has been the subject of theoretical and experimental interest. In the present investigation Fe–Cr-alloys containing 0, 2, 4 and 6% Mo were prepared and were fully transformed to the σ-phase by isothermally annealing the samples for various periods at temperatures of 600–800ºC. After each annealing cycle room temperature CEMS-spectra were recorded and micro-hardness tests were performed. The micro-hardness increases with annealing time and temperature, in accordance with the fraction of σ-phase present, and ranged from about 140 HV to 200 HV. From the measurements, activation energies were also deduced.

Decomposition of an Fe-Cr 38 wt.% chromium stainless steel at 475 °C

The physical changes occurring at 475 °C in an Nb and Al stabilized stainless steel containing 38 wt.% Cr have been studied by Mössbauer spectroscopy. The specimen started to decompose when held at 475 °C. Decomposition still continues after 500 hours into a paramagnetic Cr-rich α′-phase and a nearly pure Fe α-phase. The magnetic field distribution of the initial sample indicates that considerable decomposition has already taken place into an α′-phase with an estimated 15 at.% iron, and an iron-rich α-phase with an estimated 18 at.% Cr.

Characterization of natural chromite ((Mg,FeII)(Al,FeIIICr)2O4) samples from the Bushveld Complex, South Africa

Fe3+/Fe2+ ratios were determined for a suite of chromite samples from the Bushveld Complex, South Africa, using57Fe Mössbauer spectroscopy and a ceric sulphate titration method. Two distinctly different types of chromite were identified. Type I chromites have low Fe3+/Fe2+ ratios (between 0.2 and 0.4), and contain 3 cations for every 4 oxygen ions in the chemical formula unit. Type II chromites, on the other hand, have Fe3+/Fe2+ ratios between 0.7 and 1.3, and contain considerably less than 3 cations per 4 oxygen ions in the formula unit. Type II chromites result from the oxidation of type I chromites and can be reduced back to the latter by heating in a gas mixture containing 1% CO and 99% CO2 at 1000 °C.

An investigation of the mechanism of detoxification in asbestos

Abstract57Fe Mössbauer effect spectroscopy studies were conducted on native crocidolite and on a sample which had been detoxified by means of a chemical treatment with ferric salts. This allows for a comparison between the crystal chemistry of the untreated and treated samples. Significant chemical changes in the treated sample have been inferred from the Fe site-population analysis after conducting a temperature-dependent study of both samples in the range 300 K down to 90 K. These results, together with the results from complementary Electron Spin Resonance (ESR) studies, may help to elucidate the mechanism of toxicity in these asbetiforms.