Structural and magnetic properties, methods of synthesis, and applications of seven iron(III) oxide polymorphs, including rare beta, epsilon, amorphous, and high-pressure forms, are reviewed. Thermal transformations resulting in the formation of iron oxides are classified according to different parameters, and their mechanisms are discussed. 57Fe Mossbauer spectroscopy is presented as a powerful tool for the identification, distinction, and characterization of individual polymorphs. The advantages of Mossbauer spectroscopy are demonstrated with two examples related to the study of the thermally induced solid-state reactions of Fe2(SO4)3.

Iron(III) Oxides from Thermal ProcessesSynthesis, Structural and Magnetic Properties, Mössbauer Spectroscopy Characterization, and Applications†

Since the determination of the crystal structure of the mineral spinel, MgAl2O4, by Bragg,1 and Nishikawa2 in 1915, many metal oxides and mixed metal oxides have been found to possess the same structure,3 and these are usually referred to as spinel compounds or spinels. Such materials, and in particular those containing iron, exhibit suitable combined electrical and magnetic properties and therefore soon became extremely important for many technological applications, such as high-frequency devices, memory cores, and magnetic recording. The rapid expansion of these technologies after World War II was a great stimulus for many researchers to focus their efforts on this kind of ferrite materials. Consequently, the fifties and the early sixties were characterized by intensive investigations of spinel systems with a large variety of compositions. Examples in this respect are the works of Gorter,4,5 followed by that of Blasse,6 reporting structural and magnetic properties of hundreds of new synthetic spinel compounds. The ability of the spinel structure to contain different kinds of cations, which can, moreover, gradually be substituted, made these materials also very suitable for experiments on certain magnetic and structural concepts, new at that time, such as Neel’s ferrimagnetism, ligand field stabilization, and the cooperative Jahn-Teller effect.

Mössbauer Effect Studies of Oxidic Spinels

http://argonlab.gl.ntu.edu.tw/papers/Lo-ChemGeol2000-BioDegas.pdf

Argon release mechanisms of biotite in vacuo and the role of short-circuit diffusion and recoil

https://yly-mac.gps.caltech.edu/Venus/FegleyVenus/N17ZolotovSilicate97.pdf

Hydrous Silicates and Water on Venus

Micas have been long known to play a crucial role in most petrologic and petrogenetic processes, both in magmatic and metamorphic environments. Understanding the crystal-chemical and thermoelastic behavior of this mineral family with variations in pressure ( P ), temperature ( T ) and composition ( X ) is crucial to provide a reliable basis for further interpretation and prediction of phase equilibria, transformations and most reactions occurring in a variety of rocks. In addition, the development of geothermometers and geobarometers has extensively made use of compositional (Fe/Mg+Fe ratio, K/Na+K ratio, Tschermak substitution, etc.) and structural (cation ordering, polytypic occurrence, etc.) properties of micas, or of their occurrence in assemblages at equilibrium with other phases. A few studies by in situ high-pressure, high-temperature and high-pressure–high-temperature experiments are available (see e.g., Chapter 8 in Reviews in Mineralogy and Geochemistry , Volume 41). The objectives of these studies are to determine structural and thermoelastic properties of micas and to understand the microscopic mechanisms ruling their responses to thermobaric stress.

Following these objectives, we partition such investigations into two categories:



1. Those which do not lead to a breakdown or permanent change of the structure of the reactant phase or of its chemical composition.

Behavior of Micas at High-Pressure and High-Temperature

A Mössbauer study of the effects of heating biotite, phlogopite and vermiculite

The angular variation of the quadrupole doublet ratio for the two resolved sites in biotite single crystals is studied by Mossbauer absorption. It is shown that the angular variation is consistent with the assumption that the EFG principal Z-axis for the prominent M2 site is distributed on a cone of half angle 20°, the axis lying normal to the layer. This model is justified from the known poly-typism of biotite. The lower and higher energy lines of the quadrupole doublet are identified with I3/2 and I1/2 respectively, indicating that the quadrupole coupling constant in M2 site is negative.



Es wird die Winkelanderung der Quadrupol-Dublettaufspaltung fur zwei aufgeloste Platze in Biotit-Einkristallen mit Mosbauerspektroskopie untersucht. Es wird gezeigt, das die Winkelanderung mit der Annahme konsistent ist, das dieEFG-Haupt-Z-Achse fur den besonderen M2-Platz auf einem Kegel mit dem Halbwinkel 20° verteilt ist, dessen Achse zur Schicht normal liegt. Dieses Modell wird mit dem bekannten Polytypismus von Biotit gerechtfertigt. Die Nieder- und Hoch-energielinie des Quadrupoldubletts wird mit I3/2 bzw. I1/2 identifiziert, was darauf hinweist, das die Quadrupolkopplungskonstante der M2-Platze negativ ist.

Electric field gradient in biotite mica

A search for anisotropy of Mossbauer factor ƒ in biotite single crystal is made by observing the angular variation of Mossbauer γ-absorption directed at various angles to the normal to the plane of the sheet and also by looking for the Goldanskii-Karyagin effect in the powdered specimen. It is suggested that the absence of any detectable anisotropy of the ƒ-values in biotite, and indeed in other Fe containing compounds, in general, is possibly due to the large value of the ƒ-fraction. This is in contrast to tin- and rare- earth compounds where such anisotropy is detectable even at room temperatures.



Es wird die Anisotropie des Mosbauerfaktors ƒ in Biotit-Einkristallen mittels der Beobachtung der Winkelanderung der Mosbauerabsorption von γ-Strahlen, die unter verschiedenen Winkeln senkrecht zur Ebene der Plattchen gerichtet sind, sowie durch die Beobachtung des Goldanskii-Karyagin-Effects in pulverisierten Proben untersucht. Es wird angenommen, das das Fehlen jeder nachweisbaren Anisotropie des ƒ-Faktors in Biotit und tatsachlich auch in anderen Fehaltigen Verbindungen im allgemeinen wahrscheinlich durch den grosen Wert des ƒ-Anteils verursacht wird. Dies steht im Gegensatz zu Zinn- und Seltenen-Erd-Verbindungen, wo derartige Anisotropien sogar bei Zimmertemperatur nachweisbar sind.

Search for Anisotropy of Mössbauer Absorption in Biotite

A Mossbauer study of systems Co
 x
 Mn3−x−y
 Fe
 y
 O4 and Ni
 x
 Mn3−x−y
 Fe
 y
 O4 for values ofx=0·1, 0·5, 1·0 andy ranging from 0·1 to 2·0 in steps of 0·2 have been made. At room temperature samples fory values ranging in between 0·1 to 0·5 exhibit paramagnetic behaviour while all spectra for values ofy between 0·6 to 0·8 show relaxation effects. Well-defined hyperfine Zeeman spectra are observed for all the samples withy>0·8 and resolved in two sextets corresponding to octahedral and tetrahedral site symmetries and a central doublet probably due to the presence of super-paramagnetic particles in the system. The hyperfine field at57Fe nucleus reduces with decreasing iron cobalt and nickel concentration. These observations have been explained in terms of site preference of cations and exchange interactions.

S. Lokanathan

Papers

A Mössbauer study of the effects of heating biotite, phlogopite and vermiculite

Electric field gradient in biotite mica

Search for Anisotropy of Mössbauer Absorption in Biotite

A Mössbauer study of hyperfine interaction in the systems Co x Mn3−x−y Fe y O4 and Ni x Mn3−x−y Fe y O4

Mössbauer Study of Jahn-Teller Distortion in Systems CoxMn2.9−xFe0.1O4 and NixMn2.9−xFe0.1O4