Clays and clay minerals: The firing process

⁵⁷Fe Mössbauer spectroscopy reveals changes in iron valence and iron site geometry when clays and clay minerals are heated, and allows a distinction to be made between paramagnetic and magnetically ordered phases. Mössbauer spectra can thus reveal the extent of iron retention in silicate structures upon heating, the identity of iron oxides initially present or formed during the heating process and their transformations, and the character of the atmosphere under which heating was carried out. This makes Mössbauer spectroscopy the most effective tool for the characterization of changes induced by heating phyllosilicates and iron oxides.     

Clays and clay minerals: What can Mössbauer spectroscopy do to help understand them?

Mössbauer spectroscopy is a powerful technique for the characterization of materials formed in the weathering environment. Mössbauer studies of clay-sized phyllosilicates, however, are burdened with several problems: the samples are rarely monomineralic, they may be poor in iron, and only few iron-rich species order magnetically above 4.2 K. Site occupancies are difficult to determine, and cis and trans octahedral-OH site assignments are normally not possible. Unequivocal information that can be gained from such work thus is often restricted to the determination of the oxidation state of iron and average structural site distortions. Mössbauer data on iron oxides are generally more straightforward to interpret because these can be studied in the magnetically ordered state. A further asset of Mössbauer spectroscopy when studying iron oxides lies in its high sensitivity for magnetically ordered phases. Adverse effects ensuing from small particle size, interparticle interactions, non-stoichiometry and foreign-element substitution that often affect the Mössbauer parameters of iron oxides occurring in clays and soils can be at least partly offset by taking spectra at low temperatures.     

Mössbauer spectroscopic study of China clay samples collected from different Indian sites

The paper describes part of the study carried out on the iron bearing phases of china clays collected from different china clay reserves of the country. Chemical assays of the clays as well as the “panned” impurities were determined by standard methods and the rational mineral data was calculated from the composition. XRD and thermal analysis also were carried out to get the mineralogy of the samples. Mössbauer spectral studies were conducted on the iron rich phases separated from the clays by “panning” to get information on the different iron minerals present in these clays. It was observed that Mössbauer spectra of the impurities from the low iron containing clays were qualitatively much different from those of the impurities from the high iron containing clays.     

Mössbauer study of illitic clays containing iron-rich impurities

The iron mineralogy of nineteen illitic clays from eastern Bavaria was studied by Mössbauer spectroscopy and X-ray powder diffraction. Mössbauer spectra of the <2 μm fraction were taken at RT, 120 K and 4.2 K. The clays contain both paramagnetic Fe³⁺ and Fe²⁺. Superparamagnetic oxides are frequently present. The Fe²⁺ quadrupole splitting and the ratio of Fe³⁺ at 4.2 K to Fe²⁺ at 120 K are correlated and define two groups distinguished by their mineral content. The samples were heated systematically for 48 h up to 1250°C in steps of 50°C. One clay which is rich in chlorite and Fe(II) was studied in detail after firing in air and following a reduction for 3 h at 800°C with charcoal. The transformations of the mineral phases with temperature as shown by X-ray diffraction are also evident in the Mössbauer spectra.     

Mössbauer study of brick: Spectral analyses and temperature dependence of Fe ratio

Six clay samples, each fired to six different temperatures in the range of 1000–1250°C, were studied by Mössbauer spectroscopy and powder x-ray diffraction. Hematite, mullite, and a glass phase were identified as the dominant species which contain iron in these fired brick samples. In all the samples there is a partial substitution of Al(III), which reduces the effective magnetic field. The iron is in the Fe(III) state and is predominantly in the tetrahedral site. Two of the samples were examined at both room temperature and 77K. The magnetic field increases in the lowering of the temperature, but the quadrupole splitting remains constant. This indicates that there is no Morin transition between 77K and room temperature. The physical properties of the resulting fired bricks were examined to determine the quality of each brick. Two of the samples were identified as clay not suitable for the production of good bricks. These two particular clays can be distinguished from the others by their Mössbauer parameters.     

Clay: An important raw material for prehistoric man

Early techniques of making pottery can be investigated by ⁵⁷Fe Mössbauer spectroscopy. Iron is generally present in unpurified clays in concentrations of several percent. During firing, the iron undergoes characteristic changes of its chemical and physical state, depending on the kiln atmosphere and on the maximum firing temperature reached. These changes can be followed by Mössbauer spectroscopy. Firing techniques can often be reconstructed when spectra of laboratory and field fired samples are compared with those observed in ancient sherds.     

Crystal chemistry and Mössbauer spectroscopic analysis of clays around Riyadh for brick industry

A total of 30 clay samples were collected from the area around Riyadh city, Saudi Arabia. A complete chemical analysis was carried out using different techniques. X-ray diffraction studies showed that the clay samples were mainly of the smectite group with traces of the kaolinite one. The samples studied were classified as nontronite clay minerals. One of the clay fraction has been studied by Mössbauer spectroscopy as raw clay fraction and after being fired at 950–1,000 °C. The Mössbauer spectra showed accessory iron compounds in the form of hematite and goethite. The structural iron contents disintegrate on firing transforming into magnetic iron oxide and a paramagnetic small particles iron oxide.     

The characterization of soils, clays, and clay firing products

Mössbauer spectroscopic studies of soils are generally centered around the characterization of Fe³⁺ oxides, making use of distinctive magnetic properties of the different oxides and the high sensitivity of the technique for magnetically ordered phases. Most work on clays and clay minerals, in contrast, focuses on the determination of the oxidation state of iron, iron structural site occupancies and distortions, and the transformations that take place during clay firing.     

Characterization by Mössbauer Spectroscopy and Electron Paramagnetic Resonance of Peruvian Obsidians for Provenance Studies: A Preliminary Study

We report on an investigation of several ancient clays which were used for pottery making in northern coastal Peru at a kiln site from the Formative period (ca. 2000–800 BC) in the Poma Canal and at a Middle Sicán pottery workshop in use between ca. AD 950 and 1050 at Huaca Sialupe in the lower La Leche valley. Neutron activation analysis, ⁵⁷Fe Mössbauer spectroscopy and X-ray diffraction were used for the characterisation of the clays. The changes that occur in iron-bearing compounds in the clays depending on the kiln atmosphere and on the maximum firing temperature were studied by Mössbauer spectroscopy and X-ray diffraction. Laboratory firing series under varying controlled conditions were performed to obtain a basic understanding of the different reactions taking place in the clays during firing. The results can be used as models in the interpretation of the Mössbauer spectra observed in ancient ceramics from the same context.     

Mössbauer Spectra of Clays and Ceramics

The physical, chemical and mineralogical aspects of the use of Mössbauer spectroscopy in studies of clay-based ceramics are described. Mössbauer spectra of pottery clays fired under oxidising, reducing and changing conditions are explained, and the possibilities of using Mössbauer spectra to derive information on the firing temperatures and the kiln atmosphere during firing in antiquity are discussed and illustrated by examples.     

In situ Mössbauer studies of electrochemical processes

Recent electrochemical studies involving in situ Mössbauer spectroscopy are reviewed. The principal application has been the development of new electrodes for lithium-ion batteries, where Mössbauer spectroscopy plays an important complementary role to XRD in characterizing, and usually identifying, nanocrystalline and intermediate phases of iron and tin. Other applications include the crystallography of Prussian Blue, taken using a recently developed spectrometer for acquiring multiple spectra during continuous cyclic voltammetry, valence interconversion in clay minerals and electrochemical oxygen reduction.     

57Fe Mössbauer studies of oxisols

Mössbauer spectroscopy is a valuable complement to the standard analytical techniques of soil science for characterizing the iron content of soils, and for studying the mineralogical transformations that occur during pedogenesis from the point of view of the iron. The uses and limitations of Mössbauer spectroscopy in this field are illustrated by five selected examples. In each case, the starting point is a different question relating to the soil colour, mineralogy, magnetization or mode of formation.     

Mössbauer study of illite associated with iron oxi-hydroxides

An illite separated from mylonites from the southern rim of the Bohemian massif was studied by Mössbauer spectroscopy at temperatures between 300 and 1.8 K. The iron is mainly contained in oxides associated with the illite. Only 13% of the iron are structural Fe³⁺ and Fe²⁺ within the clay mineral matrix. The transformations of the oxidic iron during heating at temperatures up to 1250°C were followed by Mössbauer measurements at 300 and 4.2 K.     

Some comments on the study of iron-poor soils by Mössbauer spectroscopy

We discuss the use of MS and XRD in the study of an iron-poor soil. We have performed Mössbauer spectroscopy and careful XRD analysis on < 2 μm, < 50 μm and <2 mm sized samples, treated with different Fe-extraction methods. Since the iron content of the coarser fractions is comparable to that of the clay fractions, we show that the usual Mössbauer sample preparation procedures used to overcome the texture effects and to make background absorption corrections are insufficient and that additional homogenization is necessary. By milling the samples for a short interval we achieved spectral area values which are in agreement with Atomic Absorption results and allow to follow the behaviour of the extraction methods.     

Mössbauer Study of Nanocrystalline ε-Fe3−x Co x N System

Five clays from four different deposits in the Ivory Coast (Adattié, Nieki, Grand-Bassam, Nigui-Saff) were studied by ⁵⁷Fe Mössbauer spectroscopy and electron paramagnetic resonance (EPR) at room temperature. Their chemical compositions were determined by X-ray fluorescence and the most important crystalline phases were identified by X-ray diffraction. In these natural clays, kaolinite is the dominant mineral phase with minor amounts of other minerals including quartz, haematite, goethite, lepidocrocite and illite. Room-temperature ⁵⁷Fe Mössbauer spectra of the samples from Nieki, Adattié and Nigui-Saff show mainly a broad quadrupole-split doublet which indicates that iron is essentially present as Fe(III). Samples from Grand-Bassam contain important amounts of goethite (α-FeOOH) and lepidocrocite (γ-FeOOH). Some samples from Nigui-Saff contain large amounts of haematite. Clays from Nieki and Adattié are characterised by very low concentrations of iron oxides or hydroxides, and by small amounts of structural Fe(II) in the kaolinite lattice. Oxidation states and lattice sites of structural iron in the kaolinites were determined by ⁵⁷Fe Mössbauer spectra recorded after iron extraction with dithionite-citrate-bicarbonate (DCB). The results were confirmed by electron paramagnetic resonance (EPR) measurements.     

Perspectives in physical metallurgy

Physical metallurgy experienced a tremendous, impact with the advent of the Mössbauer effect. Some of these developments will be reviewed, in particular the role of⁵⁷Fe as the star performer in Mössbauer spectroscopy. Also, one must realize that in most casen metals are involved, either in the source, the absorber, or both. Mössbauer spectroscopy in its different variations (conversion electron-, conversion x-ray-, γ-transmission Mössbauer spectroacopy) is well suited for the analysis of iron based alloys. An instrument has been developed which allows simultancous triple radiation Mössbauer spectroscopy (STRMS). The backscalttering geometry of CEMS and CXMS in conjunction with the transmission mode permits non-destructive testing of surfaces and depth analysis.     

Mössbauer spectroscopy on the surface of Mars. Why?

A Mössbauer spectrometer is included in the preliminary payload of a rover to be placed on the surface of Mars in the Soviet mission to the planct in 1996/1,2/. In counection with the American planctary program it has also been suggested to construct a Mössbauer spectrometer to be landed on Mars /3, 4/. The objective is to study the iron compounds of the Martian soil and rocks by backscattering Mössbauer spectroscopy. The paper describes the significance of the element iron in the study of the evolution of the planetary system and what we might expect to learn from Mössbauer spectroscopy of the surface materials of Mars. The study of Mars is expected to expand substantially in the coming decades, probably culminating with a manned flight to the planet. The international Mössbauer community may contribute significantly to the preparation of these events.     

Study of the structural modifications in activated clays by Mössbauer spectroscopy and X-ray diffractometry

In this work we study the changes induced on the structure of a smectite clay by chemical acid activation with HCl using X-ray diffractometry (XRD) and transmission Mössbauer spectroscopy (TMS) techniques. By XRD we were able to determine the mineralogical composition of the clay samples and measure the changes in the interplanar distance associated to the structural modifications in the clays. We measured a reduction in the interplanar distance and reflection intensity as the acid concentration in the activation process increased. TMS allowed us identify and characterize the structural sites occupied by ferric and ferrous iron cations. In addition, we were able to monitor the effects caused by the chemical acid activation on the valence state of the iron cations that occupy these structural sites in the clay. For the treatment at low acid concentration, keeping time and temperature of activation constant, our results showed a strong effect on the ferrous and ferric iron sites, reducing and increasing their adsorption relative areas respectively.     

Artificial neural networks in Mössbauer material science

Mössbauer spectroscopy is a useful technique for characterizing the valencies, electronic and magnetic states, coordination symmetries and site occupancies of the cation. The Mössbauer parameters of isomer shift and quadrupole splitting are useful to distinguish paramagnetic ferrous and ferric iron in several substances, while the internal magnetic field provides information on the crystallinity. In recent years artificial neural networks have shown to be a powerful technique to solve problems of pattern recognition of a mineral from its Mössbauer spectrum, Mössbauer parameters data bank, crystalline structure and magnetic phases of soil from Mössbauer parameters. A computer software named Mössbauer Effect Assistant has been developed. It uses learning vector quantization neural network linked to a Mössbauer data bank that contains Mössbauer parameters of isomer shift, quadrupole spliting, internal magnetic field and the references of the substances. The program identifies the substance under study and/or its crystalline structure when fed with experimental Mössbauer parameters. It can also list the references from the literature by feeding the name of the substance or the author of the publication. Typical application of Mössbauer Effect Assistant in iron-bearing materials Mössbauer spectroscopy is present in user friendly Microsoft Windows environment.