Transition metals in silicate glasses: the cases of zinc, hafnium and tantalum

Many analytical and structural techniques, typically characteristic of the physical sciences, are more and more being used in materials science, biology and geology, to better understand structure, composition and other properties of compounds of different types, from organic to inorganic, from amorphous to crystalline, in all the different states of matter, solid or liquid or gas (for a review, see, p.e., .Hawthorne 1988 and Beran et Libowitzky 2004). The studies are mainly based on the use of spectroscopic methods which utilize a wide range of the electromagnetic spectrum (from microwaves to X-rays), but there is an extensive use of other techniques as well, like neutron spectroscopy or scattering and magnetic resonance to solve a variety of problems, examining different properties, on the basis of frequency (wavelength) and intensity of the signals. Composition, chemical and physical structure, electrochemical properties, are some of the information which can be obtained. In geology some of these techniques have been used already for a long time, others are new, with a faster and faster development and always new applications. Among them, most powerful are the techniques based on the use of X-rays, as very strong and tuneable X-ray radiation sources are available all around the world (mostly synchrotrons, but even Free Electron Laser facilities will be available for particular fast and powerful spectrometry) where spectra can be collected quickly and efficiently, together with less intense but more diffuse ones (X-ray tubes or even natural sources). X-rays also have a great flexibility: absorption and emission spectroscopy, diffraction, tomography can be applied and give very complete information on the same sample, respectively about composition, geometry and 3D structure, and can be performed sometime almost at the same time, using the suitable optics and detectors, thanks to the new synchrotron radiation facilities. For example, X-ray Absorption Spectroscopy (XAS) can be used to study different properties: chemistry, geometry, surface structure, and so on. Theory and experiments are growing together to get better results and to interpret them in an efficient and more coherent way. Nowadays the interpretation of spectra is much more standardized than in the past, many procedures are becoming routinely and several software products are available to extract physical and chemical data. However, the typical experiment to be performed on samples of geological relevance has its own intrinsic problems: samples can be very small, requiring the use of microbeams, or the element of interest is very light or very diluted, requiring special beamlines and experimental set-ups. In most cases these problems are summed together, since it is very common to have the need to analyse samples with at least two of the above mentioned characteristics. Moreover, the researcher has to deal sometimes with samples of complex chemical composition or is in the need of analysing samples at high temperature or high pressure or variable redox conditions, just to try to reproduce the environment where a sample (e.g., mineral or magma) can form. The use of XAS is able to solve many of the experimental problems relevant for the Earth sciences, certainly not all of them, and in the years it has been applied to a variety of compounds and topics. XAS is based on absorption cross section in X rays frequencies, strongly dependent on atomic species. The spectrum has an edge corresponding to typical frequencies (absorption edge, due to energy gap between energy of two electronic bound levels, or between a bound electron and continuum energy, that is when electron is no more bound to the atom), whose position is mainly, but not only, dependent on the atom, but also slightly on the surrounding environment. In fact, typical energies and spectrum features of an absorption spectrum vary not only depending on the chemical species of the absorber, but, even if the absorber is the same, there are some differences due to composition and structure, chemical and physical features which make the spectra useful to determine characteristics of the absorbing atom like the oxidation state, coordination geometry, bond distances, disorder, chemical environment. X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) are respectively the name of spectroscopy techniques referring to the pre- and near edge part of the spectrum and the post-edge spectrum. They can give different but complementary information about geometry and chemistry of central atom (photoabsorber) and its neighbourhood. To extract structural information, powerful and complete theories about X-ray absorption have been developed since 1970s, and nowadays there are software programs suitable to give tools to compare data with theory in an efficient way. In this research we examined the results of XANES and EXAFS measurements carried out at the European Synchrotron Radiation Facility in Grenoble (F) on synthetic silicate glasses containing transition metals (in particular Zinc, Hafnium and Tantalum), following a research line already open in our research group. The study of these elements in a series of glasses of different bulk composition, reproducing the major rock types and magma compositions (from a granitic to basaltic), is aimed at determining the geochemical and structural role of three transition elements in these glasses, considered as synthetic analogue of different melt types. Zn is associated to the genesis of ore deposits, therefore it can be an abundant component but, in spite of this, its structural role in the melts has never been determined by direct methods. The structural data obtained in this work have been also compared with data concerning the physical properties (viscosity and density) already determined on the same set of samples which show the relationships between Zn, glass composition and these properties. In the XAS experiments the glass used are S-free compositions, in a first approximation, to study how Zn behaves in presence of Oxygen when Sulfur is absent. In a following study we will synthesize S-bearing samples to investigate the variations introduced by the adding of this chemical species. In analyzing Ta and Hf , using again a set of different glass composition, we approached a different type of problem, which is related to the use of samples with a very low concentration to simulate the amounts usually related to these elements in magmas. The results, based on the EXAFS refinements and the comparison with theory, are however satisfying and produced structural models of their geochemical role, to be compared with data on Nb and Zr, elements to whom they are usually related due to their geochemical affinity. The study of glasses to solve geochemical problems is particularly useful since they can be considered a good approximation of a melt. A lot of information can be extracted from their study to be extrapolated, with the necessary precautions, to the interpretation of their geochemical behaviour, the magmatic processes, the chemical partitioning, the crystallization processes. In some cases, the comparison between structural and physical properties can help in giving a structural interpretation of the only observed variation in properties, exploring their mutual relationship. Moreover, these elements and the glass compositions systems above mentioned are more and more used in important and frequent technological applications, so that each improvement in knowledge of their behaviour can be of great interest. During this study in fact, it has been found that most of the literature found on these topics are related to materials science and technologically relevant applications of glasses and transition elements. We are certain therefore that the results of this study, when published, will contribute not only to geologically relevant problems but also to a wider field of discipline, from physics to chemistry. The exposition of the arguments treated in this research is so divided: - in chapter 1: a brief description of the structure and properties of silicate melts and their importance in scientific or applied studies is reported. Details on the network models for amorphous structures are reported, considering geometric description, structural parameters and crystal chemistry. Glasses are the samples we have been dealing with, so their general characteristics have been studied and treated in more detail, describing correlations with the silicate melts. The behaviour of the analyzed elements in glasses/melts is our final aim, so some generalities about elements geochemical classification, mainly applied to trace elements, and how they behave in glasses is reported. - in chapter 2 we introduce the physical and computational framework we are moving in: spectroscopic methods applied to material sciences, some generalities about X-rays, XAS theory in general and EXAFS/XANES in detail. How is possible to convert theory in practice, modelling and approximating complete theory to have consistent and reasonable calculations, the experimental techniques and procedures and tools used to extract physical data from raw spectra. A brief description of computational tool used in this work (GNXAS) is then given, without deepening on technical and computational aspects, but considering the model which it is based on and a block scheme of data manipulation. Also very few elements about synchrotron radiation and X-rays measurement facilities and techniques are briefly given. In the last part of this chapter, application to geology, and particularly to glasses and melts are described, with a short history, a bibliography and references about the state of the art of the specific topics. Some generalities about how and why XAS can be applied to amorphous materials conclude the chapter. - in chapter 3 we give all the data concerning the Zn-bearing samples, together with the XAS procedures and results about Zinc in the glasses. We start with the literature data available on Zinc glasses and compounds, with details concerning the group of samples used as standard materials, and how XAS has been applied to understand microscopic structure. Finally the XAS spectra have been interpreted by building up theorethical models using clusters of different types and dimensions which have been compared to the experiments. - in chapter 4 and 5 the complete results and interpretation of data regarding respectively Hafnium and Tantalum, using the same procedure as Zinc, with particular emphasis on EXAFS analysis and the building of suitable clusters. - the conclusions, with a summary of the most important results obtained from the analysis of the glasses with the three elements studied.