Models and figments in the “puzzle” approach to geo-crystal-chemistry. (Abstract/Comunicazione in atti di convegno)

Type
Label
  • Models and figments in the “puzzle” approach to geo-crystal-chemistry. (Abstract/Comunicazione in atti di convegno) (literal)
Anno
  • 2006-01-01T00:00:00+01:00 (literal)
Alternative label
  • Roberta Oberti1 and Simona Quartieri2 (2006)
    Models and figments in the “puzzle” approach to geo-crystal-chemistry.
    in 35° Congresso Nazionale AIC, Ferrara, 18-21 settembre 2006
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • Roberta Oberti1 and Simona Quartieri2 (literal)
Pagina inizio
  • Atti, MS1-KL1 (literal)
Note
  • Comunicazione (literal)
  • Abstract (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#affiliazioni
  • 1 CNR-Istituto di Geoscienze e Georisorse (IGG), Unità di Pavia 2 Dipartimento di Scienze della Terra, Università di Messina, e CNR-IGG (literal)
Titolo
  • Models and figments in the “puzzle” approach to geo-crystal-chemistry. (literal)
Abstract
  • In the last few years, we have applied crystal-chemical analysis to problems which are relevant to trace- and minor-element geochemistry. Our aim was to provide a reliable framework and an accurate method to be used to interpret and model mineral/mineral and mineral/melt elemental partitioning. In fact, the available experimental partition coefficients for complex mineral families (such as amphiboles and garnets) often have an anomalous behaviour, which may reflect a strong control of the crystal structure. In particular, there are evidences for: (1) different site preference and partitioning as a function of the overall composition and cation ordering; (2) different local coordination geometries for the same cation at the same site as a function of the overall composition. All these issues have important implications for both geochemical interpretation of the data and thermodynamic modelling of solid solutions. Due to the low concentrations of the elements investigated, geo-crystal-chemistry is a real challenge; we learnt at our own expenses that the subtle changes we can detect can be either sound clues to solve the enigma, or figments raising from uncertainties in the methods. Thus, a combination of different complementary techniques is required to compose the puzzle. However, each technique has his own peculiarities, advantages and limitations, which must be clearly understood to avoid the frequent pitfalls. Structure refinement, especially when used in a comparative perspective, can provide an accurate long-range picture of site geometry and populations. However, it fails in determining the site preference of very minor and trace elements, and does not give any information on the local geometry of a dilute element. Also, subtle changes in the refined site-scattering values may simply derive from the inadequacy of the model or the low quality of the data collected from very small (often zoned) synthetic crystals. EXAFS analysis can provide direct information on the site preference and local environment of many mineralogically and geologically relevant elements, as well as on clustering effects in solid solutions. However, the analysis of EXAFS spectra must be based on a reliable crystal-chemical starting model, typically derived by detailed diffraction studies. An important limitation of EXAFS, is the difficulty in obtaining reliable information on the local coordination of those atoms whose radial distribution function is highly disordered. This feature can derive from the absorbing atom being distributed over multiple sites and/or from the presence of a range of closely spaced interatomic distances in the coordination polyhedron. Atomistic simulations (whether semi-empirical or ab initio) give reliable results for end-member compositions, but are less convincing for solid-solution terms. Their accuracy and capability of modelling and predicting, strongly depend on the choice of the set of potentials or on the number and selection of the atomic arrangements considered. Even if a continuous information feed-back is applied, the resulting models may not appear consistent. A critical cross-check of the results, which takes into account the peculiarities and the limitations of each technique, may allow figments to disappear and the correct model to emerge. Examples will be given, commenting the results obtained from chemical, structural, spectroscopic analysis and atomistic simulations of natural and synthetic garnets solid-solutions, containing minor and trace elements of geochemical relevance. (literal)
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