MOLECULAR-SCALE APPROACHES IN MINERALOGY: BRIDGING THE GAP FROM MICROSCOPIC TO MACROSCOPIC

Spectroscopic methods and large user facilities have led to an explosion of instrumental, experimental and theoretical approaches in mineralogy and geochemistry. In particular, this has provided new tools for understanding structure-property relationships in minerals and glass/melt systems. This molecular-scale vision is suited to the study of the properties of "real minerals", e.g. the local structure of highly disordered and chemically complex materials, or the speciation of substituted trace/minor elements and radiation-induced defects. This provides a vivid illustration of the Pauling rules (Pauling was the 1967 Roebling medalist) that govern element behavior in natural and synthetic systems. This interdisciplinarity perfectly supports the concept of "Geochemistry of solids" that was advocated in 1964 by Bill Fyfe, the 1995 Roebling medalist. With the starting, at almost the same time, of Orsay/LURE and Stanford/SSRL synchrotrons, a continuous collaboration began with the group of Gordon Brown, the 2007 Roebling medalist.

These concepts may be illustrated illustrate by examples taken from different fields.

(1) The first focus is on glass structure, showing how glass properties may depend on cation speciation, sometimes with original cationic sites in glasses. This allows understand the structural mechanisms that control glass stability or, on the contrary, facilitate glass nucleation. This helps rationalize the evolution of nuclear glasses during aging (alteration, irradiation), or the formation processes of glass-ceramics.

(2) A second example concerns the mineralogical control of the concentration and dissemination of heavy elements such as As or U in geochemical anomalies and sites contaminated through industrial/mining activities. More recently, this expertise has been directed towards understanding the concentration of critical metals, such as Sc or Nb, in lateritic soils.

(3) A final example shows how radiation-induced defects in minerals may be used to detect short-lived uranium daughter elements. The high specific area of clays makes them sensitive to ground-level radiation doses, providing information on the past transfer of radionuclides in the geosphere and helping to model the migration of uranium-bearing fluids.