Local versus average structure around cations in minerals from spectroscopic and diffraction measurements

The average structure determined from diffraction data often differs from the local information gained from spectroscopic methods. Three kinds of examples are illustrated in this paper. Binary solid solutions show that Vegard law is not observed at the atomic scale due to relaxation processes during atomic substitution. The non-random intracrystalline distribution of transition elements, which cannot be obtained from diffraction, has been recently evidenced in various minerals. Intersite distribution of trace elements may also be obtained from spectroscopic methods, and the development of spatially-resolved approaches such as cathodoluminescence opens the possibility of studying zoned minerals. Such progresses give a more accurate description of the actual structure of minerals corresponding to specific conditions of formation and evolution of these phases.