Characterization of trace Nd and Ce site preference and coordination in natural melanites: a combined X-ray diffraction and high-energy XAFS study

Abstract

Natural melanite garnets from carbonatitic rocks have been studied by a multitechnique approach based on both experimental (chemical, diffractometric and spectroscopic) methods and full-multiple scattering calculations of the X-ray absorption near-edge structure. In particular, the site location and geometry of trace amounts of neodimium (from 176 to 1074 ppm) and cerium (791 ppm) in natural garnets have been studied by fluorescence-detected X-ray absorption fine-structure spectroscopy (XAFS) at high energy. The measurements, done at both Nd K (43569 eV) and Ce K (40443 eV) edges, demonstrate that, in all the samples, the trace elements are located in the dodecahedral X site and not in ill-defined defects. The local geometry around the two rare-earth elements is compatible with their ionic radius and is compared with that of Ca, the major element at the X site, as determined by single-crystal X-ray diffraction data. This work represents the first example of direct investigation of trace-level REE coordination in natural garnets, and confirms the great relevance for the Earth Sciences of the use of fluorescence XAFS at high energy.