The local structure of Ca-Na pyroxenes. II. XANES studies at the Mg and Al K edges

Abstract

X-ray absorption spectra at the Mg and Al K edges have been recorded using synchrotron radiation on synthetic end member diopside (Di) and jadeite (Jd) and on a series of natural Fe-poor Ca-Na clinopyroxenes compositionally straddling the Jd-Di join. The spectra of C2/c end members and intermediate members of the solid solution series (C-omphacites) are different from those of the intermediate members having P2/n symmetry (P-omphacites). Differences can be interpreted and explained by comparing the experimental spectra with theoretical spectra calculated via the full multiple-scattering formalism, starting from the atomic positional parameters determined by single-crystal X-ray diffraction structure refinement on the same samples. Atomic clusters with at least 89 atoms, extending to more than 0.60 nm away from the Mg or Al absorbers, are needed to reproduce the experimental spectra. This shows that in the clinopyroxene systems XANES detects medium- rather than short-range order-disorder relationships. Theoretical spectra match the experimental ones well for all features in the regions from 16 to 60 eV above threshold. Experimental near-edge features in the first 16 eV are also reproduced, albeit less accurately. Certain near-edge features of C-omphacites reflect the octahedral arrangement of the back-scattering six O atoms nearest neighbours of the probed atom (Mg or Al) located at site M1 of the crystal structure, thus being indicators of short-range order. Others arise again from medium-range order. P-omphacites show more complicated spectra than C-omphacites. Their additional features reflect the increased complexity of the structure and the greater local disorder around the probed atom induced by the two alternative M1, M11 configurations of the six O atoms forming the first coordination shells. Mg and Al are confirmed to be preferentially partitioned in the M1 and M11 site of the P-omphacite crystal structure, however with a certain degree of local disorder. The relative heights of certain prominent features are directly related to sample composition in terms of Di:Jd ratio in the Al K-edge spectra, whereas they show abrupt variations in the Mg K-edge spectra. They demonstrate that XANES is directly related to composition and may be used to distinguish C- from P-omphacites.