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

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 s traddling the Jd-Di join. The spectra of C2/c end members and intermediate members of the solid solution series (C-omphacites) are different fi om tho se of the intermediate members having P2/n symmetry (P-omphacites). Differe nces call be interpreted and explained by comparing the experimental spectr a with theoretical spectra calculated lin the full multiple-scattering form alism, 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 t he Mg or Al absorbers, are needed to reproduce the experimental spectra. Th is shows that in the clinopyroxene systems XANES detects medium-rather than short-range order-disorder relationships. Theoretical spectra match the ex perimental 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 rep roduced, albeit less accurately. Certain near-edge features of C-omphacites reflect the octahedral arrangement of the back-scattering six O atoms near est neighbours of the probed atom (Mg or Al) located at site M1 of the crys tal structure, thus being indicators of short-range order. Others arise aga in from medium-range order. P-omphacites show more complicated spectra than C-omphacites. Their additional features reflect the increased complexity o f the structure and the greater local disorder around the probed atom induc ed by the two alternative M1, M11 configurations of the six O atoms forming the first coordination shells. Mg and Al are confirmed to be preferentiall y 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 ce rtain prominent features are directly related to sample composition in term s of Di:Jd ratio in the Al It-edge spectra, whereas they show abrupt variat ions in the Mg It-edge spectra. They demonstrate that XANES is directly rel ated to composition and may be used to distinguish C- from P-omphacites.