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

X-ray absorption spectra at the Mg and Al K edges have been recorded on synthetic endmember 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 members of the series (C-omphacites) are different from having P2/n symmetry (P-omphacites). Differences can be explained by theoretical spectra calculated via the multiple-scattering formalism on atomic clusters with at least 89 atoms, extending to ca. 0.62 nm away from the Mg viz. Al absorber: XANES detects in these systems medium- rather than short-range order-disorder relationships. Near-edge features of C-omphacites reflect the single-type of octahedral arrangement of the back scattering nearest-neighbours (six O atoms) around the absorber (Mg resp. Al) at the centre of the cluster (site M1). Others arise again from medium-range order. P-omphacites show more complicated spectra than C-omphacites. Their additional features reflect the increased local disorder around the probed atom (Mg resp. Al) induced by the two alternative M1, M11 configurations of the six O atoms forming the first co-ordination spheres. Mg and Al are confirmed to be preferentially partitioned in the M1 resp. M11 site of the P-omphacite crystal structure, however never exclusively, but in a ratio close to 85:15 (±10%) that implies a certain degree of local disorder. Changes in the relative heights of some prominent features are more evident in the Al than in the Mg K-edge spectra. They are diagnostic to qualitatively distinguish C- from P-omphacites