The local structure of Ca-Na pyroxenes. II. XANES studies at the Mg and Al K edges |
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Authors: | A Mottana T Murata A Marcelli Z Y Wu G Cibin E Paris G Giuli |
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Institution: | (1) Dipartimento di Fisica and INFM, Università "La Sapienza", Piazzale A. Moro 2, 00185 Rome, Italy;(2) LURE, CNRS, Commissariat a l'Energie Atomique, MEN, Bâtiment 209D, 91405 Orsay, France;(3) Dipartimento di Medicina Sperimentale and INFM, Università de L'Aquila, 67100 L'Aquila, Italy;(4) ISM-CNR, Via Fosso del Cavaliere, 00133 Rome, Italy;(5) Faculty of Physics, Rostov State University, 5 Sorge, 344090 Rostov-Don , Russia; |
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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.
Received: 24 November 1998 / Revised, accepted: 10 June 1999 |
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Keywords: | Ca-Na pyroxenes XANES Mg and Al K-edges Synchrotron Radiation |
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