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The local structure of Ca-Na pyroxenes. I. XANES study at the Na K-edge |
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| Authors: | Annibale Mottana Takatoshi Murata Ziyu Wu Augusto Marcelli Eleonora Paris |
| Institution: | Dipartimento di Scienze Geologiche, Università di Roma Tre, Via Ostiense 169, I-00154 Roma RM, Italy, Fax: +39-6-5737-2827, E-mail: mottana@uniroma3.it, IT Department of Physics, Kyoto University of Education, 1 Fujinomori-cho, Fukakusa, Fushimi-ku, Kyoto 612, Japan, JP I.N.F.N. – Laboratori Nazionali di Frascati, Via Enrico Fermi 13, I-00044 Frascati RM, Italy, IT Dipartimento di Scienze della Terra, Università di Camerino, Via Gentile III da Varano, I-62032 Camerino MC, Italy, IT |
| Abstract: | X-ray absorption Na K-edge spectra have been recorded on synthetic endmember jadeite and on a series of natural Ca-Na pyroxenes compositionally straddling the Jd-Di join. The C2/c members of the series are systematically different from the P2/n members. Differences can be interpreted and explained by comparing the experimental spectra with theoretical spectra. These have been calculated by the multiple-scattering formalism from the atomic positional parameters determined by single-crystal X-ray diffraction structure refinement on the same samples. In the full multiple scattering region of the spectra (1075 to 1090 eV) C-pyroxenes exhibit three features which reflect the 6-2 configuration of the O back-scattering atoms around the Na absorber located at the center of the cluster (site M2 of the jadeite structure). P-pyroxenes show more complicated spectra in which at least four (possibly five) features can be recognized; they reflect the two types of configuration (6-2 and 4-2-2) of O around Na in the two independent M2 and M21 eight-fold coordinated sites of the omphacite structure. A weak, sometimes poorly resolved peak at 1079 eV is diagnostic and discriminates C- from P-pyroxenes. The Garnet Ridge C2/c impure jadeite exhibits a spectrum which is intermediate between those of jadeite and omphacite. The Hedin-Lundqvist potential proves best for these insulating materials and allows multiple-scattering calculations agreeing well with experiments. Received: July 11, 1996/Revised, accepted: October 21, 1996 |
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