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Satoshi Sasaki, Yoshio Takéuchi, Kiyoshi Fujino, Syun-iti Akimoto

Electron-density distributions of three orthopyroxenes, Mg₂Si₂O₆, Co₂Si₂O₆, and Fe₂Si₂O₆

The crystal structures, net atomic charges and electron-density distributions of three synthetic orthopyroxens, Mg₂Si₂0₆ (orthoenstatite), Co₂Si₂0₆ , and Fe₂Si₂0₆ (orthoferrosilite), have been studied using single crystal X-ray diffraction intensities. In accordance with the fact that the size of Co²⁺ is intermediate between those of Mg²⁺ and Fe²⁺, the structure of Co₂Si₂0₆, newly determined, is in every respect intermediate between those of orthoenstatite and orthoferrosilite. An average of the net charges of Co in CO₂Si₂0₆ has been found to be +0.95 and that of Fe in orthoferrosilite + 1.12. These values are considerably smaller than + 1.82 for Mg in orthoenstatite. In the same species of orthopyroxene, which has two distinct octahedral sites M1 and M2, the cation at M1 tends to be more ionic than that at M2; in the case of C0₂Si₂0₆, the former has a charge of + 1.3 and the latter + 0.6, both with estimated errors of &plusm;0.1. The net charges determined for Si in the three materials yield an average value of + 2.25. Residual Fourier-peaks occurring between Si and bridge oxygen atoms are significantly different, in peak height and distances from Si, from those between Si and non-bridge oxygen atoms. Those occurring around Fe atoms at the M1 and M2 sites of orthoferrosilite, each having a point symmetry 1, have features which are similar respectively to those around M2 and Ml of fayalite, Fe₂Si0₄, the former having point symmetry m and the latter i.

Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0044-2968
Volume: 158, 03/1982
Pages: 279 - 297

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