FERROUS ions in crystal structures of silicates frequently prefer octahedrally coordinated sites with a distinct local distortion. Typical examples are found in chain silicates. For example, in crystal structures of the orthopyroxene system (Mg,Fe)SiO3, ferrous ions show preference for the distorted, octahedrally coordinated M2 position, whereas the magnesium ions occur predominantly at the octahedrally coordinated M1 position which exhibits a more regular local symmetry. An analogous situation exists in the cummingtonite-grunerite system (Mg,Fe)7-Si8O22(OH)2 where iron is predominantly located at the octahedrally coordinated, strongly distorted M4 position, whereas magnesium prefers the less distorted M1, M2 and M3 octahedral positions. Olivines of the composition (Mg,Fe)SiO4 should therefore show similar differential cation distributions over the octahedrally coordinated sites. But the difference in the distortion between the two nonequivalent octahedral sites M1 and M2 is less than that between M1 and M2 in pyroxenes or M1, M2, M3 and M4 in amphiboles. Thus ordering should be less pronounced compared with the aforementioned chain silicates. In fact, recent structural refinements of four natural olivines close to the Mg2SiO4-Fe2SiO4 join by Birle et al.1 did not reveal cation ordering within the experimental error.
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BUSH, W., HAFNER, S. & VIRGO, D. Some Ordering of Iron and Magnesium at the Octahedrally Coordinated Sites in a Magnesium-rich Olivine. Nature 227, 1339–1341 (1970) doi:10.1038/2271339b0
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