Structure refinement data of natural and heated orthopyroxenes with different Fe/(Fe + Mg) ratios were used for a systematic analysis of structural variations as a function of chemical composition and ordering degree. The mean T–O bond distances do not vary significantly with increasing iron content and therefore any change of 〈TB–O〉 distances may provide a measure of Si/AlIV substitution, which occurs only at the TB site. The mean M1–O distance increases linearly with iron content as a consequence of a homogeneous expansion of M1 site. The mean M2–O distance also increases with iron content, but the linear plot is the sum of very different trends of the individual bond lengths; among these, the M2–03 distances exhibit a peculiar behavior consistent with a 4 + 2 coordination of the M2 polyhedron. The R3+ cations, which enter the structure with the Si/AlIV substitution, appear to order totally in the M1 site.

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