Cation and anion substitutions in the humite minerals

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Abstract
Summary: The humites are structurally analogous to olivine wherein the replacement of four oxygen by four (F,OH) anions in the slightly distorted, hexagonal close-packed array is balanced by the replacement of one tetrahedrally coordinated Si by a tetrahedral void, according to the general formula Mg2xSix-1O4x-4(F,OH)4 where x = 3, 5, 7, 9. In humites the key structural units are not ‘olivine and sellaite (or brucite) layers’, as previously assumed, but are zigzag chains of edge-sharing octahedra, just as in olivines. It is shown that for humites and olivines alike the unit cell parameters a, b, and d001/n and the cell volume (normalized to one-half the mean anion-anion distance along the normal to (001)) vary linearly with the average radius of the octahedrally coordinated cation in the chain.Substitutions of (F,OH) for O and vacancies for Si have second-order effects on the unit cell parameters, causing a linear decrease of the normalized cell volume with increase in F/O ratio in the synthetic series forsterite-humite-norbergite. Comparison of the crystal structures of forsterite and norbergite shows that the polyhedral distortions in norbergite are smaller than in forsterite in accord with the decreased number of shared edges: the fluorines in norbergite are bonded to three Mg atoms whereas all anions in forsterite are bonded to three Mg and one Si atom.

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