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Cation and anion substitutions in the humite minerals

Published online by Cambridge University Press:  14 March 2018

P. H. Ribbe ,
G. V. Gibbs  and
Norris W. Jones
P. H. Ribbe
Affiliation:
Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia, U.S.A.
G. V. Gibbs
Affiliation:
Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia, U.S.A.
Norris W. Jones
Affiliation:
Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia, U.S.A.
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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.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 36 , Issue 283 , September 1968 , pp. 966 - 975
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1968

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