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A refinement of the dickite structure and some remarks on polymorphism in kaolin minerals1

Published online by Cambridge University Press:  14 March 2018

Robert E. Newnham
Robert E. Newnham*
Affiliation:
Cavendish Laboratory, Cambridge University and Laboratory for Insulation Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A.
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Summary

The crystal structure of the clay mineral dickite (Al2Si2H4O9) has been refined to a greater accuracy than that reported in an earlier analysis. Improved lattiçe parameters are: a 5·15±0·001, b 8·940±0·001, c 14·424 ± 0·002Å., β 96° 44′± 1′. The dickite structure shows several significant distortions from the geometry of the idealized kaolin layer, including deformation and rotation of the silica tetra-hedra. The most striking features of the octahedral layer are the extremely short shared edges of 2·37 Å. Although the analysis was not sufficiently accurate to position the hydrogen atoms with certainty, a model consistent with the infrared absorption spectra is proposed. The stacking sequences of kaolin-layer minerals have been considered with reference to the structural features observed in dickite. There are thirty-six ways of superposing two kaolin layers commensurate with the OH-O bonds found in kaolinite, dickite, and nacrite. The twelve sequences showing the least amount of cation-cation superposition between consecutive kaolin layers can be used to construct two one-layer cells, kaolinite and its mirror image, and twelve two-layer cells, including dickite and nacrite. The distortions of the kaolin layer introduce secondary variations in the interlayer bonding that suggest that dickite and nacrite are the most stable of the kaolin layer structures, since they possess the shortest oxygen-hydroxyl contacts.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 32 , Issue 252 , March 1961 , pp. 683 - 704
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1961

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Footnotes

1

Supported by the Nuffield Foundation, I.C.I. Ltd., and the U.S. Army Signal Corps, Office of Naval Research, and Air Force

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