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Tem and X-Ray Diffraction Evidence for Cristobalite and Tridymite Stacking Sequences in Opal

Published online by Cambridge University Press:  28 February 2024

Jessica M. Elzea  and
Stephen B. Rice
Jessica M. Elzea
Affiliation:
Thiele Kaolin Company, P.O. Box 1056, Sandersville, GA 31082
Stephen B. Rice
Affiliation:
McCrone Associates, 850 Pasquinelli Drive, Westmont, IL 60559
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Abstract

In an attempt to resolve the structure of opal-CT and opal-C more precisely, 24 opal samples from bentonites, Fuller's Earths, zeolite tuffs, biogenic silicas and silicified kaolins have been analyzed by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). Results of this examination demonstrate that opal-C and opal-CT are part of a continuous series of intergrowths between end-member cristobalite and tridymite stacking sequences.

These findings are consistent with Flörke's (1955) interpretation of the most intense opal peak at ~4 Å as a combination of the (101) cristobalite and (404) tridymite peaks. The position and width of this peak are controlled by the relative volume of the two stacking types and the mean crystallite size. Direct evidence obtained by HRTEM provides data showing various stacking sequences in opals. Broadening due to crystallite size alone was determined by directly measuring crystallite size by TEM and comparing the measured size to the apparent size calculated using the Scherrer equation. XRD peak broadening is also described in terms of various contributions from structural disorder. The mean opal crystallite size ranges from 120 to 320 Å. For samples at either end of the size range, the crystallite size plays a larger role, relative to stacking disorder, in controlling peak broadening.

Keywords

OpalTransmission Electron MicroscopyX-Ray Diffraction
Type
Research Article
Information
Clays and Clay Minerals , Volume 44 , Issue 4 , August 1996 , pp. 492 - 500
Copyright
Copyright © 1996, The Clay Minerals Society

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