Coexistence of two polytypic groups in cronstedtite from Lostwithiel, England

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doi: 10.1346/000986002320514226
Authors:Kogure, Toshihiro; Hybler, Jiri; Yoshida, Hideto
Author Affiliations:Primary:
University of Tokyo, Department of Earth and Planetary Science, Tokyo, Japan
Other:
Czech Republic Academy of Science, Czech Republic
Volume Title:Clays and Clay Minerals
Source:Clays and Clay Minerals, 50(4), p.504-513. Publisher: Clay Minerals Society, Clarkson, NY, United States. ISSN: 0009-8604
Publication Date:2002
Note:In English. 17 refs.; illus.
Summary:Cronstedtite from Lostwithiel, Cornwall, England, in which two polytypic groups (A and C) are present within the same crystal, has been investigated using various techniques to reveal the distribution of the two groups and the relationship between polytypic structure and chemical composition. X-ray precession photographs from cleaved fragments of pyramidal crystals revealed the variable proportions of the two groups from the top to the base. Near the top, the crystal consists entirely of group C, with 1 T as the dominant polytype. Near the base, both groups A and C are present. Back-scattered electron images from cross-sections parallel to the pyramidal axis showed mosaic contrast near the base, suggesting that two compositionally-different domains of several tens of microns in size are present, whereas the contrast was uniform near the top. Electron microprobe chemical analysis indicated the compositions (Fe2+2.31Fe3+0.69)(Si1.31Fe3+0.69)O5(OH)4 and (Fe2+2.16Fe3+0.84)O5(OH)4. Electron back-scattered patterns (EBSPs) confirmed that the domain with Si-rich composition belongs to group C and that with Si-poor composition to group A. This is the first evidence that specific chemical compositions are related to the polytypic structures in cronstedtite. Transmission electron microscopy revealed that intergrowth of groups A and C at the monolayer level is also present as stacking disorder in both domains near the base, whereas such intergrowth was not observed in the region near the top.
Sections:Crystal structure
Subsections:Specific structures: silicates
Subjects:Clay minerals; Coexisting minerals; Cronstedtite; Electron diffraction data; Kaolinite; Polytypism; Serpentine; Serpentine group; Sheet silicates; Silicates; X-ray analysis; Cornwall England; England; Europe; Great Britain; United Kingdom; Western Europe; Lostwithiel England
Abstract Numbers:03M/199
Record ID:2002073367
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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