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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100 1 |a Kogure, Toshihiro  |e analytic author  |u University of Tokyo, Department of Earth and Planetary Science, Tokyo 
245 1 0 |a Coexistence of two polytypic groups in cronstedtite from Lostwithiel, England 
300 |a p. 504-513 
500 |a In English. 17 refs. 
500 |a Abstract number: 03M/199 
500 |a Category Section: Crystal structure 
500 |a Category Subsection: Specific structures: silicates 
500 |a Affiliation: University of Tokyo, Department of Earth and Planetary Science; Tokyo; JPN; Japan 
500 |a Affiliation: Czech Republic Academy of Science; ; CZE; Czech Republic 
500 |a Key title: Clays and Clay Minerals 
500 |a Source note: Clays and Clay Minerals, 50(4), p.504-513. Publisher: Clay Minerals Society, Clarkson, NY, United States. ISSN: 0009-8604 
500 |a Publication type: journal article 
504 |b 17 refs. 
510 3 |a GeoRef, Copyright 2019 American Geosciences Institute. 
520 |a 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 (Fe>2+`<2.31`Fe>3+`<0.69`)(Si<1.31`Fe>3+`<0.69`)O<5`(OH)<4` and (Fe>2+`<2.16`Fe>3+`<0.84`)O<5`(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. 
650 7 |a Clay minerals  |2 georeft 
650 7 |a Coexisting minerals  |2 georeft 
650 7 |a Cronstedtite  |2 georeft 
650 7 |a Electron diffraction data  |2 georeft 
650 7 |a Kaolinite  |2 georeft 
650 7 |a Polytypism  |2 georeft 
650 7 |a Serpentine  |2 georeft 
650 7 |a Serpentine group  |2 georeft 
650 7 |a Sheet silicates  |2 georeft 
650 7 |a Silicates  |2 georeft 
650 7 |a X-ray analysis  |2 georeft 
651 7 |a Cornwall England  |2 georeft 
651 7 |a England  |2 georeft 
651 7 |a Europe  |2 georeft 
651 7 |a Great Britain  |2 georeft 
651 7 |a United Kingdom  |2 georeft 
651 7 |a Western Europe  |2 georeft 
653 |a Lostwithiel England 
700 1 |a Hybler, Jiri,  |e analytic author 
700 1 |a Yoshida, Hideto,  |e analytic author 
773 0 |t Clays and Clay Minerals  |d Clarkson, NY : Clay Minerals Society, Aug. 2002  |x 0009-8604  |y CLCMAB  |n Clays and Clay Minerals, 50(4), p.504-513. Publisher: Clay Minerals Society, Clarkson, NY, United States. ISSN: 0009-8604 Publication type: journal article  |g Vol. 50, no. 4  |h illus. 
856 |u urn:doi: 10.1346/000986002320514226