Refinement of the crystal structure of cronstedtite-2H2

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doi: 10.1346/000986002320679332
Authors:Hybler, Jiri; Petricek, Vaclav; Fabry, Jan; Durovic, Slavomil
Author Affiliations:Primary:
Academy of Sciences of the Czech Republic, Institute of Physics, Prague, Czech Republic
Other:
Slovak Academy of Sciences, Slovakia
Volume Title:Clays and Clay Minerals
Source:Clays and Clay Minerals, 50(5), p.601-613. Publisher: Clay Minerals Society, Clarkson, NY, United States. ISSN: 0009-8604
Publication Date:2002
Note:In English. 71 refs.; illus., incl. 6 tables
Summary:The crystal structure of cronstedtite-2H2 was refined in a hexagonal cell, space group P63, Z = 2, using two acicular crystals from Wheal Maudlin, Cornwall, England, and from Pribram, Czech Republic. The Wheal Maudlin sample has the chemical composition (Fe2+2.291Fe3+0.709) (Si1.298Fe3+0.707Al0.004)O5(OH)4 and the Pribram sample has the composition (Fe2+2.269Fe3+0.731) (Si1.271 Fe3+0.724 Al0.005)O5(OH)4. The results of refinements are as follows: a = 5.500 (1), c = 14.163(2) Å, V = 371.08(8) Å3, R = 3.83%, from 381 independent reflections, and a = 5.4927 (1), c = 14.1481(2) Å, V = 369.70(4) Å3, R = 4.77%, from 1088 independent reflections for the Wheal Maudlin and Pribram samples, respectively. The best Fo vs. Fc agreement was achieved when the structure was interpreted as merohedral twin; several possible twinning laws are discussed. The cronstedtite layer consists of one tetrahedral sheet and one octahedral sheet. There is one octahedral (M1) position, occupied by Fe only, and two tetrahedral (T1, T2) positions in the structure. Refinement of occupancy of tetrahedral sites led to values Si:Fe = 0.45:0.55(1) (Wheal Maudlin) and 0.432:0.568(8) (Pribram) in T1, and Si : Fe = 0.99:0.01(1) (Wheal Maudlin) and 0.888:0.112 (7) (Pribram) in T2. Whereas the size of T1 is reasonable (average dT1-o = 1.693 Å (Wheal Maudlin), 1.691 Å (Pribram)), T2 is unusually large: (dT2-o = 1.740 Å (Wheal Maudlin), 1.737 Å (Pribram)) with respect to the small or almost zero Fe content. As an explanation, an alternative structure model comprising a certain amount of vacancies in T2 is presented. The tetrahedral rotation angle α is highly positive (+12.1° and +12.5° for the Wheal Maudlin and Pribram samples, respectively), and the layer belongs to the Franzini type A. Distortion parameters of octahedra and tetrahedra are given for both samples. One hydrogen atom engaged in the hydrogen bond was located in the Wheal Maudlin sample.
Sections:Crystal structure
Subsections:Specific structures: silicates
Subjects:Clay mineralogy; Clay minerals; Cronstedtite; Crystal growth; Crystal structure; Morphology; Order-disorder; Polyhedra; Polytypism; Refinement; Serpentine group; Sheet silicates; Silicates; Twinning; X-ray diffraction data; Bohemia; Central Europe; Cornwall England; Czech Republic; England; Europe; Great Britain; Pribram Czech Republic; United Kingdom; Western Europe; Wheal Maudlin England
Coordinates:N500000 N505000 W0041500 W0063000
Abstract Numbers:03M/200
Record ID:2003005685
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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