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REFINEMENT OF THE CRYSTAL STRUCTURE OF CRONSTEDTITE-2H₂

Jií Hybler^1,*, Václav Petíek¹, Jan Fábry¹ and Slavomil urovi²

¹ Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-18221 Praha 8, Czech Republic
² Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-84236 Bratislava, Slovakia

^* E-mail address of corresponding author: hybler{at}fzu.cz

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 Píbram, Czech Republic. The Wheal Maudlin sample has the chemical composition (Fe_2.291²⁺Fe_0.709³⁺) (Si_1.298Fe_0.707³⁺Al_0.004)O₅(OH)₄ and the Píbram sample has the composition (Fe_2.269²⁺Fe_0.731³⁺) (Si_1.271Fe_0.724³⁺Al_0.005)O₅(OH)₄. The results of refinements are as follows: a = 5.500(1), c = 14.163(2) Å , V = 371.08(8) ų, R = 3.83%, from 381 independent reflections, and a = 5.4927(1), c = 14.1481(2) Å, V = 369.70(4) ų, R = 4.77%, from 1088 independent reflections for the Wheal Maudlin and Píbram samples, respectively. The best F_o vs. F_c 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) (Píbram) in T1, and Si : Fe = 0.99:0.01(1) (Wheal Maudlin) and 0.888:0.112(7) (Píbram) in T2. Whereas the size of T1 is reasonable (average d_T1–O = 1.693 Å (Wheal Maudlin), 1.691 Å (Píbram)), T2 is unusually large: (d_T2–O = 1.740 Å (Wheal Maudlin), 1.737 Å (Píbram)) 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 is highly positive (+12.1° and +12.5° for the Wheal Maudlin and Píbram 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.

Key Words: Cronstedtite • Layer Silicate • Order-Disorder (OD) • Polytypism • Twinning • X-ray Diffraction

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