Wenkite was found in the marbles of Candoglia (Ivrea zone, Alps, Italy), by Papageorgakis (1959, 1962), who studied its physical and optical properties, determined its hexagonal lattice constants and proposed the chemical formula: Ba4.5Ca4.5Al9Si12O42(OH)5(SO4)2. Wenk (1966) refined the lattice constants, obtaining a = 13.511(1), c = 7.462(1) Å, and determined P-62m as the most probable space group. Merlino (1973) and Wenk (1973) independently solved and refined the crystal structure of wenkite, which showed a disordered distribution of Si and Al within the tetrahedral sites, a positional disorder of the sulfate groups and additional disorder in the extraframework cations. Merlino (1973) obtained the crystal chemical formula (Ba,K)4(Ca,Na)6(Si,Al)20O41(OH)2(SO4)3, and discussed the structural relationships between wenkite and offretite. These two phases have very similar unit cell parameters and both structures are characterized by columns of polyhedral cages [4665], with point-group symmetry -62m. However, they show different structures, related to the different positions of the mirror planes in offretite (space group P-6m2) and in wenkite (space group P-62m). A new data collection was carried out on a crystal of wenkite, which resulted in a better refinement of the disordered structure. Moreover, the X-ray data collections revealed a superstructure corresponding to a hexagonal unit cell with a = 23.400(3) Å, c = 14.903(2) Å. This supercell is different from that described by Lee (1976), who observed in transmission electron microscopy additional superstructure reflections which doubled both the a and the c parameters. A possible model for the superstructure of wenkite is proposed. Lee, F. (1976): The submicroscopis structure of wenkite. in ”Electron microscopy in mineralogy”, H.R. Wenk, ed., Springer, Heidelberg, 361-370. Merlino, S. (1973): The crystal structure of wenkite. Acta Crystallogr., B30, 1262-1266. Papageorgakis, J. (1959): Ein neues Bariummineral des Marmors von Candoglia in der unteren Val d’Ossola. Schweiz. Mineral. Petrog. Mitt., 39, 333-335. Papageorgakis, J. (1962): Wenkit, ein neues Mineral von Candoglia. Schweiz. Mineral. Petrog. Mitt., 42, 269-274. Wenk, H.R. (1966): New X-ray data for wenkite. Schweiz. Mineral. Petrog. Mitt., 46, 85-88. Wenk, H.R. (1973): The structure of wenkite. Z. Kristallogr., 137, 113-126.

Wenkite, a revised structure

BONACCORSI, ELENA
2016-01-01

Abstract

Wenkite was found in the marbles of Candoglia (Ivrea zone, Alps, Italy), by Papageorgakis (1959, 1962), who studied its physical and optical properties, determined its hexagonal lattice constants and proposed the chemical formula: Ba4.5Ca4.5Al9Si12O42(OH)5(SO4)2. Wenk (1966) refined the lattice constants, obtaining a = 13.511(1), c = 7.462(1) Å, and determined P-62m as the most probable space group. Merlino (1973) and Wenk (1973) independently solved and refined the crystal structure of wenkite, which showed a disordered distribution of Si and Al within the tetrahedral sites, a positional disorder of the sulfate groups and additional disorder in the extraframework cations. Merlino (1973) obtained the crystal chemical formula (Ba,K)4(Ca,Na)6(Si,Al)20O41(OH)2(SO4)3, and discussed the structural relationships between wenkite and offretite. These two phases have very similar unit cell parameters and both structures are characterized by columns of polyhedral cages [4665], with point-group symmetry -62m. However, they show different structures, related to the different positions of the mirror planes in offretite (space group P-6m2) and in wenkite (space group P-62m). A new data collection was carried out on a crystal of wenkite, which resulted in a better refinement of the disordered structure. Moreover, the X-ray data collections revealed a superstructure corresponding to a hexagonal unit cell with a = 23.400(3) Å, c = 14.903(2) Å. This supercell is different from that described by Lee (1976), who observed in transmission electron microscopy additional superstructure reflections which doubled both the a and the c parameters. A possible model for the superstructure of wenkite is proposed. Lee, F. (1976): The submicroscopis structure of wenkite. in ”Electron microscopy in mineralogy”, H.R. Wenk, ed., Springer, Heidelberg, 361-370. Merlino, S. (1973): The crystal structure of wenkite. Acta Crystallogr., B30, 1262-1266. Papageorgakis, J. (1959): Ein neues Bariummineral des Marmors von Candoglia in der unteren Val d’Ossola. Schweiz. Mineral. Petrog. Mitt., 39, 333-335. Papageorgakis, J. (1962): Wenkit, ein neues Mineral von Candoglia. Schweiz. Mineral. Petrog. Mitt., 42, 269-274. Wenk, H.R. (1966): New X-ray data for wenkite. Schweiz. Mineral. Petrog. Mitt., 46, 85-88. Wenk, H.R. (1973): The structure of wenkite. Z. Kristallogr., 137, 113-126.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/838070
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