New insights into the crystal chemistry of agardite-(Ce): refinement of the crystal structure, hydrogen bonding, and epitaxial intergrowths with the Sb-analogue of auriacusiteстатья

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Дата последнего поиска статьи во внешних источниках: 7 марта 2018 г.

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[1] New insights into the crystal chemistry of agardite-(ce): refinement of the crystal structure, hydrogen bonding, and epitaxial intergrowths with the sb-analogue of auriacusite / S. M. Aksenov, N. V. Chukanov, J. Göttlicher et al. // Physics and Chemistry of Minerals. — 2018. — Vol. 45, no. 1. — P. 39–50. Agardite-(Ce) from Clara Mine, Schwarzwald, Germany, has been investigated by means of electron microprobe analysis, single-crystal X-ray analysis, XANES spectroscopy and IR spectroscopy. Hexagonal unit-cell parameters are: a = 13.598(6), c = 5.954(3) Å; V = 953.5(2) Å3; space group P63/m. The structure has been solved and refined to final R 1 = 3.87%, wR 2 = 5.02 for 786 I > 3σ(I). Hydrogen atoms have been localized. The crystal-chemical formula is (Z = 2): A(1)(Ce0.82Ca0.14Sr0.04)Σ1.00 A(2)(Ca0.03Ce0.02)Σ0.05 [Cu5.75(Fe3+, Mn)0.20]Σ5.95 [ T(1)(AsO4) 2.96 T(2) (SbO4)0.04)]Σ3.00 (OH)5.96O0.04·3H2O. Hydrogen bonding in agardite-series minerals has been characterized for the first time. IR spectra of agardite-(Ce) and agardite-(Nd) from Lavrion used for comparison, as well as structural data indicate the presence of isolated H+ cations that do not form strong covalent bonds with coordinating O atoms. Agardite-(Ce) from Clara Mine forms epitaxial growths with the Sb-analogue of auriacusite. The latter mineral was characterized by EDS analyses; its typical empirical formulae are Ca0.06Ce0.04Fe3+1.06Cu0.89[(SbO4)0.58(AsO4)0.38(SiO4)0.04]Σ1.00(O,OH) and Ca0.075Ce0.04Fe3+0.93Cu0.97[(SbO4)0.59(AsO4)0.35(SiO4)0.06]Σ1.00(O,OH). The formation of uniaxial growths of the Sb-analogue of auriacusite and agardite-(Ce) is caused by the close values of their c parameters (for auriacusite s.s. c  =  5.9501(5) Å). Three-valence state of iron and five-valence of antimony in both minerals has been validated by means of Fe K- and Sb L 2,3-edge XANES spectroscopy. [ DOI ]

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