Reference: ▪ Yang, Z. & Giester, G. (2016): Hydrogen-bonding system in amarillite, NaFe(SO4)2(H2O)6: the structure refinement. European Journal of Mineralogy, 28, 953-958.
Abstract: The crystal structure of amarillite, NaFe(SO4)2(H2O)6, was redetermined from a single crystal from Xitieshan, Qinghai Province, China. In complement to previous work, all non-H atoms were refined with anisotropic displacement parameters and H-atoms were located by difference Fourier methods and refined from X-ray diffraction data. The structure can be formally described by octahedral–tetrahedral “sheets” of corner-sharing [NaO4(H2O)2] octahedra and (SO4) tetrahedra, which constitute a structural sheet with composition [Na(SO4)2(H2O)2]3- extending parallel to (001). The resultant sheets are held together by interstitial Fe3+ cations and hydrogen bonds. The [NaO4(H2O)2] octahedra donate four hydrogen bonds to the oxygen atoms of neighboring (SO4) tetrahedra, thus strengthening connections in three dimensions. The interstitial [FeO2(H2O)4] octahedra are linked by eight hydrogen bonds to the vertices of the adjacent octahedral–tetrahedral sheets, further stabilizing the crystal structure of amarillite. The FTIR spectrum of amarillite shows a strong absorption between 3000 cm–1 and 3500 cm–1 with maxima at ∼3110cm–1 and ∼3442 cm–1, which is in accordance with the O–H···O distances derived from structure data.