Zincoberaunite from Krásno, Czech Republic
| Authors | |
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| Year of publication | 2019 |
| Type | Conference abstract |
| MU Faculty or unit | |
| Citation | |
| Description | Results of the complex XDR structure and chemical study of a newly obtained specimen of zincoberaunite from Krásno shed light onto peculiar behavior of Al3+ in the beraunite structures and give a better idea on hydrogen bonding scheme in beraunite related minerals. According to single-crystal X-ray data, zincoberaunite from Krásno is monoclinic, space group C2/c, with a = 20.3440(19) A, b = 5.1507(3) A, c = 19.1361(15) A, beta = 93.568(8)°, V = 2001.3(3) A3, Z = 4. The general structural architecture given in previous studies on beraunite-related structures has been confirmed. Crystal structure of studied sample contains three distinct H2O groups playing a different role: while O11(Wa) and O12(-Wa) are transformer groups with [3]O atom, O14(Wa) localized in the channels running parallel to b, is non-transformer group with [4]O atom. Two OH groups help to propagate bond-valence within the framework. Interestingly, based on site-scattering refinement, Al3+ is distributed over the M-sites without preferential ordering. Based on refined occupancies and bond-valence considerations the structural formula of studied zincoberaunite is (Zn0.81Al0.19)(OH)2(Fe0.61Al0.39)(OH)2(H2[3]O)2(Fe1.52Al0.48)(H2[3]O)2(Fe1.72Al0.28)(OH)(PO4)4(H2[4]O)2, Z = 4. Electron microprobe analyses support the obtained results. However, keeping the same cations occupancy in the M2-M4 sites, the ratio of Al3+ to Me2+ in the M1 position requires the presence of divalent cations as follows: (Fe0.24Zn0.57Al0.19)1.00(Fe3.84Al1.16)5.00[(PO4)3.88(AsO4)0.10(SiO4)0.01]4.00[O0.16(OH)4.60F0.24]5.00·6H2O. Individual zincoberaunite crystals exhibit a zonality manifested by increasing Fe and decreasing Zn and Al contents from cores to margins. Accordingly, there are observed good correlations between Zn-Al (r = 0.7), Fe-Zn (r = -0.7) and Fe-Al (r = -0.9). In the low-frequency region of the Raman spectra, the strongest peak at 997 cm–1 is attributed to the v1(PO4)3- symmetric stretching vibrations. The other main bands observed are assigned to the v3(PO4)3-antisymmetric stretching vibrations (1038 cm-1), v4(PO4)3- antisymmetric bending modes (580, 603 and 677 cm–1), dM–OH bending vibrations (843 cm–1) and metal-oxygen stretching ones (206, 247 and 316 cm–1). |
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