New insights into the crystal chemistry of zemannite: Trigonal rather than hexagonal symmetry due to ordering within the host-guest structure

The microporous crystal structure of zemannite, Mg(H 2 O) 6 [Zn 2+ Fe 3+ (TeO 3 ) 3 ] 2 · n H 2 O, n ≤ 3, was re-investigated based on single-crystal X-ray diffraction data measured at 298 ± 0.5 K, 200 ± 1 K and 100 ± 3 K. So far, zemannite was described in space group P 6 3 exhibiting a pronounced pseudo symmetry ( P 6 3 / m ). All refinements confirm the [Zn 2+ Fe 3+ (TeO 3 ) 3 ] 1− framework topology with the extra-framework constituents (Mg atoms and H 2 O molecules) being located within the channels along [001]. Measurements on a sample from the type locality revealed the unexpected occurrence of 00 l reflections with l = 2 n + 1, which clearly violate the 6 3 screw-axis symmetry. The minor but significant intensities of the low-order 00 l reflections are assigned to the small differences in the scattering power between the Fe and Zn atoms; thus, the Zn and Fe cations are partly ordered between crystallographically distinct sites within the framework. In addition, the low symmetry allows a full order of the extra-framework atoms for the first time. A series of comparative refinement models were performed in the space groups P 6 3 / m , P 6 3 , P 6 ¯ , and P 3. A fully ordered arrangement of the extra-framework guest atoms confirms the earlier postulated theoretical structure model with a hexahydrated Mg 2+ ion besides additional interstitial H 2 O molecules. The final refinements in space group P 3 yield R1 ≤ 0.025 for the entire data sets measured at the distinct temperatures (2θ max = 101.4°, MoKα radiation). The polarity of the arrangement in the channels is restricted to individual domains of equal twin fractions related by a mirror plane parallel to (0001).