New insights into the crystal chemistry of zemannite

Autor(en)

Herta Silvia Effenberger, Martin Ende, Ronald Miletich

Abstrakt

The microporous crystal structure of zemannite, Mg(H₂O)₆[Zn²⁺Fe³⁺(TeO₃)₃]₂·nH₂O, 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 P6₃ exhibiting a pronounced pseudosymmetry (P6₃/m). All refinements confirm the [Zn²⁺Fe³⁺(TeO₃)₃]¹⁻ framework topology with the extra-framework constituents (Mg atoms and H₂O

molecules) being located within the channels along [001]. Measurements

on a sample from the type locality revealed the unexpected occurrence of

00l reflections with l = 2n + 1, which clearly violate the 6₃ screw-axis symmetry. The minor but significant intensities of the low-order 00l 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 P6₃/m, P6₃, P‾6, and P3.

A fully ordered arrangement of the extra-framework guest atoms confirms

the earlier postulated theoretical structure model with a hexahydrated

Mg²⁺ ion besides additional interstitial H₂O molecules. The final refinements in space group P3 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).

Organisation(en)

Institut für Mineralogie und Kristallographie

Journal

Mineralogy and Petrology

Band

117

Seiten

117-131

Anzahl der Seiten

15

ISSN

0930-0708

Publikationsdatum

05-2023

Peer-reviewed

Ja

ÖFOS 2012

105113 Kristallographie, 105116 Mineralogie

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/a916f46f-e72d-43a8-8d88-02f91075014a