The structures of mcgovernite and carlfrancisite reconsidered: static disorder or ordered defects and twinning?
The minerals mcgovernite and carlfrancisite possess an extraordinary rhombohedral unit cell [mcgovernite: a = b = 8.206 Å and c = 204.118 Å, with the complex formula unit M<sup>2+</sup><sub>19</sub>Zn<sub>3</sub>(OH)<sub>21</sub>(AsO<sub>3</sub>)(AsO<sub>4</sub>)<sub>3</sub>(SiO<sub>4</sub>)<sub>3</sub> (M = Mn, Mg, Zn); carlfrancisite similar]. Hawthorne [(2018), Mineral. Mag. 82, 1101-1118] reported a single-crystal study using a centrosymmetric model (R3c) containing disorder on three cationic sites and an As<sup>III</sup> lone-pair site for both mcgovernite and carlfrancisite. A solution of the structure of mcgovernite from high-resolution synchrotron powder diffraction data suggests a reinterpretation of the crystal structures of mcgovernite and carlfrancisite as noncentrosymmetric R3c with ordered planar defects which allows for merohedral twinning.
Authors
- R. B. Von Dreele (ORCID: https://orcid.org/0000-0002-5339-0634)
- Charles H. Lake
Institutions
- Argonne National Laboratory (US)
- Indiana University of Pennsylvania (US)
Publication Details
- Journal
- Acta Crystallographica Section B Structural Science Crystal Engineering and Materials
- Published
- 2025-11-08
- DOI
- https://doi.org/10.1107/s2052520625009175
- Primary Topic
- Zeolite Catalysis and Synthesis
- Type
- article
- Field-Weighted Citation Impact
- 0.00