The crystal structures of the low-temperature phases of leonite-type compounds K2Me(SO4)2ž4H2O (Me2+ = Mg, Mn, Fe)
- Author(s)
- Birgit Hertweck, Gerald Giester, Eugen Libowitzky
- Abstract
Recent optical and differential scanning calorimetry measurements indicate phase transitions in leonite-type compounds at low temperatures. The crystal structures of these phases, i.e., leonite, K2Mg(SO4)2ž4H2O, "Mn-leonite", K2Mn(SO4)2ž4H2O, and mereiterite, K2FE(SO4)2ž4H2O, have been determined at low temperatures. The leonite structure (space group C2/m at room temperature) is composed of sulfate tetrahedra and MeO6 octahedra which are interconnected by K cations and hydrogen bonds of the H2O molecules. Previous structure investigations at room temperature have shown that one of the sulfate groups is disordered. Refinements of single-crystal X-ray data at ambient and low temperatures indicate that the dynamic disorder in leonite and "Mn-leonite" is "frozen" in two steps and thus results in two new, ordered structures at low temperatures. In mereiterite only one transition from the dynamically disordered to the ordered structure is observed. The two low-temperature crystal structures of leonite have been refined to R=0.0236 at 170 K (space group 12la, Z=8, a.780(2) A°,b=9.486(2) A°,c.730(4) A°, ?•.23(3)°, V!95.6 A°3), and to R=0.0230 at 100 K (space group P2,la, Z=4, a.778(1) A°,b=9.469(1) A°, c= 9.851(2) A°, ?•.26(1)°, V94.01 A°3). The two low-temperature crystal structures of "Mn-leonite" have been refined to R=0.0272 at 185 K (space group 12/a, Z=8, a.035(2) A°, b= 9.549(2) A°, c.839(4) A°, ?”.99(3)°, V"71.3 A°3), and to R=0.0237 at 110 K (space group P21/a, Z=4,a.031(1) A°, b=9.531(1) A°, c=9.902(1) A°, ?•.02(1)°, V31.08 A°3). The low-temperature crystal structure of mereiterite has been refined to R=0.0219 at 185 K (space group P21/a, Z=4, a= 11.834(2) A°, b=9.502(1) A°, c=9.913(2) A°, ?”.87(1)°, V10.66, A°3). The different behavior of mereiterite (i.e., stability range, sequence of transitions) in comparison to the Mg and Mn endmembers may be explained by more distorted MeO6 octahedra and by strongly different hydrogen bond lengths around the disordered sulfate groups.
- Organisation(s)
- Department of Mineralogy and Crystallography
- External organisation(s)
- University of Vienna
- Journal
- American Mineralogist: an international journal of earth and planetary materials
- Volume
- 86
- Pages
- 1282-1292
- No. of pages
- 11
- ISSN
- 0003-004X
- Publication date
- 2001
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 1051 Geology, Mineralogy
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/b269aea6-a6bf-400c-acf4-42da1b93d933