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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

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1945-3027
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Volume 86, Issue 10

Issues

The crystal structures of the low-temperature phases of leonite-type compounds, K2Me(SO4)2·4H2O (Me2+ = Mg, Mn, Fe)

Birgit Hertweck
  • Institut für Mineralogie und Kristallographie, Universität Wien–Geozentrum, 1090 Wien, Austria
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Gerald Giester
  • Institut für Mineralogie und Kristallographie, Universität Wien–Geozentrum, 1090 Wien, Austria
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Eugen Libowitzky
Published Online: 2015-03-26 | DOI: https://doi.org/10.2138/am-2001-1016

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)2⋅4H2O, “Mn-leonite”, K2Mn(SO4)2⋅4H2O, and mereiterite, K2Fe(SO4)2⋅4H2O, 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 I2/a, Z = 8, a = 11.780(2) Å, b = 9.486(2) Å, c = 19.730(4) Å, β = 95.23(3)°, V = 2195.6 Å3), and to R = 0.0230 at 100 K (space group P21/a, Z = 4, a = 11.778(1) Å, b = 9.469(1) Å, c = 9.851(2) Å, β = 95.26(1)°, V = 1094.01 Å3). The two low-temperature crystal structures of “Mnleonite” have been refined to R = 0.0272 at 185 K (space group I2/a, Z = 8, a = 12.035(2) Å, b = 9.549(2) Å, c = 19.839(4) Å, β = 94.99(3)°, V = 2271.3 Å3), and to R = 0.0237 at 110 K (space group P21/a, Z = 4, a = 12.031(1) Å, b = 9.531(1) Å, c = 9.902(1) Å, β = 95.02(1)°, V = 1131.08 Å3). 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) Å, b = 9.502(1) Å, c = 9.913(2) Å, β = 94.87(1)°, V = 1110.66 Å3).

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.

About the article

Received: 2000-11-27

Accepted: 2001-06-01

Published Online: 2015-03-26

Published in Print: 2001-10-01


Citation Information: American Mineralogist, Volume 86, Issue 10, Pages 1282–1292, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2001-1016.

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© 2015 by Walter de Gruyter Berlin/Boston.

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