Structure Refinement and Thermal Stability Studies of the Uranyl Carbonate Mineral Andersonite, Na2Ca[(UO2)(CO3)3]·(5+x)H2O
1
Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia
2
Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN 46556, USA
3
Kola Science Centre, Russian Academy of Sciences, Fersmana 14, 184209 Apatity, Russia
4
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA
*
Author to whom correspondence should be addressed.
Minerals 2018, 8(12), 586; https://doi.org/10.3390/min8120586
Received: 21 November 2018 / Revised: 4 December 2018 / Accepted: 10 December 2018 / Published: 11 December 2018
(This article belongs to the Special Issue Actinide Mineralogy and Crystallography)
A sample of uranyl carbonate mineral andersonite, Na2Ca[(UO2)(CO3)3]·5−6H2O, originating from the Cane Springs Canyon, San Juan Co., UT, USA was studied using single-crystal and powder X-ray diffraction at various temperatures. Andersonite is trigonal, R−3m, a = 17.8448(4), c = 23.6688(6) Å, V = 6527.3(3) Å3, Z = 18, R1 = 0.018. Low-temperature SCXRD determined the positions of H atoms and disordered H2O molecules, arranged within the zeolite-like channels. The results of high-temperature PXRD experiments revealed that the structure of andersonite is stable up to 100 °C; afterwards, it loses crystallinity due to release of H2O molecules. Taking into account the well-defined presence of H2O molecules forming channels’ walls that to the total of five molecules p.f.u., we suggest that the formula of andersonite is Na2Ca[(UO2)(CO3)3]·(5+x)H2O, where x ≤ 1. The thermal behavior of andersonite is essentially anisotropic with the lowest values of the main thermal expansion coefficients in the direction perpendicular to the channels (plane (001)), while the maximal expansion is observed along the c axis—in the direction of channels. The thermal expansion around 80 °C within the (001) plane becomes negative due to the total release of “zeolitic” H2O molecules. The information-based structural complexity parameters of andersonite were calculated after the removal of all the disordered atoms, leaving only the predominantly occupied sites, and show that the crystal structure of the mineral should be described as complex, possessing 4.535 bits/atom and 961.477 bits/cell, which is comparative to the values for another very common natural uranyl carbonate, liebigite.
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Keywords:
andersonite; uranium; carbonate; crystal structure; minerals; X-ray diffraction; structural complexity
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Gurzhiy, V.V.; Krzhizhanovskaya, M.G.; Izatulina, A.R.; Sigmon, G.E.; Krivovichev, S.V.; Burns, P.C. Structure Refinement and Thermal Stability Studies of the Uranyl Carbonate Mineral Andersonite, Na2Ca[(UO2)(CO3)3]·(5+x)H2O. Minerals 2018, 8, 586.
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