One of Nature's Puzzles Is Assembled: Analog of the Earth's Most Complex Mineral, Ewingite, Synthesized in a Laboratory.

Through the combination of low-temperature hydrothermal synthesis and room-temperature evaporation, a synthetic phase similar in composition and crystal structure to the Earth's most complex mineral, ewingite, was obtained. The crystal structures of both natural and synthetic compounds are based on supertetrahedral uranyl-carbonate nanoclusters that are arranged according to the cubic body-centered lattice principle. The structure and composition of the uranyl carbonate nanocluster were refined using the data on synthetic material. Although the stability of natural ewingite is higher (according to visual observation and experimental studies), the synthetic phase can be regarded as a primary and/or metastable reaction product which further re-crystallizes into a more stable form under environmental conditions.

Chemically Induced Polytypic Phase Transitions in the Mg[(UO2)(TO4)2(H2O)](H2O)4 (T = S, Se) System.

Chemically induced polytypic phase transitions have been observed during experimental investigations of crystallization in the mixed uranyl sulfate-selenate Mg[(UO2)(TO4)2(H2O)](H2O)4 (T = S, Se) system. Three different structure types form in the system, depending upon the Se:S ratio in the initial aqueous solution. The phases with the Se/(Se + S) ratios (in mol %) in the ranges 0-9, 16-47, and 58-100 crystallize in the space groups P21, Pmn21, and P21/c, respectively. The structures of the phases are based upon the same type of uranyl-based sulfate/selenate chains that, through hydrogen bonds, are linked into pseudolayers of the same topological type. The layers are linked into three-dimensional structures via interlayer Mg-centered octahedra. The three structure types contain the same layers but with different stacking sequences that can be conveniently described as belonging to the 1M, 2O, and 2M polytypic modifications. The Se-for-S substitution demonstrates a strong selectivity with preferential incorporation of Se into less tightly bonded T1 site. The larger ionic radius of Se6+ relative to S6+ induces rotation of (T1O4) tetrahedra in the adjacent layers and reconstruction of the structure types. From the information-theoretic viewpoint, the intermediate Pmn21 structure type is more complex than the monoclinic end-member structure types.