Crystal Structure of Tengchongite with a Revised Chemical Formula Ca(UO2)6(MoO4OH)2O2(OH)4·9H2O

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Abstract
Tengchongite, a uranyl molybdate mineral from Tengchong County, Yunnan Province, China, was originally described as orthorhombic, with space group A2122, unit-cell parameters a = 15.616(4) Å, b = 13.043(6) Å, c = 17.716(14) Å, V = 3608 Å3, and an ideal chemistry CaO·6UO2·2MO3·12H2O. Its ideal chemical formula is given as Ca(UO2)6(MoO4)2O5·12H2O in the current IMA-CNMNC List of Mineral Names. Tengchongite is the only mineral with a U:Mo ratio of 3:1, the second-highest ratio of all natural and synthetic uranyl molybdate materials, but its crystal structure remained undetermined until now. This study reports the structure determination of tengchongite from the type sample and a revision of its chemical formula to Ca(UO2)6(MoO4OH)2O2(OH)4·9H2O. Tengchongite is orthorhombic, with space group C2221, Z = 4, a = 13.0866(8) Å, b = 17.6794(12) Å, c = 15.6800(9) Å, and V = 3627.8(4) Å3. Its crystal structure was refined from single-crystal X-ray diffraction data to R1 = 0.0323 for 6055 unique observed reflections. The fundamental building blocks of the tengchongite structure are sheets consisting of six-membered clusters of edge-sharing UO7 pentagonal bipyramids, which are connected by sharing vertices among them, as well as edges and vertices with MoO5 trigonal bipyramids. These sheets, parallel to [010], are linked together by Ca2+ and H2O groups. Tengchongite represents a new type of structural connectivity between U- and Mo-polyhedra for uranyl molybdate minerals.