Synthetic bayleyite, Mg2[UO2(CO3)3].18H2O: thermochemistry, crystallography and crystal structure

Thermochemistry, morphology, optical properties and crystal structure of synthetic bayleyite, Mg2[UO2(CO3)3] centredot 18H2O, monoclinic, have been studied. Incongruent melting at 55 deg, three steps of dehydration and two steps of decarboxylation have been found by thermochemic investigations. Morphology: prisms along [001] with (100), (110), (210), (001), (401), (021), (211), (111) and (3-bar11) as the most important forms. Optical data: nsub(α) = 1.453, nsub(β) = 1.498, nsub(γ) = 1.499, 2Vsub(x) = 16 deg, Y = b, X angle c = 11 deg. Crystal structure: space group P21/a, a = 26.560(3), b = 15.256(2), c = 6.505(1) A, β = 92.90(1) deg, Z = 4; R = 0.029 for 5126 independent reflections measured with MoKsub(α)-radiation. The structure is built up from isolated Mg(H2O)6 octahedra. UO2(CO3)3 units and lattice water molecules, all held together by hydrogen bonds only. (Author)