The (3 + 3) commensurately modulated structure of the uranyl silicate mineral swamboite-(Nd), Nd0.333[(UO2)(SiO3OH)](H2O)2.41.

The uranyl mineral swamboite has been redefined to swamboite-(Nd) and its structure has been solved and refined as a commensurate structure in sixdimensional superspace. The structure is monoclinic, superspace group P21/m(a1, b1, g1)00(-a1, b1, g1)00 (a2,0,g2)0s, cell parameters a = 6.6560(3), b = 6.9881(5), c = 8.8059(6), c = 11.3361(16) Å, ß = 102.591(5)°, modulation wave-vectors q1 = 1/3 1/3 0; q2 = -1/3 1/3 0; q3 = 1/2 0 1/2. The structure was refined from 8717 reflections to a final R = 0.0610. The model includes modulation both of atomic positions and displacement parameters, as well as occupancy waves. The structure is based upon uranyl-silicate sheets of uranophane topology alternating with an interlayer of partly occupied Nd3+ sites and H2O molecules. The strong (3 + 3) dimensional modulation of the structure originates from the distribution of the Nd-dominated sites and further accommodation of the suitable geometry within the sheets and charge distribution within the structure. The separation distances between the corresponding occupied Nd sites are rationals of the super-cell vectors corresponding to the modulation vectors of the structure. The case of swamboite-(Nd) is the first example of a modulated structure within the oxysalts of U6+. [ABSTRACT FROM AUTHOR]