The role of water in the structures of synthetic hallimondite, [Pb.sub.2][(U[O.sub.2])[(As[O.sub.4]).sub.2]] [([H.sub.2]O).sub.n] and synthetic parsonsite, [Pb.sub.2][(U[O.sub.2])[(P[O.sub.4]).sub.2] [([H.sub.2]O).sub.n], 0 [less than or equal to] n [less than or equal to] 0.5

The crystal structures of synthetic hallimondite and synthetic parsonsite have been refined by full-matrix least-squares techniques to agreement indices (hallimondite, parsonsite) w[R.sub.2], of 5.5, and 7.6% for all data, and R1 of 2.7 and 3.4%, calculated for 339l and 3181 unique observed reflections (|[F.sub.0]| [greater than or equal to] 4[[sigma].sub.F]), respectively. Hallimondite is triclinic, space group P[bar.1], Z = 2, a = 7.1153(8), b = 10.4780(12), c = 6.8571(8) A, [alpha] = 101.178(3)[degrees], [beta] = 95.711 (3)[degrees], [gamma] = 86.651 (3)[degrees], V = 498.64(3) [A.sup.3], and is isostructural with parsonsite, triclinic, space group P[bar.1], Z = 2, a = 6.8432(5), b = 10.4105(7), c = 6.6718(4) A, [alpha] = 101.418(1)[degrees], [beta] = 98.347(2)[degrees], [gamma] = 86.264(2)[degrees], V = 460.64(5) [A.sup.3]. In both structures, hexavalent uranium occurs as a uranyl pentagonal bipyramid. The uranyl polyhedra share an edge, forming dimers that are linked by edge- and vertex-sharing with arsenate or phosphate tetrahedra to form chains along [001]. Two symmetrically distinct Pb positions connect the chains. In hallimondite, a partially occupied oxygen atom is located in the cavity between the uranyl arsenate chains and Pb positions, and is attributed to an [H.sub.2]O group. The crystal of synthetic parsonsite investigated does not have appreciable electron density at this position, but its structural cavity is large enough to contain [H.sub.2]O. The presence of [H.sub.2]O in synthetic hallimondite, and its absence in synthetic parsonsite, are supported by the results of FTIR spectroscopy. In conjunction with thermogravimetric results from the literature, we suggest that the formula of parsonsite should be considered [Pb.sub.2][(U[O.sub.2])[(P[O.sub.4).sub.2]][([H.sub.2]O).sub.n], and hallimondite, [Pb.sub.2][(U[O.sub.2])[(As[O.sub.4).sub.2]][([H.sub.2]O).sub.n], with 0 [less than or equal to] n [less than or equal to] 0.5 in each case.