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The new triclinic phase Ca5(HAsO4)2(AsO4)2 · 5 H2O, polymorph of monoclinic vladimirite, was obtained by dehydration of Ca5(HAsO4)2(AsO4)2 · 9H2O (ferrarisite) at 60°C. Unit-cell data are: a = 8.286(4), b = 6.673(3), c = 9.743(4) Å, α = 86.58(3), β = 111.10(3), γ = 99.74(3)°; Z = 1, space group P[unk]. The structure was solved by direct methods and least-squares refined to R = 0.088, using 1200 observed counter reflections (MoKα radiation). One Ca atom and three water molecules are disordered over centrosymmetrically-related positions. Isostructural (001) layers of Ca and As coordination polyhedra are observed in the structures of both nona- and penta-hydrated phases; however, they differ by a relative shift and sandwich ordered and disordered Ca octahedra, respectively, in the two cases. Dehydration of ferrarisite involves the loss of inter-layer water molecules alternatively from either set of centrosymmetrically-related positions, while layers slide and approach one another. The sandwiched Ca atoms are left attached alternatively to either side of facing layers, and replace their broken coordination bonds by new ones on the side of the shifted layer. This random character of the dehydration reaction is the cause of disorder in the penta-hydrated phase. Crystal-chemical properties of the M5(HAsO4)2(AsO4)2 · n H2O phases are analyzed.

Mg3(P04h' 22 H20-II crystallizes in space group P f, with a 6.937(3),b = 6.932(3), c = 16.132(5)A, a = 82.15(3), f3 = 89.72(3), y 119.49(3) , Z = 1. The structure was solved (direct methods) and refined anisotropically (least squares) to R = 0.042, using 1826 observed reflections measured on a single-crystal diffractometer (MoKa radiation). All hydrogen atoms were located from difference maps and included in the refinement. (001)layers formed by Mg(H20)6 and P04 polyhedra and by lattice water moleculesare linked by hydrogen bonds and stacked with the. . . A BB' A. . . scheme.A structure previously studied by other authors has been recognized as a polytype, Mg3(P04h . 22 H20-I, where the pair of layers AB is nearly identical as in II, but the B' layer is oriented differently with respect to B. The two orientations are related by a mirror pseudo-symmetry of the B layer. Differencesof the hydrogen bonding scheme in the two cases are pointed out.