Mechanism of the reaction Ca5H2(AsO4)4 · 9 H2O (ferrarisite) → Ca5H2(AsO4)4 · 5 H2O) (dimorph of vladimirite), and structure of the latter phase

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.