Three-dimensional crystal structure analysis followed by full-matrix parameter refinement elucidated the crystal chemistry of synadelphite. The reliability index, Rhkl, converged to 0.089 for 1249 nonequivalent reflections.

Synadelphite is orthorhombic, space group Pnma, a 10.754(11), b 18.865(17), c 9.884 (14) Å, with composition Mn2+9(OH)9(H2O)2(AsO3)(AsO4)2, Z = 4. The atomic arrangement is based on “double” hexagonal close-packed oxygen atoms stacked perpendicular to the c-axis. Voids appear in the oxygen close-packing so that 88 oxygen atoms, instead of 96 for complete densest packing, occur in the unit cell. The ligands include (AsO3)3- trigonal pyramids with point symmetry m, (AsO3)3- tetrahedra, (OH)- ions and H2O molecules. Synadelphite is the first example of a known structure which contains arsenic in two states of oxidation.

The Mn2+ cations occupy octahedral voids. The key to the arrangement is a wedgeshaped cluster of nine edge-sharing Mn-O octahedra in a plane with point symmetry m. This cluster, when submitted to the axial glide and inversion operations, generates the three-dimensional edifice through corner- and edge-sharing with symmetry equivalent wedges.

Classical wet chemical analyses consistently have failed to show the presence of mixed valence states of arsenic in synadelphite. This is attributed to hydrolysis of As3+ in acid solution to form As5+. Such a mechanism also accounts for consistently lower water contents in chemical analyses in comparison with the crystal structure. Observations on rotation and Weissenberg films and a crystal-chemical examination of the data of Hurlbut (1937) for “plumbosynadelphite” (the red outer shell on Langban synadelphites) indicate that this variety must contain some of the cations in higher oxidation states.

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