Synthesis and Characterization of the Synthetic Minerals Villyaellenite and Sarkinite, Mn₅(AsO₄)₂(HAsO₄)₂ · 4H₂O and Mn₂(AsO₄)(OH)

Using hydrothermal methods, two manganese arsenates have been synthesized and characterized by single crystal X-ray diffraction. The products Mn₅(AsO₄)₂(HAsO₄)₂ ?4H₂O (1) and Mn₂AsO₄(OH) (2), the Mn end-members of the minerals villyaellenite and sarkinite, respectively, have been obtained (crystal data 1: monoclinic, C2/c, a = 18.109(4), b = 9.332(2), c = 9.809(2) Å, β = 96.172(4)?, Z = 4; 2: monoclinic, P2₁/c, a = 10.219(2), b = 13.613(2), c = 12.780(2) Å, β = 108.834(2)?, Z = 16). In both compounds a three-dimensional framework of edge-sharing MnO polyhedra is observed. Based on the availability of the all Mn2containing form of villyaellenite (1), the ordering scheme of the impurity cations of the natural samples could be confirmed. Magnetic susceptibility measurements of 1 indicate the presence of high-spin Mn²⁺ ions. The comparison of the data on sarkinite (2) with the data obtained from the natural sample indicates that the mineral has either a very high Mn content, or an absence of impurity cation ordering.