Abstract: | 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. |