The Crystal Structure of Ramsdellite from Pirika Mine

Chemical composition and crystal structure of ramsdellite from Pirika mine, Hokkaido, Japan were studied. Chemical composition, determined by EPMA, shows that ramsdellite is pure manganese oxide and does not contain any other element. The crystallographic data, obtained by Rietveld analysis, indicate that ramsdellite has orthorhombic symmetry. Space group is Pnma and unit cell parameters are: a=4.513(1)Å, b=9.264(1)Å and c=2.859(1)Å. The structure consists of MnO6 double chains (Byström, 1950). There are 4 symmetry-dependent MnO6 octahedral in an unit cell and 2 symmetry-independent oxygen atoms in an MnO6 octahedron. The difference of inter-atomic distances between Mn-O(1) and Mn-O(2) is larger than other manganese dioxide minerals. The distortion of MnO6 octahedron is quite similar to that of groutite (α-MnOOH). These data suggest that ramsdellite was formed by oxidation of groutite.