Refinement of the crystal structure of neltnerite, a member of the bixbyite-braunite group of minerals

The crystal structure of neltnerite CaMn (sub 6) SiO (sub 12) was confirmed by Patterson methods and refined to an R - 0.03 based on 763 observed reflections. The mineral is tetragonal with refined cell constants a = 9.468(1) A and c = 18.862(2) A and with space group I4 (sub 1) /acd. The structure corresponds closely to that of braunite, with Ca in distorted cubic coordination, Si in tetrahedral coordination, and three Mn atoms in octahedral coordination showing Jahn-Teller distortion. Mean interatomic distances are 2.377 A for Ca-O ; 2.041, 2.066 and 2.046 A for Mn-O ; and 1.621 A for Si-O. Significant differences between the coordination polyhedra in neltnerite and braunite are present only for the cubic coordination polyhedron. This polyhedron is less distorted in neltnerite than in braunite but more distorted than in braunite-II. The reported discontinuous solid solution between braunite and neltnerite is probably due to insufficient analytical data and not to any apparent crystal chemical reason.