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

The crystal structure of neltnerite CaMn₆SiO₁₂ 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) Å and c = 18.862(2) Å and with space group I4₁/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 Å for Ca-O; 2.041, 2.066 and 2.046 Å for Mn-O; and 1.621 Å for Si-O. -from Authors