The sorosilicate andremeyerite is monoclinic pseudo-orthorhombic, a = 7.488, b = 13.785, c = 7.085 Å, β = 118.23°, space group P21/c; it always shows multiple (100) twinning. The crystal structure was determined by Patterson and Fourier syntheses and refined by least-squares cycles with isotropic and anisotropic temperature factors. The final R values for 1430 observed reflections were 0.10 and 0.04 (0.06 for all the 1883 measured reflections), respectively, before and after the correction of the structure amplitudes for twinning. In the structure, Si2O7 groups bridge chains of flattened FeO4 tetrahe-dra: Ba atoms fill cavities between four silicate groups. The atomic arrangement in Si2O7, is very near to an ideal arrangement in which the atoms are related by the symmetry mm2: the Si-O-Si angle is 127°, and the mean Si-O distances are 1.656 and 1.619 Å for bridging and nonbridging oxygens, respectively. Two independent Fe sites have four nearest neighbors with oxygens: mean Fe-O distances are 1.987 and 2.034 Å, respectively; the latter Fe site has a fifth neighbor at a greater distance (2.425 Å). The Ba polyhedron is a rough trigonal prism with a centered lateral face (mean Ba-O distance: 2.856 Å). The crystal-chemical formula unit of andremeyerite is BaFe(Fe,Mn,Mg)Si2O7, Z = 4.