Attakolite, a silico-phosphate mineral from the Västanå mine, Sweden, has been submitted to chemical and X-ray structure analysis in order to establish its crystal structure and chemical formula. The mineral is monoclinic, space group C2/m, with cell parameters refined from X-ray powder diffraction; a = 17.188(4), b = 11.477(8), c = 7.322(5) Å, and, β = 113.83(4). Complete X-ray powder diffraction and optical data are given. An electron microprobe analysis gave SiO2 5.9, P2O5 33.6, Al2O3 26.8, Fe2O3 3.9, MnO 9.7, CaO 6.6, SrO 3.0, and H2O (difference) 10.5. This analysis in conjunction with the crystal-structure determination yields the simplest formula, (Ca,Sr)Mn(Al,Fe3+)4[HSiO4][PO4]3(OH)4, wirh Z = 4. The structure has been refined to R = 2.6% and Rw = 2.3%. The layer composed of chains of [AlO4(OH)2] octahedra with independent [PO4] and [SiO3(OH)] tetrahedra in attakolite is compared with similar structural features in carminite, palermoite, tsumcorite, and the descloizite and adelite groups. As the silicate anion is a stronger Lewis base than the phosphate anion, it predictably is the hydrogenated anionic species.