Crystals of Co7.0As3.6O16(1.8 × 3.84 CoO.As2O5) are orthorhombic with unit cell parametersa = 10.526(5),b = 5.985(2), andc = 4.871(2) Å. The space group isPnmawithZ = 1 and, except for fractionally occupied cation sites, the crystals have a structure closely related to that of the mineral olivine. The structure was refined by full-matrix least-squares with isotropic thermal parameters, using 1175 independent reflections measured with MoKα radiation, to a finalRvalue of 0.075. The composition was determined by site population analysis. The structure is based upon a hexagonally close-packed arrangement of oxygen layers with As in tetrahedrally and Co in octahedrally coordinated interstices. The mean As—O bond length is 1.676 Å and the mean Co—O bond lengths are 2.139 Å for the site withmsymmetry and 2.174 Å for the site showing inversion symmetry. These bond lengths are uncorrected for the effects of fractional occupancy of some of the cation sites.