Cd2Mn3O8 crystallizes in the monoclinic C2/m space group. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with six equivalent Cd(1)O6 pentagonal pyramids, edges with four equivalent Mn(2)O6 octahedra, and edges with two equivalent Cd(1)O6 pentagonal pyramids. In the second Mn site, Mn(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form MnO6 octahedra that share corners with four equivalent Cd(1)O6 pentagonal pyramids, edges with two equivalent Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with two equivalent Cd(1)O6 pentagonal pyramids. Cd(1) is bonded to one O(1), one O(2), and four equivalent O(3) atoms to form distorted CdO6 pentagonal pyramids that share corners with three equivalent Mn(1)O6 octahedra, corners with four equivalent Mn(2)O6 octahedra,  an edgeedge with one Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with two equivalent Cd(1)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 19-55°. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Mn(2) and one Cd(1) atom. In the second O site, O(2) is bonded to one Mn(1), two equivalent Mn(2), and one Cd(1) atom to form distorted OMn3Cd trigonal pyramids that share  a cornercorner with one O(2)Mn3Cd trigonal pyramid, corners with four equivalent O(3)Mn2Cd2 trigonal pyramids,  an edgeedge with one O(2)Mn3Cd trigonal pyramid, and edges with four equivalent O(3)Mn2Cd2 trigonal pyramids. In the third O site, O(3) is bonded to one Mn(1), one Mn(2), and two equivalent Cd(1) atoms to form distorted OMn2Cd2 trigonal pyramids that share corners with two equivalent O(2)Mn3Cd trigonal pyramids, corners with six equivalent O(3)Mn2Cd2 trigonal pyramids, edges with two equivalent O(3)Mn2Cd2 trigonal pyramids, and edges with two equivalent O(2)Mn3Cd trigonal pyramids.
Cd2Mn3O8 crystallizes in the monoclinic C2/m space group. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with six equivalent Cd(1)O6 pentagonal pyramids, edges with four equivalent Mn(2)O6 octahedra, and edges with two equivalent Cd(1)O6 pentagonal pyramids. Both Mn(1)-O(2) bond lengths are 1.91 Å. All Mn(1)-O(3) bond lengths are 1.89 Å. In the second Mn site, Mn(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form MnO6 octahedra that share corners with four equivalent Cd(1)O6 pentagonal pyramids, edges with two equivalent Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with two equivalent Cd(1)O6 pentagonal pyramids. Both Mn(2)-O(1) bond lengths are 1.85 Å. Both Mn(2)-O(2) bond lengths are 1.98 Å. Both Mn(2)-O(3) bond lengths are 1.89 Å. Cd(1) is bonded to one O(1), one O(2), and four equivalent O(3) atoms to form distorted CdO6 pentagonal pyramids that share corners with three equivalent Mn(1)O6 octahedra, corners with four equivalent Mn(2)O6 octahedra,  an edgeedge with one Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with two equivalent Cd(1)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 19-55°. The Cd(1)-O(1) bond length is 2.21 Å. The Cd(1)-O(2) bond length is 2.40 Å. There are two shorter (2.22 Å) and two longer (2.38 Å) Cd(1)-O(3) bond lengths. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Mn(2) and one Cd(1) atom. In the second O site, O(2) is bonded to one Mn(1), two equivalent Mn(2), and one Cd(1) atom to form distorted OMn3Cd trigonal pyramids that share  a cornercorner with one O(2)Mn3Cd trigonal pyramid, corners with four equivalent O(3)Mn2Cd2 trigonal pyramids,  an edgeedge with one O(2)Mn3Cd trigonal pyramid, and edges with four equivalent O(3)Mn2Cd2 trigonal pyramids. In the third O site, O(3) is bonded to one Mn(1), one Mn(2), and two equivalent Cd(1) atoms to form distorted OMn2Cd2 trigonal pyramids that share corners with two equivalent O(2)Mn3Cd trigonal pyramids, corners with six equivalent O(3)Mn2Cd2 trigonal pyramids, edges with two equivalent O(3)Mn2Cd2 trigonal pyramids, and edges with two equivalent O(2)Mn3Cd trigonal pyramids.