MgMn3Cd2O8 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form MgO6 octahedra that share edges with two equivalent Mn(1)O6 octahedra and edges with four equivalent Mn(2)O6 octahedra. 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 edges with two equivalent Mg(1)O6 octahedra and edges with four equivalent Mn(2)O6 octahedra. 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 edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Mn(1)O6 octahedra, and edges with two equivalent Mn(2)O6 octahedra. Cd(1) is bonded in a 6-coordinate geometry to one O(1), one O(2), and four equivalent O(3) atoms. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), two equivalent Mn(2), and one Cd(1) atom to form OMgMn2Cd trigonal pyramids that share corners with four equivalent O(3)MgMn2Cd2 trigonal bipyramids,  a cornercorner with one O(1)MgMn2Cd trigonal pyramid, edges with four equivalent O(3)MgMn2Cd2 trigonal bipyramids, and  an edgeedge with one O(1)MgMn2Cd trigonal pyramid. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Mn(1), two equivalent Mn(2), and one Cd(1) atom. In the third O site, O(3) is bonded to one Mg(1), one Mn(1), one Mn(2), and two equivalent Cd(1) atoms to form distorted OMgMn2Cd2 trigonal bipyramids that share corners with five equivalent O(3)MgMn2Cd2 trigonal bipyramids, corners with two equivalent O(1)MgMn2Cd trigonal pyramids, edges with four equivalent O(3)MgMn2Cd2 trigonal bipyramids, and edges with two equivalent O(1)MgMn2Cd trigonal pyramids.
MgMn3Cd2O8 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form MgO6 octahedra that share edges with two equivalent Mn(1)O6 octahedra and edges with four equivalent Mn(2)O6 octahedra. Both Mg(1)-O(1) bond lengths are 2.08 Å. All Mg(1)-O(3) bond lengths are 2.07 Å. 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 edges with two equivalent Mg(1)O6 octahedra and edges with four equivalent Mn(2)O6 octahedra. Both Mn(1)-O(2) bond lengths are 2.22 Å. All Mn(1)-O(3) bond lengths are 1.99 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 edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Mn(1)O6 octahedra, and edges with two equivalent Mn(2)O6 octahedra. Both Mn(2)-O(1) bond lengths are 1.97 Å. Both Mn(2)-O(2) bond lengths are 1.96 Å. Both Mn(2)-O(3) bond lengths are 2.13 Å. Cd(1) is bonded in a 6-coordinate geometry to one O(1), one O(2), and four equivalent O(3) atoms. The Cd(1)-O(1) bond length is 2.27 Å. The Cd(1)-O(2) bond length is 2.36 Å. There are two shorter (2.32 Å) and two longer (2.59 Å) Cd(1)-O(3) bond lengths. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), two equivalent Mn(2), and one Cd(1) atom to form OMgMn2Cd trigonal pyramids that share corners with four equivalent O(3)MgMn2Cd2 trigonal bipyramids,  a cornercorner with one O(1)MgMn2Cd trigonal pyramid, edges with four equivalent O(3)MgMn2Cd2 trigonal bipyramids, and  an edgeedge with one O(1)MgMn2Cd trigonal pyramid. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Mn(1), two equivalent Mn(2), and one Cd(1) atom. In the third O site, O(3) is bonded to one Mg(1), one Mn(1), one Mn(2), and two equivalent Cd(1) atoms to form distorted OMgMn2Cd2 trigonal bipyramids that share corners with five equivalent O(3)MgMn2Cd2 trigonal bipyramids, corners with two equivalent O(1)MgMn2Cd trigonal pyramids, edges with four equivalent O(3)MgMn2Cd2 trigonal bipyramids, and edges with two equivalent O(1)MgMn2Cd trigonal pyramids.