MgMn4O8 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form MgO6 octahedra that share corners with four equivalent Mn(1)O5 trigonal bipyramids, edges with four equivalent Mn(2)O6 octahedra, and edges with two equivalent Mn(1)O5 trigonal bipyramids. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(3) and four equivalent O(1) atoms to form distorted MnO5 trigonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra, corners with six equivalent Mn(2)O6 octahedra,  an edgeedge with one Mg(1)O6 octahedra, and edges with two equivalent Mn(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 49-58°. 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 six equivalent Mn(1)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Mn(2)O6 octahedra. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mn(2), and two equivalent Mn(1) atoms to form a mixture of corner and edge-sharing OMgMn3 trigonal pyramids. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1) and two equivalent Mn(2) atoms. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Mn(1) and two equivalent Mn(2) atoms.
MgMn4O8 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form MgO6 octahedra that share corners with four equivalent Mn(1)O5 trigonal bipyramids, edges with four equivalent Mn(2)O6 octahedra, and edges with two equivalent Mn(1)O5 trigonal bipyramids. Both Mg(1)-O(2) bond lengths are 1.97 Å. All Mg(1)-O(1) bond lengths are 2.10 Å. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(3) and four equivalent O(1) atoms to form distorted MnO5 trigonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra, corners with six equivalent Mn(2)O6 octahedra,  an edgeedge with one Mg(1)O6 octahedra, and edges with two equivalent Mn(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 49-58°. The Mn(1)-O(3) bond length is 2.09 Å. There are two shorter (1.97 Å) and two longer (2.00 Å) Mn(1)-O(1) bond lengths. 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 six equivalent Mn(1)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Mn(2)O6 octahedra. Both Mn(2)-O(1) bond lengths are 2.00 Å. Both Mn(2)-O(2) bond lengths are 1.90 Å. Both Mn(2)-O(3) bond lengths are 1.94 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mn(2), and two equivalent Mn(1) atoms to form a mixture of corner and edge-sharing OMgMn3 trigonal pyramids. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1) and two equivalent Mn(2) atoms. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Mn(1) and two equivalent Mn(2) atoms.