MgMn2O4 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to one O(1), one O(2), and four equivalent O(3) atoms to form MgO6 octahedra that share corners with three equivalent Mn(2)O6 octahedra, corners with three equivalent Mn(3)O6 octahedra,  an edgeedge with one Mn(2)O6 octahedra,  an edgeedge with one Mn(3)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 12-15°. There are three inequivalent Mn sites. In the first Mn site, Mn(1) 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 Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, edges with two equivalent Mn(3)O6 octahedra, and edges with four equivalent Mg(1)O6 octahedra. In the second Mn site, Mn(2) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Mn(3)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 12-15°. In the third Mn site, Mn(3) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 12-15°. There are three inequivalent O sites. In the first O site, O(1) is bonded in a see-saw-like geometry to one Mg(1), one Mn(2), and two equivalent Mn(1) atoms. In the second O site, O(2) is bonded in a see-saw-like geometry to one Mg(1), one Mn(3), and two equivalent Mn(1) atoms. In the third O site, O(3) is bonded to two equivalent Mg(1), one Mn(1), one Mn(2), and one Mn(3) atom to form a mixture of corner and edge-sharing OMg2Mn3 square pyramids.
MgMn2O4 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to one O(1), one O(2), and four equivalent O(3) atoms to form MgO6 octahedra that share corners with three equivalent Mn(2)O6 octahedra, corners with three equivalent Mn(3)O6 octahedra,  an edgeedge with one Mn(2)O6 octahedra,  an edgeedge with one Mn(3)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 12-15°. The Mg(1)-O(1) bond length is 2.11 Å. The Mg(1)-O(2) bond length is 2.12 Å. All Mg(1)-O(3) bond lengths are 2.11 Å. There are three inequivalent Mn sites. In the first Mn site, Mn(1) 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 Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, edges with two equivalent Mn(3)O6 octahedra, and edges with four equivalent Mg(1)O6 octahedra. Both Mn(1)-O(1) bond lengths are 1.96 Å. Both Mn(1)-O(2) bond lengths are 1.96 Å. Both Mn(1)-O(3) bond lengths are 1.94 Å. In the second Mn site, Mn(2) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Mn(3)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 12-15°. Both Mn(2)-O(1) bond lengths are 2.24 Å. All Mn(2)-O(3) bond lengths are 2.22 Å. In the third Mn site, Mn(3) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 12-15°. Both Mn(3)-O(2) bond lengths are 2.23 Å. All Mn(3)-O(3) bond lengths are 2.22 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a see-saw-like geometry to one Mg(1), one Mn(2), and two equivalent Mn(1) atoms. In the second O site, O(2) is bonded in a see-saw-like geometry to one Mg(1), one Mn(3), and two equivalent Mn(1) atoms. In the third O site, O(3) is bonded to two equivalent Mg(1), one Mn(1), one Mn(2), and one Mn(3) atom to form a mixture of corner and edge-sharing OMg2Mn3 square pyramids.