MgMn6O12 crystallizes in the monoclinic P2 space group. Mg(1) is bonded to two equivalent O(1) and four equivalent O(4) atoms to form MgO6 octahedra that share corners with four equivalent Mn(4)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, edges with four equivalent Mn(2)O6 octahedra, and edges with two equivalent Mn(4)O5 trigonal bipyramids. There are four inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form edge-sharing MnO6 octahedra. In the second Mn site, Mn(2) is bonded to one O(2), one O(4), two equivalent O(1), and two equivalent O(3) atoms to form MnO6 octahedra that share corners with two equivalent Mn(4)O5 trigonal bipyramids, 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. In the third Mn site, Mn(3) is bonded to two equivalent O(6) and four equivalent O(5) atoms to form MnO6 octahedra that share edges with two equivalent Mn(3)O6 octahedra and edges with four equivalent Mn(4)O5 trigonal bipyramids. In the fourth Mn site, Mn(4) is bonded to one O(5), two equivalent O(4), and two equivalent O(6) atoms to form distorted MnO5 trigonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Mn(2)O6 octahedra,  an edgeedge with one Mg(1)O6 octahedra, edges with two equivalent Mn(3)O6 octahedra, and edges with two equivalent Mn(4)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 33-59°. There are six inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1) and two equivalent Mn(2) atoms. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Mn(2) and two equivalent Mn(1) atoms. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Mn(1) and two equivalent Mn(2) atoms. In the fourth O site, O(4) is bonded to two equivalent Mg(1), one Mn(2), and two equivalent Mn(4) atoms to form a mixture of distorted edge and corner-sharing OMg2Mn3 trigonal bipyramids. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Mn(4) and two equivalent Mn(3) atoms. In the sixth O site, O(6) is bonded in a distorted trigonal non-coplanar geometry to one Mn(3) and two equivalent Mn(4) atoms.
MgMn6O12 crystallizes in the monoclinic P2 space group. Mg(1) is bonded to two equivalent O(1) and four equivalent O(4) atoms to form MgO6 octahedra that share corners with four equivalent Mn(4)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, edges with four equivalent Mn(2)O6 octahedra, and edges with two equivalent Mn(4)O5 trigonal bipyramids. Both Mg(1)-O(1) bond lengths are 1.98 Å. There are two shorter (2.15 Å) and two longer (2.16 Å) Mg(1)-O(4) bond lengths. There are four inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form edge-sharing MnO6 octahedra. Both Mn(1)-O(3) bond lengths are 1.94 Å. All Mn(1)-O(2) bond lengths are 1.94 Å. In the second Mn site, Mn(2) is bonded to one O(2), one O(4), two equivalent O(1), and two equivalent O(3) atoms to form MnO6 octahedra that share corners with two equivalent Mn(4)O5 trigonal bipyramids, 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. The Mn(2)-O(2) bond length is 1.93 Å. The Mn(2)-O(4) bond length is 2.08 Å. Both Mn(2)-O(1) bond lengths are 1.92 Å. Both Mn(2)-O(3) bond lengths are 1.95 Å. In the third Mn site, Mn(3) is bonded to two equivalent O(6) and four equivalent O(5) atoms to form MnO6 octahedra that share edges with two equivalent Mn(3)O6 octahedra and edges with four equivalent Mn(4)O5 trigonal bipyramids. Both Mn(3)-O(6) bond lengths are 1.97 Å. All Mn(3)-O(5) bond lengths are 1.93 Å. In the fourth Mn site, Mn(4) is bonded to one O(5), two equivalent O(4), and two equivalent O(6) atoms to form distorted MnO5 trigonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Mn(2)O6 octahedra,  an edgeedge with one Mg(1)O6 octahedra, edges with two equivalent Mn(3)O6 octahedra, and edges with two equivalent Mn(4)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 33-59°. The Mn(4)-O(5) bond length is 2.09 Å. There is one shorter (2.06 Å) and one longer (2.08 Å) Mn(4)-O(4) bond length. There is one shorter (1.91 Å) and one longer (1.93 Å) Mn(4)-O(6) bond length. There are six inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1) and two equivalent Mn(2) atoms. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Mn(2) and two equivalent Mn(1) atoms. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Mn(1) and two equivalent Mn(2) atoms. In the fourth O site, O(4) is bonded to two equivalent Mg(1), one Mn(2), and two equivalent Mn(4) atoms to form a mixture of distorted edge and corner-sharing OMg2Mn3 trigonal bipyramids. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Mn(4) and two equivalent Mn(3) atoms. In the sixth O site, O(6) is bonded in a distorted trigonal non-coplanar geometry to one Mn(3) and two equivalent Mn(4) atoms.