MgMn3Cu3O8 crystallizes in the trigonal R3m space group. Mg(1) is bonded in a 4-coordinate geometry to one Cu(3) and three equivalent O(3) atoms. Mn(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form MnO6 octahedra that share edges with four equivalent Mn(1)O6 octahedra and  an edgeedge with one Cu(3)MgO3 tetrahedra. There are three inequivalent Cu sites. In the first Cu site, Cu(1) is bonded in a trigonal planar geometry to three equivalent O(3) atoms. In the second Cu site, Cu(2) is bonded in a trigonal planar geometry to three equivalent O(2) atoms. In the third Cu site, Cu(3) is bonded to one Mg(1) and three equivalent O(2) atoms to form distorted CuMgO3 tetrahedra that share edges with three equivalent Mn(1)O6 octahedra. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal non-coplanar geometry to three equivalent Mn(1) atoms. In the second O site, O(2) is bonded in a rectangular see-saw-like geometry to two equivalent Mn(1), one Cu(2), and one Cu(3) atom. In the third O site, O(3) is bonded in a rectangular see-saw-like geometry to one Mg(1), two equivalent Mn(1), and one Cu(1) atom. In the fourth O site, O(4) is bonded in a trigonal non-coplanar geometry to three equivalent Mn(1) atoms.
MgMn3Cu3O8 crystallizes in the trigonal R3m space group. Mg(1) is bonded in a 4-coordinate geometry to one Cu(3) and three equivalent O(3) atoms. The Mg(1)-Cu(3) bond length is 2.47 Å. All Mg(1)-O(3) bond lengths are 1.96 Å. Mn(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form MnO6 octahedra that share edges with four equivalent Mn(1)O6 octahedra and  an edgeedge with one Cu(3)MgO3 tetrahedra. The Mn(1)-O(1) bond length is 1.97 Å. The Mn(1)-O(4) bond length is 1.96 Å. Both Mn(1)-O(2) bond lengths are 2.15 Å. Both Mn(1)-O(3) bond lengths are 2.13 Å. There are three inequivalent Cu sites. In the first Cu site, Cu(1) is bonded in a trigonal planar geometry to three equivalent O(3) atoms. All Cu(1)-O(3) bond lengths are 1.89 Å. In the second Cu site, Cu(2) is bonded in a trigonal planar geometry to three equivalent O(2) atoms. All Cu(2)-O(2) bond lengths are 1.86 Å. In the third Cu site, Cu(3) is bonded to one Mg(1) and three equivalent O(2) atoms to form distorted CuMgO3 tetrahedra that share edges with three equivalent Mn(1)O6 octahedra. All Cu(3)-O(2) bond lengths are 1.91 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal non-coplanar geometry to three equivalent Mn(1) atoms. In the second O site, O(2) is bonded in a rectangular see-saw-like geometry to two equivalent Mn(1), one Cu(2), and one Cu(3) atom. In the third O site, O(3) is bonded in a rectangular see-saw-like geometry to one Mg(1), two equivalent Mn(1), and one Cu(1) atom. In the fourth O site, O(4) is bonded in a trigonal non-coplanar geometry to three equivalent Mn(1) atoms.