MgMn3Cu3O8 crystallizes in the trigonal R-3m space group. Mg(1) is bonded to six equivalent O(2) atoms to form MgO6 octahedra that share corners with six equivalent Cu(2)O6 octahedra and edges with six equivalent Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles are 10°. Mn(1) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form MnO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Cu(2)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded in a distorted rectangular see-saw-like geometry to one O(1) and three equivalent O(2) atoms. In the second Cu site, Cu(2) is bonded to six equivalent O(2) atoms to form CuO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra and edges with six equivalent Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles are 10°. There are two inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Mn(1) and one Cu(1) atom to form distorted OMn3Cu trigonal pyramids that share corners with nine equivalent O(2)MgMn2Cu2 trigonal bipyramids, corners with three equivalent O(1)Mn3Cu trigonal pyramids, and edges with three equivalent O(2)MgMn2Cu2 trigonal bipyramids. In the second O site, O(2) is bonded to one Mg(1), two equivalent Mn(1), one Cu(1), and one Cu(2) atom to form OMgMn2Cu2 trigonal bipyramids that share corners with six equivalent O(2)MgMn2Cu2 trigonal bipyramids, corners with three equivalent O(1)Mn3Cu trigonal pyramids, edges with six equivalent O(2)MgMn2Cu2 trigonal bipyramids, and  an edgeedge with one O(1)Mn3Cu trigonal pyramid.
MgMn3Cu3O8 crystallizes in the trigonal R-3m space group. Mg(1) is bonded to six equivalent O(2) atoms to form MgO6 octahedra that share corners with six equivalent Cu(2)O6 octahedra and edges with six equivalent Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles are 10°. All Mg(1)-O(2) bond lengths are 2.09 Å. Mn(1) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form MnO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Cu(2)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. Both Mn(1)-O(1) bond lengths are 2.01 Å. All Mn(1)-O(2) bond lengths are 2.05 Å. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded in a distorted rectangular see-saw-like geometry to one O(1) and three equivalent O(2) atoms. The Cu(1)-O(1) bond length is 1.85 Å. All Cu(1)-O(2) bond lengths are 2.21 Å. In the second Cu site, Cu(2) is bonded to six equivalent O(2) atoms to form CuO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra and edges with six equivalent Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles are 10°. All Cu(2)-O(2) bond lengths are 2.29 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Mn(1) and one Cu(1) atom to form distorted OMn3Cu trigonal pyramids that share corners with nine equivalent O(2)MgMn2Cu2 trigonal bipyramids, corners with three equivalent O(1)Mn3Cu trigonal pyramids, and edges with three equivalent O(2)MgMn2Cu2 trigonal bipyramids. In the second O site, O(2) is bonded to one Mg(1), two equivalent Mn(1), one Cu(1), and one Cu(2) atom to form OMgMn2Cu2 trigonal bipyramids that share corners with six equivalent O(2)MgMn2Cu2 trigonal bipyramids, corners with three equivalent O(1)Mn3Cu trigonal pyramids, edges with six equivalent O(2)MgMn2Cu2 trigonal bipyramids, and  an edgeedge with one O(1)Mn3Cu trigonal pyramid.