MgFe5CuO8 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(1)O6 octahedra and edges with six equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles are 4°. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded in a distorted rectangular see-saw-like geometry to one O(1) and three equivalent O(2) atoms. In the second Fe site, Fe(2) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form FeO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Cu(1)O6 octahedra, and edges with four equivalent Fe(2)O6 octahedra. Cu(1) 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 Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles are 4°. There are two inequivalent O sites. In the first O site, O(1) is bonded to one Fe(1) and three equivalent Fe(2) atoms to form distorted OFe4 trigonal pyramids that share corners with nine equivalent O(2)MgFe3Cu square pyramids, corners with three equivalent O(1)Fe4 trigonal pyramids, and edges with three equivalent O(2)MgFe3Cu square pyramids. In the second O site, O(2) is bonded to one Mg(1), one Fe(1), two equivalent Fe(2), and one Cu(1) atom to form OMgFe3Cu square pyramids that share corners with six equivalent O(2)MgFe3Cu square pyramids, corners with three equivalent O(1)Fe4 trigonal pyramids, edges with six equivalent O(2)MgFe3Cu square pyramids, and  an edgeedge with one O(1)Fe4 trigonal pyramid.
MgFe5CuO8 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(1)O6 octahedra and edges with six equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles are 4°. All Mg(1)-O(2) bond lengths are 2.14 Å. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded in a distorted rectangular see-saw-like geometry to one O(1) and three equivalent O(2) atoms. The Fe(1)-O(1) bond length is 1.88 Å. All Fe(1)-O(2) bond lengths are 1.97 Å. In the second Fe site, Fe(2) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form FeO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Cu(1)O6 octahedra, and edges with four equivalent Fe(2)O6 octahedra. Both Fe(2)-O(1) bond lengths are 2.02 Å. All Fe(2)-O(2) bond lengths are 2.25 Å. Cu(1) 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 Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles are 4°. All Cu(1)-O(2) bond lengths are 2.25 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded to one Fe(1) and three equivalent Fe(2) atoms to form distorted OFe4 trigonal pyramids that share corners with nine equivalent O(2)MgFe3Cu square pyramids, corners with three equivalent O(1)Fe4 trigonal pyramids, and edges with three equivalent O(2)MgFe3Cu square pyramids. In the second O site, O(2) is bonded to one Mg(1), one Fe(1), two equivalent Fe(2), and one Cu(1) atom to form OMgFe3Cu square pyramids that share corners with six equivalent O(2)MgFe3Cu square pyramids, corners with three equivalent O(1)Fe4 trigonal pyramids, edges with six equivalent O(2)MgFe3Cu square pyramids, and  an edgeedge with one O(1)Fe4 trigonal pyramid.