MgFe5CuO8 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form MgO6 octahedra that share corners with six equivalent Fe(3)O6 octahedra, edges with two equivalent Cu(1)O6 octahedra, and edges with four equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 4-8°. There are three inequivalent Fe sites. In the first Fe site, Fe(1) is bonded in a rectangular see-saw-like geometry to one O(1), one O(2), and two equivalent O(3) atoms. In the second Fe site, Fe(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form FeO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(2)O6 octahedra, edges with two equivalent Fe(3)O6 octahedra, and edges with two equivalent Cu(1)O6 octahedra. In the third Fe site, Fe(3) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form FeO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Cu(1)O6 octahedra, and edges with four equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 4-8°. Cu(1) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form CuO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(3)O6 octahedra, and edges with four equivalent Fe(2)O6 octahedra. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Fe(1), one Fe(3), and two equivalent Fe(2) atoms to form OMgFe4 trigonal bipyramids that share corners with two equivalent O(3)MgFe3Cu square pyramids, corners with three equivalent O(2)Fe3Cu tetrahedra, corners with four equivalent O(1)MgFe4 trigonal bipyramids, edges with six equivalent O(3)MgFe3Cu square pyramids, and  an edgeedge with one O(2)Fe3Cu tetrahedra. In the second O site, O(2) is bonded to one Fe(1), two equivalent Fe(2), and one Cu(1) atom to form OFe3Cu tetrahedra that share corners with six equivalent O(3)MgFe3Cu square pyramids, corners with three equivalent O(2)Fe3Cu tetrahedra, corners with three equivalent O(1)MgFe4 trigonal bipyramids, edges with two equivalent O(3)MgFe3Cu square pyramids, and  an edgeedge with one O(1)MgFe4 trigonal bipyramid. In the third O site, O(3) is bonded to one Mg(1), one Fe(1), one Fe(2), one Fe(3), and one Cu(1) atom to form OMgFe3Cu square pyramids that share corners with five equivalent O(3)MgFe3Cu square pyramids, corners with three equivalent O(2)Fe3Cu tetrahedra,  a cornercorner with one O(1)MgFe4 trigonal bipyramid, edges with three equivalent O(3)MgFe3Cu square pyramids,  an edgeedge with one O(2)Fe3Cu tetrahedra, and edges with three equivalent O(1)MgFe4 trigonal bipyramids.
MgFe5CuO8 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form MgO6 octahedra that share corners with six equivalent Fe(3)O6 octahedra, edges with two equivalent Cu(1)O6 octahedra, and edges with four equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 4-8°. Both Mg(1)-O(1) bond lengths are 2.07 Å. All Mg(1)-O(3) bond lengths are 2.15 Å. There are three inequivalent Fe sites. In the first Fe site, Fe(1) is bonded in a rectangular see-saw-like geometry to one O(1), one O(2), and two equivalent O(3) atoms. The Fe(1)-O(1) bond length is 2.12 Å. The Fe(1)-O(2) bond length is 1.92 Å. Both Fe(1)-O(3) bond lengths are 2.14 Å. In the second Fe site, Fe(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form FeO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(2)O6 octahedra, edges with two equivalent Fe(3)O6 octahedra, and edges with two equivalent Cu(1)O6 octahedra. Both Fe(2)-O(1) bond lengths are 2.13 Å. Both Fe(2)-O(2) bond lengths are 1.99 Å. Both Fe(2)-O(3) bond lengths are 2.06 Å. In the third Fe site, Fe(3) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form FeO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Cu(1)O6 octahedra, and edges with four equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 4-8°. Both Fe(3)-O(1) bond lengths are 2.34 Å. All Fe(3)-O(3) bond lengths are 2.22 Å. Cu(1) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form CuO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(3)O6 octahedra, and edges with four equivalent Fe(2)O6 octahedra. Both Cu(1)-O(2) bond lengths are 1.94 Å. All Cu(1)-O(3) bond lengths are 2.23 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Fe(1), one Fe(3), and two equivalent Fe(2) atoms to form OMgFe4 trigonal bipyramids that share corners with two equivalent O(3)MgFe3Cu square pyramids, corners with three equivalent O(2)Fe3Cu tetrahedra, corners with four equivalent O(1)MgFe4 trigonal bipyramids, edges with six equivalent O(3)MgFe3Cu square pyramids, and  an edgeedge with one O(2)Fe3Cu tetrahedra. In the second O site, O(2) is bonded to one Fe(1), two equivalent Fe(2), and one Cu(1) atom to form OFe3Cu tetrahedra that share corners with six equivalent O(3)MgFe3Cu square pyramids, corners with three equivalent O(2)Fe3Cu tetrahedra, corners with three equivalent O(1)MgFe4 trigonal bipyramids, edges with two equivalent O(3)MgFe3Cu square pyramids, and  an edgeedge with one O(1)MgFe4 trigonal bipyramid. In the third O site, O(3) is bonded to one Mg(1), one Fe(1), one Fe(2), one Fe(3), and one Cu(1) atom to form OMgFe3Cu square pyramids that share corners with five equivalent O(3)MgFe3Cu square pyramids, corners with three equivalent O(2)Fe3Cu tetrahedra,  a cornercorner with one O(1)MgFe4 trigonal bipyramid, edges with three equivalent O(3)MgFe3Cu square pyramids,  an edgeedge with one O(2)Fe3Cu tetrahedra, and edges with three equivalent O(1)MgFe4 trigonal bipyramids.