MgCr4(CuO4)2 crystallizes in the monoclinic C2 space group. Mg(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form distorted MgO6 octahedra that share corners with six equivalent Cr(2)O6 octahedra, edges with two equivalent Cr(3)O6 octahedra, and edges with four equivalent Cr(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 10-12°. There are three inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(1), one O(2), two equivalent O(3), and two equivalent O(4) atoms to form CrO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Cr(1)O6 octahedra, edges with two equivalent Cr(2)O6 octahedra, and edges with two equivalent Cr(3)O6 octahedra. In the second Cr site, Cr(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form distorted CrO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Cr(3)O6 octahedra, and edges with four equivalent Cr(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 10-12°. In the third Cr site, Cr(3) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form CrO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Cr(2)O6 octahedra, and edges with four equivalent Cr(1)O6 octahedra. Cu(1) is bonded in a distorted bent 150 degrees geometry to one O(2) and one O(4) atom. There are four inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Cr(1), one Cr(2), and one Cr(3) atom. In the second O site, O(2) is bonded to one Mg(1), one Cr(1), one Cr(2), one Cr(3), and one Cu(1) atom to form OMgCr3Cu trigonal bipyramids that share  a cornercorner with one O(2)MgCr3Cu trigonal bipyramid, corners with three equivalent O(4)Cr3Cu trigonal pyramids,  an edgeedge with one O(2)MgCr3Cu trigonal bipyramid, and  an edgeedge with one O(4)Cr3Cu trigonal pyramid. In the third O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Cr(2), and two equivalent Cr(1) atoms. In the fourth O site, O(4) is bonded to one Cr(3), two equivalent Cr(1), and one Cu(1) atom to form OCr3Cu trigonal pyramids that share corners with three equivalent O(2)MgCr3Cu trigonal bipyramids, corners with three equivalent O(4)Cr3Cu trigonal pyramids, and  an edgeedge with one O(2)MgCr3Cu trigonal bipyramid.
MgCr4(CuO4)2 crystallizes in the monoclinic C2 space group. Mg(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form distorted MgO6 octahedra that share corners with six equivalent Cr(2)O6 octahedra, edges with two equivalent Cr(3)O6 octahedra, and edges with four equivalent Cr(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 10-12°. Both Mg(1)-O(1) bond lengths are 2.16 Å. Both Mg(1)-O(2) bond lengths are 2.33 Å. Both Mg(1)-O(3) bond lengths are 2.19 Å. There are three inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(1), one O(2), two equivalent O(3), and two equivalent O(4) atoms to form CrO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Cr(1)O6 octahedra, edges with two equivalent Cr(2)O6 octahedra, and edges with two equivalent Cr(3)O6 octahedra. The Cr(1)-O(1) bond length is 2.01 Å. The Cr(1)-O(2) bond length is 2.04 Å. There is one shorter (2.02 Å) and one longer (2.07 Å) Cr(1)-O(3) bond length. There is one shorter (2.00 Å) and one longer (2.03 Å) Cr(1)-O(4) bond length. In the second Cr site, Cr(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form distorted CrO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Cr(3)O6 octahedra, and edges with four equivalent Cr(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 10-12°. Both Cr(2)-O(1) bond lengths are 2.10 Å. Both Cr(2)-O(2) bond lengths are 2.23 Å. Both Cr(2)-O(3) bond lengths are 2.13 Å. In the third Cr site, Cr(3) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form CrO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Cr(2)O6 octahedra, and edges with four equivalent Cr(1)O6 octahedra. Both Cr(3)-O(1) bond lengths are 2.04 Å. Both Cr(3)-O(2) bond lengths are 2.01 Å. Both Cr(3)-O(4) bond lengths are 1.98 Å. Cu(1) is bonded in a distorted bent 150 degrees geometry to one O(2) and one O(4) atom. The Cu(1)-O(2) bond length is 1.85 Å. The Cu(1)-O(4) bond length is 1.82 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Cr(1), one Cr(2), and one Cr(3) atom. In the second O site, O(2) is bonded to one Mg(1), one Cr(1), one Cr(2), one Cr(3), and one Cu(1) atom to form OMgCr3Cu trigonal bipyramids that share  a cornercorner with one O(2)MgCr3Cu trigonal bipyramid, corners with three equivalent O(4)Cr3Cu trigonal pyramids,  an edgeedge with one O(2)MgCr3Cu trigonal bipyramid, and  an edgeedge with one O(4)Cr3Cu trigonal pyramid. In the third O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Cr(2), and two equivalent Cr(1) atoms. In the fourth O site, O(4) is bonded to one Cr(3), two equivalent Cr(1), and one Cu(1) atom to form OCr3Cu trigonal pyramids that share corners with three equivalent O(2)MgCr3Cu trigonal bipyramids, corners with three equivalent O(4)Cr3Cu trigonal pyramids, and  an edgeedge with one O(2)MgCr3Cu trigonal bipyramid.