MgFeCu5S4 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one S(1) and three equivalent S(3) atoms to form distorted MgS4 tetrahedra that share corners with three equivalent Cu(4)S4 tetrahedra, corners with six equivalent Mg(1)S4 tetrahedra, and edges with three equivalent Cu(4)S4 tetrahedra. Fe(1) is bonded in a trigonal planar geometry to three equivalent S(1) atoms. There are five inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one S(4) and three equivalent S(2) atoms to form distorted corner-sharing CuS4 tetrahedra. In the second Cu site, Cu(2) is bonded in a trigonal planar geometry to three equivalent S(4) atoms. In the third Cu site, Cu(3) is bonded in a 4-coordinate geometry to one S(2) and three equivalent S(1) atoms. In the fourth Cu site, Cu(4) is bonded to one S(4) and three equivalent S(3) atoms to form distorted CuS4 tetrahedra that share  a cornercorner with one Cu(1)S4 tetrahedra, corners with three equivalent Mg(1)S4 tetrahedra, corners with six equivalent Cu(4)S4 tetrahedra, and edges with three equivalent Mg(1)S4 tetrahedra. In the fifth Cu site, Cu(5) is bonded in a trigonal non-coplanar geometry to three equivalent S(2) atoms. There are four inequivalent S sites. In the first S site, S(1) is bonded in a 7-coordinate geometry to one Mg(1), three equivalent Fe(1), and three equivalent Cu(3) atoms. In the second S site, S(2) is bonded in a 7-coordinate geometry to one Cu(3), three equivalent Cu(1), and three equivalent Cu(5) atoms. In the third S site, S(3) is bonded to three equivalent Mg(1) and three equivalent Cu(4) atoms to form distorted SMg3Cu3 octahedra that share corners with three equivalent S(4)Cu5 trigonal bipyramids and edges with six equivalent S(3)Mg3Cu3 octahedra. In the fourth S site, S(4) is bonded to one Cu(1), one Cu(4), and three equivalent Cu(2) atoms to form SCu5 trigonal bipyramids that share corners with three equivalent S(3)Mg3Cu3 octahedra and corners with six equivalent S(4)Cu5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 64-66°.
MgFeCu5S4 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one S(1) and three equivalent S(3) atoms to form distorted MgS4 tetrahedra that share corners with three equivalent Cu(4)S4 tetrahedra, corners with six equivalent Mg(1)S4 tetrahedra, and edges with three equivalent Cu(4)S4 tetrahedra. The Mg(1)-S(1) bond length is 2.50 Å. All Mg(1)-S(3) bond lengths are 2.51 Å. Fe(1) is bonded in a trigonal planar geometry to three equivalent S(1) atoms. All Fe(1)-S(1) bond lengths are 2.22 Å. There are five inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one S(4) and three equivalent S(2) atoms to form distorted corner-sharing CuS4 tetrahedra. The Cu(1)-S(4) bond length is 2.27 Å. There is one shorter (2.43 Å) and two longer (2.45 Å) Cu(1)-S(2) bond lengths. In the second Cu site, Cu(2) is bonded in a trigonal planar geometry to three equivalent S(4) atoms. There are two shorter (2.22 Å) and one longer (2.23 Å) Cu(2)-S(4) bond length. In the third Cu site, Cu(3) is bonded in a 4-coordinate geometry to one S(2) and three equivalent S(1) atoms. The Cu(3)-S(2) bond length is 2.26 Å. There is one shorter (2.61 Å) and two longer (2.63 Å) Cu(3)-S(1) bond lengths. In the fourth Cu site, Cu(4) is bonded to one S(4) and three equivalent S(3) atoms to form distorted CuS4 tetrahedra that share  a cornercorner with one Cu(1)S4 tetrahedra, corners with three equivalent Mg(1)S4 tetrahedra, corners with six equivalent Cu(4)S4 tetrahedra, and edges with three equivalent Mg(1)S4 tetrahedra. The Cu(4)-S(4) bond length is 2.27 Å. There are two shorter (2.45 Å) and one longer (2.46 Å) Cu(4)-S(3) bond length. In the fifth Cu site, Cu(5) is bonded in a trigonal non-coplanar geometry to three equivalent S(2) atoms. All Cu(5)-S(2) bond lengths are 2.30 Å. There are four inequivalent S sites. In the first S site, S(1) is bonded in a 7-coordinate geometry to one Mg(1), three equivalent Fe(1), and three equivalent Cu(3) atoms. In the second S site, S(2) is bonded in a 7-coordinate geometry to one Cu(3), three equivalent Cu(1), and three equivalent Cu(5) atoms. In the third S site, S(3) is bonded to three equivalent Mg(1) and three equivalent Cu(4) atoms to form distorted SMg3Cu3 octahedra that share corners with three equivalent S(4)Cu5 trigonal bipyramids and edges with six equivalent S(3)Mg3Cu3 octahedra. In the fourth S site, S(4) is bonded to one Cu(1), one Cu(4), and three equivalent Cu(2) atoms to form SCu5 trigonal bipyramids that share corners with three equivalent S(3)Mg3Cu3 octahedra and corners with six equivalent S(4)Cu5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 64-66°.