MgCu4(AsS)4 crystallizes in the triclinic P1 space group. Mg(1) is bonded in a 2-coordinate geometry to two equivalent Cu(3), one As(3), two equivalent As(2), one S(1), and two equivalent S(4) atoms. There are four inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one As(2), one S(3), and two equivalent S(2) atoms to form corner-sharing CuAsS3 tetrahedra. In the second Cu site, Cu(2) is bonded to one As(1), one S(2), and two equivalent S(3) atoms to form corner-sharing CuAsS3 tetrahedra. In the third Cu site, Cu(3) is bonded in a 6-coordinate geometry to two equivalent Mg(1), one As(3), one S(1), and two equivalent S(4) atoms. In the fourth Cu site, Cu(4) is bonded to one As(4), one S(4), and two equivalent S(1) atoms to form distorted CuAsS3 tetrahedra that share  a cornercorner with one S(3)Cu3As tetrahedra, corners with two equivalent Cu(4)AsS3 tetrahedra, and corners with four equivalent As(1)CuAs2S tetrahedra. There are four inequivalent As sites. In the first As site, As(1) is bonded to one Cu(2), two equivalent As(4), and one S(1) atom to form distorted AsCuAs2S tetrahedra that share  a cornercorner with one S(2)Cu3As tetrahedra, corners with two equivalent As(1)CuAs2S tetrahedra, corners with four equivalent Cu(4)AsS3 tetrahedra, and corners with four equivalent S(3)Cu3As tetrahedra. In the second As site, As(2) is bonded in a 5-coordinate geometry to two equivalent Mg(1), one Cu(1), and two equivalent As(3) atoms. In the third As site, As(3) is bonded in a 5-coordinate geometry to one Mg(1), one Cu(3), two equivalent As(2), and one S(2) atom. In the fourth As site, As(4) is bonded in a 4-coordinate geometry to one Cu(4), two equivalent As(1), and one S(3) atom. There are four inequivalent S sites. In the first S site, S(1) is bonded in a 5-coordinate geometry to one Mg(1), one Cu(3), two equivalent Cu(4), and one As(1) atom. In the second S site, S(2) is bonded to one Cu(2), two equivalent Cu(1), and one As(3) atom to form SCu3As tetrahedra that share  a cornercorner with one As(1)CuAs2S tetrahedra, corners with two equivalent S(2)Cu3As tetrahedra, and corners with four equivalent S(3)Cu3As tetrahedra. In the third S site, S(3) is bonded to one Cu(1), two equivalent Cu(2), and one As(4) atom to form SCu3As tetrahedra that share  a cornercorner with one Cu(4)AsS3 tetrahedra, corners with two equivalent S(3)Cu3As tetrahedra, corners with four equivalent As(1)CuAs2S tetrahedra, and corners with four equivalent S(2)Cu3As tetrahedra. In the fourth S site, S(4) is bonded to two equivalent Mg(1), one Cu(4), and two equivalent Cu(3) atoms to form distorted edge-sharing SMg2Cu3 square pyramids.
MgCu4(AsS)4 crystallizes in the triclinic P1 space group. Mg(1) is bonded in a 2-coordinate geometry to two equivalent Cu(3), one As(3), two equivalent As(2), one S(1), and two equivalent S(4) atoms. There is one shorter (2.89 Å) and one longer (2.93 Å) Mg(1)-Cu(3) bond length. The Mg(1)-As(3) bond length is 3.07 Å. There is one shorter (2.86 Å) and one longer (3.06 Å) Mg(1)-As(2) bond length. The Mg(1)-S(1) bond length is 2.86 Å. There is one shorter (2.49 Å) and one longer (2.50 Å) Mg(1)-S(4) bond length. There are four inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one As(2), one S(3), and two equivalent S(2) atoms to form corner-sharing CuAsS3 tetrahedra. The Cu(1)-As(2) bond length is 2.42 Å. The Cu(1)-S(3) bond length is 2.28 Å. Both Cu(1)-S(2) bond lengths are 2.27 Å. In the second Cu site, Cu(2) is bonded to one As(1), one S(2), and two equivalent S(3) atoms to form corner-sharing CuAsS3 tetrahedra. The Cu(2)-As(1) bond length is 2.48 Å. The Cu(2)-S(2) bond length is 2.26 Å. Both Cu(2)-S(3) bond lengths are 2.31 Å. In the third Cu site, Cu(3) is bonded in a 6-coordinate geometry to two equivalent Mg(1), one As(3), one S(1), and two equivalent S(4) atoms. The Cu(3)-As(3) bond length is 2.57 Å. The Cu(3)-S(1) bond length is 2.28 Å. Both Cu(3)-S(4) bond lengths are 2.34 Å. In the fourth Cu site, Cu(4) is bonded to one As(4), one S(4), and two equivalent S(1) atoms to form distorted CuAsS3 tetrahedra that share  a cornercorner with one S(3)Cu3As tetrahedra, corners with two equivalent Cu(4)AsS3 tetrahedra, and corners with four equivalent As(1)CuAs2S tetrahedra. The Cu(4)-As(4) bond length is 2.42 Å. The Cu(4)-S(4) bond length is 2.30 Å. Both Cu(4)-S(1) bond lengths are 2.28 Å. There are four inequivalent As sites. In the first As site, As(1) is bonded to one Cu(2), two equivalent As(4), and one S(1) atom to form distorted AsCuAs2S tetrahedra that share  a cornercorner with one S(2)Cu3As tetrahedra, corners with two equivalent As(1)CuAs2S tetrahedra, corners with four equivalent Cu(4)AsS3 tetrahedra, and corners with four equivalent S(3)Cu3As tetrahedra. There is one shorter (2.55 Å) and one longer (2.56 Å) As(1)-As(4) bond length. The As(1)-S(1) bond length is 2.39 Å. In the second As site, As(2) is bonded in a 5-coordinate geometry to two equivalent Mg(1), one Cu(1), and two equivalent As(3) atoms. There is one shorter (2.49 Å) and one longer (2.51 Å) As(2)-As(3) bond length. In the third As site, As(3) is bonded in a 5-coordinate geometry to one Mg(1), one Cu(3), two equivalent As(2), and one S(2) atom. The As(3)-S(2) bond length is 2.47 Å. In the fourth As site, As(4) is bonded in a 4-coordinate geometry to one Cu(4), two equivalent As(1), and one S(3) atom. The As(4)-S(3) bond length is 2.30 Å. There are four inequivalent S sites. In the first S site, S(1) is bonded in a 5-coordinate geometry to one Mg(1), one Cu(3), two equivalent Cu(4), and one As(1) atom. In the second S site, S(2) is bonded to one Cu(2), two equivalent Cu(1), and one As(3) atom to form SCu3As tetrahedra that share  a cornercorner with one As(1)CuAs2S tetrahedra, corners with two equivalent S(2)Cu3As tetrahedra, and corners with four equivalent S(3)Cu3As tetrahedra. In the third S site, S(3) is bonded to one Cu(1), two equivalent Cu(2), and one As(4) atom to form SCu3As tetrahedra that share  a cornercorner with one Cu(4)AsS3 tetrahedra, corners with two equivalent S(3)Cu3As tetrahedra, corners with four equivalent As(1)CuAs2S tetrahedra, and corners with four equivalent S(2)Cu3As tetrahedra. In the fourth S site, S(4) is bonded to two equivalent Mg(1), one Cu(4), and two equivalent Cu(3) atoms to form distorted edge-sharing SMg2Cu3 square pyramids.