MgMn4(NiO4)2 crystallizes in the orthorhombic Imm2 space group. Mg(1) is bonded to two equivalent O(2) and two equivalent O(4) atoms to form distorted MgO4 tetrahedra that share corners with six equivalent Mn(3)O6 octahedra and corners with two equivalent Mn(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 29-61°. There are three inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to two equivalent O(3) and two equivalent O(4) atoms to form MnO4 tetrahedra that share corners with six equivalent Mn(3)O6 octahedra and corners with two equivalent Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 55-77°. In the second Mn site, Mn(2) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms. In the third Mn site, Mn(3) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form MnO6 octahedra that share corners with three equivalent Mg(1)O4 tetrahedra, corners with three equivalent Mn(1)O4 tetrahedra, and edges with two equivalent Mn(3)O6 octahedra. Ni(1) is bonded in a 5-coordinate geometry to one O(3), two equivalent O(1), and two equivalent O(4) atoms. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Mn(2), one Mn(3), and two equivalent Ni(1) atoms to form OMn2Ni2 tetrahedra that share  a cornercorner with one O(1)Mn2Ni2 tetrahedra, corners with five equivalent O(4)MgMn2Ni2 trigonal bipyramids, and  an edgeedge with one O(1)Mn2Ni2 tetrahedra. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Mg(1), one Mn(2), and two equivalent Mn(3) atoms. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Mn(1), one Mn(2), two equivalent Mn(3), and one Ni(1) atom. In the fourth O site, O(4) is bonded to one Mg(1), one Mn(1), one Mn(3), and two equivalent Ni(1) atoms to form distorted OMgMn2Ni2 trigonal bipyramids that share corners with five equivalent O(1)Mn2Ni2 tetrahedra,  a cornercorner with one O(4)MgMn2Ni2 trigonal bipyramid, and  a faceface with one O(4)MgMn2Ni2 trigonal bipyramid.
MgMn4(NiO4)2 crystallizes in the orthorhombic Imm2 space group. Mg(1) is bonded to two equivalent O(2) and two equivalent O(4) atoms to form distorted MgO4 tetrahedra that share corners with six equivalent Mn(3)O6 octahedra and corners with two equivalent Mn(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 29-61°. Both Mg(1)-O(2) bond lengths are 1.94 Å. Both Mg(1)-O(4) bond lengths are 2.05 Å. There are three inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to two equivalent O(3) and two equivalent O(4) atoms to form MnO4 tetrahedra that share corners with six equivalent Mn(3)O6 octahedra and corners with two equivalent Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 55-77°. Both Mn(1)-O(3) bond lengths are 2.09 Å. Both Mn(1)-O(4) bond lengths are 2.13 Å. In the second Mn site, Mn(2) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms. Both Mn(2)-O(1) bond lengths are 2.01 Å. Both Mn(2)-O(2) bond lengths are 2.28 Å. Both Mn(2)-O(3) bond lengths are 2.50 Å. In the third Mn site, Mn(3) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form MnO6 octahedra that share corners with three equivalent Mg(1)O4 tetrahedra, corners with three equivalent Mn(1)O4 tetrahedra, and edges with two equivalent Mn(3)O6 octahedra. The Mn(3)-O(1) bond length is 1.95 Å. The Mn(3)-O(4) bond length is 1.97 Å. Both Mn(3)-O(2) bond lengths are 2.15 Å. Both Mn(3)-O(3) bond lengths are 2.20 Å. Ni(1) is bonded in a 5-coordinate geometry to one O(3), two equivalent O(1), and two equivalent O(4) atoms. The Ni(1)-O(3) bond length is 1.99 Å. Both Ni(1)-O(1) bond lengths are 2.00 Å. Both Ni(1)-O(4) bond lengths are 2.17 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Mn(2), one Mn(3), and two equivalent Ni(1) atoms to form OMn2Ni2 tetrahedra that share  a cornercorner with one O(1)Mn2Ni2 tetrahedra, corners with five equivalent O(4)MgMn2Ni2 trigonal bipyramids, and  an edgeedge with one O(1)Mn2Ni2 tetrahedra. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Mg(1), one Mn(2), and two equivalent Mn(3) atoms. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Mn(1), one Mn(2), two equivalent Mn(3), and one Ni(1) atom. In the fourth O site, O(4) is bonded to one Mg(1), one Mn(1), one Mn(3), and two equivalent Ni(1) atoms to form distorted OMgMn2Ni2 trigonal bipyramids that share corners with five equivalent O(1)Mn2Ni2 tetrahedra,  a cornercorner with one O(4)MgMn2Ni2 trigonal bipyramid, and  a faceface with one O(4)MgMn2Ni2 trigonal bipyramid.