Mg3Co2O7 crystallizes in the orthorhombic Cmc2_1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(4), and four equivalent O(3) atoms. In the second Mg site, Mg(2) is bonded in a 6-coordinate geometry to one O(3), two equivalent O(2), and three equivalent O(1) atoms. Co(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form corner-sharing CoO6 octahedra. The corner-sharing octahedral tilt angles range from 39-43°. There are four inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Mg(2) and one Co(1) atom to form distorted OMg3Co tetrahedra that share corners with six equivalent O(1)Mg3Co tetrahedra,  a cornercorner with one O(3)Mg3Co2 trigonal bipyramid,  a cornercorner with one O(4)Mg2Co2 trigonal pyramid, and edges with two equivalent O(3)Mg3Co2 trigonal bipyramids. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Mg(1), two equivalent Mg(2), and two equivalent Co(1) atoms. In the third O site, O(3) is bonded to one Mg(2), two equivalent Mg(1), and two equivalent Co(1) atoms to form distorted OMg3Co2 trigonal bipyramids that share  a cornercorner with one O(1)Mg3Co tetrahedra, corners with two equivalent O(3)Mg3Co2 trigonal bipyramids, corners with four equivalent O(4)Mg2Co2 trigonal pyramids, edges with two equivalent O(1)Mg3Co tetrahedra, edges with three equivalent O(3)Mg3Co2 trigonal bipyramids, and  an edgeedge with one O(4)Mg2Co2 trigonal pyramid. In the fourth O site, O(4) is bonded to two equivalent Mg(1) and two equivalent Co(1) atoms to form OMg2Co2 trigonal pyramids that share corners with two equivalent O(1)Mg3Co tetrahedra, corners with eight equivalent O(3)Mg3Co2 trigonal bipyramids, corners with two equivalent O(4)Mg2Co2 trigonal pyramids, and edges with two equivalent O(3)Mg3Co2 trigonal bipyramids.
Mg3Co2O7 crystallizes in the orthorhombic Cmc2_1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(4), and four equivalent O(3) atoms. Both Mg(1)-O(2) bond lengths are 2.64 Å. There is one shorter (2.05 Å) and one longer (2.09 Å) Mg(1)-O(4) bond length. There are two shorter (2.06 Å) and two longer (2.32 Å) Mg(1)-O(3) bond lengths. In the second Mg site, Mg(2) is bonded in a 6-coordinate geometry to one O(3), two equivalent O(2), and three equivalent O(1) atoms. The Mg(2)-O(3) bond length is 2.30 Å. There is one shorter (2.03 Å) and one longer (2.20 Å) Mg(2)-O(2) bond length. There are a spread of Mg(2)-O(1) bond distances ranging from 2.00-2.05 Å. Co(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form corner-sharing CoO6 octahedra. The corner-sharing octahedral tilt angles range from 39-43°. The Co(1)-O(1) bond length is 1.86 Å. The Co(1)-O(4) bond length is 1.91 Å. There is one shorter (1.90 Å) and one longer (1.92 Å) Co(1)-O(2) bond length. There is one shorter (1.90 Å) and one longer (1.92 Å) Co(1)-O(3) bond length. There are four inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Mg(2) and one Co(1) atom to form distorted OMg3Co tetrahedra that share corners with six equivalent O(1)Mg3Co tetrahedra,  a cornercorner with one O(3)Mg3Co2 trigonal bipyramid,  a cornercorner with one O(4)Mg2Co2 trigonal pyramid, and edges with two equivalent O(3)Mg3Co2 trigonal bipyramids. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Mg(1), two equivalent Mg(2), and two equivalent Co(1) atoms. In the third O site, O(3) is bonded to one Mg(2), two equivalent Mg(1), and two equivalent Co(1) atoms to form distorted OMg3Co2 trigonal bipyramids that share  a cornercorner with one O(1)Mg3Co tetrahedra, corners with two equivalent O(3)Mg3Co2 trigonal bipyramids, corners with four equivalent O(4)Mg2Co2 trigonal pyramids, edges with two equivalent O(1)Mg3Co tetrahedra, edges with three equivalent O(3)Mg3Co2 trigonal bipyramids, and  an edgeedge with one O(4)Mg2Co2 trigonal pyramid. In the fourth O site, O(4) is bonded to two equivalent Mg(1) and two equivalent Co(1) atoms to form OMg2Co2 trigonal pyramids that share corners with two equivalent O(1)Mg3Co tetrahedra, corners with eight equivalent O(3)Mg3Co2 trigonal bipyramids, corners with two equivalent O(4)Mg2Co2 trigonal pyramids, and edges with two equivalent O(3)Mg3Co2 trigonal bipyramids.