Mg3Fe2O7 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 to one O(3), two equivalent O(2), and three equivalent O(1) atoms to form distorted MgO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with eight equivalent Mg(2)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and  a faceface with one Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-73°. Fe(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form FeO6 octahedra that share corners with two equivalent Mg(2)O6 octahedra, corners with five equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(2)O6 octahedra, and  a faceface with one Mg(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 39-63°. There are four inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Mg(2) and one Fe(1) atom to form distorted OMg3Fe tetrahedra that share corners with six equivalent O(1)Mg3Fe tetrahedra and  a cornercorner with one O(4)Mg2Fe2 trigonal pyramid. 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 Fe(1) atoms. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Mg(2), two equivalent Mg(1), and two equivalent Fe(1) atoms. In the fourth O site, O(4) is bonded to two equivalent Mg(1) and two equivalent Fe(1) atoms to form OMg2Fe2 trigonal pyramids that share corners with two equivalent O(1)Mg3Fe tetrahedra and corners with two equivalent O(4)Mg2Fe2 trigonal pyramids.
Mg3Fe2O7 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 2.69 Å. There is one shorter (2.09 Å) and one longer (2.16 Å) Mg(1)-O(4) bond length. There are two shorter (2.09 Å) and two longer (2.49 Å) Mg(1)-O(3) bond lengths. In the second Mg site, Mg(2) is bonded to one O(3), two equivalent O(2), and three equivalent O(1) atoms to form distorted MgO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with eight equivalent Mg(2)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and  a faceface with one Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-73°. The Mg(2)-O(3) bond length is 2.26 Å. There is one shorter (2.09 Å) and one longer (2.25 Å) Mg(2)-O(2) bond length. There are a spread of Mg(2)-O(1) bond distances ranging from 2.03-2.11 Å. Fe(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form FeO6 octahedra that share corners with two equivalent Mg(2)O6 octahedra, corners with five equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(2)O6 octahedra, and  a faceface with one Mg(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 39-63°. The Fe(1)-O(1) bond length is 1.91 Å. The Fe(1)-O(4) bond length is 1.97 Å. There is one shorter (1.93 Å) and one longer (2.02 Å) Fe(1)-O(2) bond length. There is one shorter (1.90 Å) and one longer (2.09 Å) Fe(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 Fe(1) atom to form distorted OMg3Fe tetrahedra that share corners with six equivalent O(1)Mg3Fe tetrahedra and  a cornercorner with one O(4)Mg2Fe2 trigonal pyramid. 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 Fe(1) atoms. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Mg(2), two equivalent Mg(1), and two equivalent Fe(1) atoms. In the fourth O site, O(4) is bonded to two equivalent Mg(1) and two equivalent Fe(1) atoms to form OMg2Fe2 trigonal pyramids that share corners with two equivalent O(1)Mg3Fe tetrahedra and corners with two equivalent O(4)Mg2Fe2 trigonal pyramids.