Mg(FeO2)8 crystallizes in the monoclinic P2/m space group. Mg(1) is bonded to two equivalent O(4) and four equivalent O(6) atoms to form MgO6 octahedra that share corners with four equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with four equivalent Fe(4)O6 octahedra, and faces with two equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles are 41°. There are four inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(4), two equivalent O(5), and two equivalent O(6) atoms to form distorted FeO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with four equivalent Fe(4)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, edges with two equivalent Fe(3)O6 octahedra, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 41-49°. In the second Fe site, Fe(2) is bonded to one O(2), one O(3), two equivalent O(7), and two equivalent O(8) atoms to form a mixture of corner and edge-sharing FeO6 octahedra. The corner-sharing octahedral tilt angles are 50°. In the third Fe site, Fe(3) is bonded to one O(5), one O(8), two equivalent O(1), and two equivalent O(2) atoms to form a mixture of corner and edge-sharing FeO6 octahedra. The corner-sharing octahedral tilt angles are 50°. In the fourth Fe site, Fe(4) is bonded to one O(6), one O(7), two equivalent O(3), and two equivalent O(4) atoms to form distorted FeO6 octahedra that share corners with four equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(2)O6 octahedra, and edges with two equivalent Fe(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-49°. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to one Fe(1) and two equivalent Fe(3) atoms. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Fe(2) and two equivalent Fe(3) atoms. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to one Fe(2) and two equivalent Fe(4) atoms. In the fourth O site, O(4) is bonded to one Mg(1), one Fe(1), and two equivalent Fe(4) atoms to form distorted corner-sharing OMgFe3 trigonal pyramids. In the fifth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Fe(3) and two equivalent Fe(1) atoms. In the sixth O site, O(6) is bonded in a 5-coordinate geometry to two equivalent Mg(1), one Fe(4), and two equivalent Fe(1) atoms. In the seventh O site, O(7) is bonded in a distorted trigonal non-coplanar geometry to one Fe(4) and two equivalent Fe(2) atoms. In the eighth O site, O(8) is bonded in a trigonal planar geometry to one Fe(3) and two equivalent Fe(2) atoms.
Mg(FeO2)8 crystallizes in the monoclinic P2/m space group. Mg(1) is bonded to two equivalent O(4) and four equivalent O(6) atoms to form MgO6 octahedra that share corners with four equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with four equivalent Fe(4)O6 octahedra, and faces with two equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles are 41°. Both Mg(1)-O(4) bond lengths are 2.00 Å. All Mg(1)-O(6) bond lengths are 2.11 Å. There are four inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(4), two equivalent O(5), and two equivalent O(6) atoms to form distorted FeO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with four equivalent Fe(4)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, edges with two equivalent Fe(3)O6 octahedra, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 41-49°. The Fe(1)-O(1) bond length is 2.04 Å. The Fe(1)-O(4) bond length is 2.25 Å. Both Fe(1)-O(5) bond lengths are 1.93 Å. Both Fe(1)-O(6) bond lengths are 2.10 Å. In the second Fe site, Fe(2) is bonded to one O(2), one O(3), two equivalent O(7), and two equivalent O(8) atoms to form a mixture of corner and edge-sharing FeO6 octahedra. The corner-sharing octahedral tilt angles are 50°. The Fe(2)-O(2) bond length is 1.99 Å. The Fe(2)-O(3) bond length is 2.11 Å. Both Fe(2)-O(7) bond lengths are 1.94 Å. Both Fe(2)-O(8) bond lengths are 1.94 Å. In the third Fe site, Fe(3) is bonded to one O(5), one O(8), two equivalent O(1), and two equivalent O(2) atoms to form a mixture of corner and edge-sharing FeO6 octahedra. The corner-sharing octahedral tilt angles are 50°. The Fe(3)-O(5) bond length is 2.02 Å. The Fe(3)-O(8) bond length is 1.99 Å. Both Fe(3)-O(1) bond lengths are 1.95 Å. Both Fe(3)-O(2) bond lengths are 1.94 Å. In the fourth Fe site, Fe(4) is bonded to one O(6), one O(7), two equivalent O(3), and two equivalent O(4) atoms to form distorted FeO6 octahedra that share corners with four equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(2)O6 octahedra, and edges with two equivalent Fe(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-49°. The Fe(4)-O(6) bond length is 2.45 Å. The Fe(4)-O(7) bond length is 2.03 Å. Both Fe(4)-O(3) bond lengths are 1.93 Å. Both Fe(4)-O(4) bond lengths are 1.97 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to one Fe(1) and two equivalent Fe(3) atoms. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Fe(2) and two equivalent Fe(3) atoms. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to one Fe(2) and two equivalent Fe(4) atoms. In the fourth O site, O(4) is bonded to one Mg(1), one Fe(1), and two equivalent Fe(4) atoms to form distorted corner-sharing OMgFe3 trigonal pyramids. In the fifth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Fe(3) and two equivalent Fe(1) atoms. In the sixth O site, O(6) is bonded in a 5-coordinate geometry to two equivalent Mg(1), one Fe(4), and two equivalent Fe(1) atoms. In the seventh O site, O(7) is bonded in a distorted trigonal non-coplanar geometry to one Fe(4) and two equivalent Fe(2) atoms. In the eighth O site, O(8) is bonded in a trigonal planar geometry to one Fe(3) and two equivalent Fe(2) atoms.