MgFeO2 crystallizes in the triclinic P1 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(4), two equivalent O(6), and two equivalent O(7) atoms to form MgO5 trigonal bipyramids that share corners with two equivalent Mg(3)O4 tetrahedra, corners with two equivalent Mg(4)O4 tetrahedra, corners with two equivalent Fe(2)O5 trigonal bipyramids, edges with two equivalent Mg(1)O5 trigonal bipyramids, and  a faceface with one Fe(2)O5 trigonal bipyramid. In the second Mg site, Mg(2) is bonded to one O(1), two equivalent O(5), and two equivalent O(8) atoms to form MgO5 trigonal bipyramids that share corners with two equivalent Mg(3)O4 tetrahedra, corners with two equivalent Mg(4)O4 tetrahedra, corners with two equivalent Fe(3)O5 trigonal bipyramids, edges with two equivalent Mg(2)O5 trigonal bipyramids, and  a faceface with one Fe(3)O5 trigonal bipyramid. In the third Mg site, Mg(3) is bonded to one O(7), one O(8), and two equivalent O(3) atoms to form MgO4 tetrahedra that share corners with two equivalent Mg(3)O4 tetrahedra, corners with two equivalent Mg(1)O5 trigonal bipyramids, corners with two equivalent Mg(2)O5 trigonal bipyramids, corners with two equivalent Fe(2)O5 trigonal bipyramids, corners with two equivalent Fe(3)O5 trigonal bipyramids, and  an edgeedge with one Fe(2)O5 trigonal bipyramid. In the fourth Mg site, Mg(4) is bonded to one O(5), one O(6), and two equivalent O(2) atoms to form MgO4 tetrahedra that share corners with two equivalent Mg(4)O4 tetrahedra, corners with two equivalent Mg(1)O5 trigonal bipyramids, corners with two equivalent Mg(2)O5 trigonal bipyramids, corners with two equivalent Fe(2)O5 trigonal bipyramids, corners with two equivalent Fe(3)O5 trigonal bipyramids, and  an edgeedge with one Fe(3)O5 trigonal bipyramid. There are four inequivalent Fe sites. In the first Fe site, Fe(1) is bonded in a square co-planar geometry to two equivalent O(4) and two equivalent O(5) atoms. In the second Fe site, Fe(2) is bonded to one O(4), two equivalent O(3), and two equivalent O(6) atoms to form distorted FeO5 trigonal bipyramids that share corners with two equivalent Mg(3)O4 tetrahedra, corners with two equivalent Mg(4)O4 tetrahedra, corners with two equivalent Mg(1)O5 trigonal bipyramids,  an edgeedge with one Mg(3)O4 tetrahedra, edges with two equivalent Fe(2)O5 trigonal bipyramids, and  a faceface with one Mg(1)O5 trigonal bipyramid. In the third Fe site, Fe(3) is bonded to one O(1), two equivalent O(2), and two equivalent O(8) atoms to form distorted FeO5 trigonal bipyramids that share corners with two equivalent Mg(3)O4 tetrahedra, corners with two equivalent Mg(4)O4 tetrahedra, corners with two equivalent Mg(2)O5 trigonal bipyramids,  an edgeedge with one Mg(4)O4 tetrahedra, edges with two equivalent Fe(3)O5 trigonal bipyramids, and  a faceface with one Mg(2)O5 trigonal bipyramid. In the fourth Fe site, Fe(4) is bonded in a square co-planar geometry to two equivalent O(1) and two equivalent O(7) atoms. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Mg(2), one Fe(3), and two equivalent Fe(4) atoms to form distorted OMgFe3 tetrahedra that share corners with two equivalent O(1)MgFe3 tetrahedra, corners with two equivalent O(5)Mg3Fe2 trigonal bipyramids, corners with two equivalent O(7)Mg3Fe2 trigonal bipyramids, and  an edgeedge with one O(7)Mg3Fe2 trigonal bipyramid. In the second O site, O(2) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(4) and two equivalent Fe(3) atoms. In the third O site, O(3) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(3) and two equivalent Fe(2) atoms. In the fourth O site, O(4) is bonded to one Mg(1), one Fe(2), and two equivalent Fe(1) atoms to form distorted OMgFe3 tetrahedra that share corners with two equivalent O(4)MgFe3 tetrahedra, corners with two equivalent O(5)Mg3Fe2 trigonal bipyramids, corners with two equivalent O(7)Mg3Fe2 trigonal bipyramids, and  an edgeedge with one O(5)Mg3Fe2 trigonal bipyramid. In the fifth O site, O(5) is bonded to one Mg(4), two equivalent Mg(2), and two equivalent Fe(1) atoms to form distorted OMg3Fe2 trigonal bipyramids that share corners with two equivalent O(1)MgFe3 tetrahedra, corners with two equivalent O(4)MgFe3 tetrahedra,  an edgeedge with one O(4)MgFe3 tetrahedra, and edges with two equivalent O(5)Mg3Fe2 trigonal bipyramids. In the sixth O site, O(6) is bonded in a 5-coordinate geometry to one Mg(4), two equivalent Mg(1), and two equivalent Fe(2) atoms. In the seventh O site, O(7) is bonded to one Mg(3), two equivalent Mg(1), and two equivalent Fe(4) atoms to form distorted OMg3Fe2 trigonal bipyramids that share corners with two equivalent O(1)MgFe3 tetrahedra, corners with two equivalent O(4)MgFe3 tetrahedra,  an edgeedge with one O(1)MgFe3 tetrahedra, and edges with two equivalent O(7)Mg3Fe2 trigonal bipyramids. In the eighth O site, O(8) is bonded in a 5-coordinate geometry to one Mg(3), two equivalent Mg(2), and two equivalent Fe(3) atoms.
MgFeO2 crystallizes in the triclinic P1 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(4), two equivalent O(6), and two equivalent O(7) atoms to form MgO5 trigonal bipyramids that share corners with two equivalent Mg(3)O4 tetrahedra, corners with two equivalent Mg(4)O4 tetrahedra, corners with two equivalent Fe(2)O5 trigonal bipyramids, edges with two equivalent Mg(1)O5 trigonal bipyramids, and  a faceface with one Fe(2)O5 trigonal bipyramid. The Mg(1)-O(4) bond length is 2.08 Å. There is one shorter (2.06 Å) and one longer (2.07 Å) Mg(1)-O(6) bond length. There is one shorter (2.14 Å) and one longer (2.15 Å) Mg(1)-O(7) bond length. In the second Mg site, Mg(2) is bonded to one O(1), two equivalent O(5), and two equivalent O(8) atoms to form MgO5 trigonal bipyramids that share corners with two equivalent Mg(3)O4 tetrahedra, corners with two equivalent Mg(4)O4 tetrahedra, corners with two equivalent Fe(3)O5 trigonal bipyramids, edges with two equivalent Mg(2)O5 trigonal bipyramids, and  a faceface with one Fe(3)O5 trigonal bipyramid. The Mg(2)-O(1) bond length is 2.09 Å. There is one shorter (2.14 Å) and one longer (2.15 Å) Mg(2)-O(5) bond length. There is one shorter (2.06 Å) and one longer (2.07 Å) Mg(2)-O(8) bond length. In the third Mg site, Mg(3) is bonded to one O(7), one O(8), and two equivalent O(3) atoms to form MgO4 tetrahedra that share corners with two equivalent Mg(3)O4 tetrahedra, corners with two equivalent Mg(1)O5 trigonal bipyramids, corners with two equivalent Mg(2)O5 trigonal bipyramids, corners with two equivalent Fe(2)O5 trigonal bipyramids, corners with two equivalent Fe(3)O5 trigonal bipyramids, and  an edgeedge with one Fe(2)O5 trigonal bipyramid. The Mg(3)-O(7) bond length is 2.01 Å. The Mg(3)-O(8) bond length is 1.95 Å. Both Mg(3)-O(3) bond lengths are 1.97 Å. In the fourth Mg site, Mg(4) is bonded to one O(5), one O(6), and two equivalent O(2) atoms to form MgO4 tetrahedra that share corners with two equivalent Mg(4)O4 tetrahedra, corners with two equivalent Mg(1)O5 trigonal bipyramids, corners with two equivalent Mg(2)O5 trigonal bipyramids, corners with two equivalent Fe(2)O5 trigonal bipyramids, corners with two equivalent Fe(3)O5 trigonal bipyramids, and  an edgeedge with one Fe(3)O5 trigonal bipyramid. The Mg(4)-O(5) bond length is 2.02 Å. The Mg(4)-O(6) bond length is 1.96 Å. Both Mg(4)-O(2) bond lengths are 1.97 Å. There are four inequivalent Fe sites. In the first Fe site, Fe(1) is bonded in a square co-planar geometry to two equivalent O(4) and two equivalent O(5) atoms. Both Fe(1)-O(4) bond lengths are 1.98 Å. Both Fe(1)-O(5) bond lengths are 2.16 Å. In the second Fe site, Fe(2) is bonded to one O(4), two equivalent O(3), and two equivalent O(6) atoms to form distorted FeO5 trigonal bipyramids that share corners with two equivalent Mg(3)O4 tetrahedra, corners with two equivalent Mg(4)O4 tetrahedra, corners with two equivalent Mg(1)O5 trigonal bipyramids,  an edgeedge with one Mg(3)O4 tetrahedra, edges with two equivalent Fe(2)O5 trigonal bipyramids, and  a faceface with one Mg(1)O5 trigonal bipyramid. The Fe(2)-O(4) bond length is 2.06 Å. Both Fe(2)-O(3) bond lengths are 2.11 Å. There is one shorter (2.28 Å) and one longer (2.30 Å) Fe(2)-O(6) bond length. In the third Fe site, Fe(3) is bonded to one O(1), two equivalent O(2), and two equivalent O(8) atoms to form distorted FeO5 trigonal bipyramids that share corners with two equivalent Mg(3)O4 tetrahedra, corners with two equivalent Mg(4)O4 tetrahedra, corners with two equivalent Mg(2)O5 trigonal bipyramids,  an edgeedge with one Mg(4)O4 tetrahedra, edges with two equivalent Fe(3)O5 trigonal bipyramids, and  a faceface with one Mg(2)O5 trigonal bipyramid. The Fe(3)-O(1) bond length is 2.04 Å. Both Fe(3)-O(2) bond lengths are 2.11 Å. There is one shorter (2.28 Å) and one longer (2.30 Å) Fe(3)-O(8) bond length. In the fourth Fe site, Fe(4) is bonded in a square co-planar geometry to two equivalent O(1) and two equivalent O(7) atoms. Both Fe(4)-O(1) bond lengths are 1.99 Å. There is one shorter (2.16 Å) and one longer (2.17 Å) Fe(4)-O(7) bond length. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Mg(2), one Fe(3), and two equivalent Fe(4) atoms to form distorted OMgFe3 tetrahedra that share corners with two equivalent O(1)MgFe3 tetrahedra, corners with two equivalent O(5)Mg3Fe2 trigonal bipyramids, corners with two equivalent O(7)Mg3Fe2 trigonal bipyramids, and  an edgeedge with one O(7)Mg3Fe2 trigonal bipyramid. In the second O site, O(2) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(4) and two equivalent Fe(3) atoms. In the third O site, O(3) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(3) and two equivalent Fe(2) atoms. In the fourth O site, O(4) is bonded to one Mg(1), one Fe(2), and two equivalent Fe(1) atoms to form distorted OMgFe3 tetrahedra that share corners with two equivalent O(4)MgFe3 tetrahedra, corners with two equivalent O(5)Mg3Fe2 trigonal bipyramids, corners with two equivalent O(7)Mg3Fe2 trigonal bipyramids, and  an edgeedge with one O(5)Mg3Fe2 trigonal bipyramid. In the fifth O site, O(5) is bonded to one Mg(4), two equivalent Mg(2), and two equivalent Fe(1) atoms to form distorted OMg3Fe2 trigonal bipyramids that share corners with two equivalent O(1)MgFe3 tetrahedra, corners with two equivalent O(4)MgFe3 tetrahedra,  an edgeedge with one O(4)MgFe3 tetrahedra, and edges with two equivalent O(5)Mg3Fe2 trigonal bipyramids. In the sixth O site, O(6) is bonded in a 5-coordinate geometry to one Mg(4), two equivalent Mg(1), and two equivalent Fe(2) atoms. In the seventh O site, O(7) is bonded to one Mg(3), two equivalent Mg(1), and two equivalent Fe(4) atoms to form distorted OMg3Fe2 trigonal bipyramids that share corners with two equivalent O(1)MgFe3 tetrahedra, corners with two equivalent O(4)MgFe3 tetrahedra,  an edgeedge with one O(1)MgFe3 tetrahedra, and edges with two equivalent O(7)Mg3Fe2 trigonal bipyramids. In the eighth O site, O(8) is bonded in a 5-coordinate geometry to one Mg(3), two equivalent Mg(2), and two equivalent Fe(3) atoms.