MgFe4(OF)4 crystallizes in the monoclinic P2 space group. Mg(1) is bonded to four equivalent O(2) and two equivalent F(1) atoms to form MgO4F2 octahedra that share corners with four equivalent Fe(3)O3F2 trigonal bipyramids, edges with two equivalent Mg(1)O4F2 octahedra, edges with four equivalent Fe(2)O2F4 octahedra, and edges with two equivalent Fe(3)O3F2 trigonal bipyramids. There are three inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to four equivalent O(1) and two equivalent F(2) atoms to form FeO4F2 octahedra that share corners with eight equivalent Fe(3)O3F2 trigonal bipyramids and edges with two equivalent Fe(1)O4F2 octahedra. In the second Fe site, Fe(2) is bonded to two equivalent O(2) and four equivalent F(1) atoms to form FeO2F4 octahedra that share corners with four equivalent Fe(3)O3F2 trigonal bipyramids, edges with two equivalent Fe(2)O2F4 octahedra, and edges with four equivalent Mg(1)O4F2 octahedra. In the third Fe site, Fe(3) is bonded to one O(1), two equivalent O(2), and two equivalent F(2) atoms to form distorted FeO3F2 trigonal bipyramids that share corners with two equivalent Mg(1)O4F2 octahedra, corners with two equivalent Fe(2)O2F4 octahedra, corners with four equivalent Fe(1)O4F2 octahedra,  an edgeedge with one Mg(1)O4F2 octahedra, and edges with two equivalent Fe(3)O3F2 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 31-62°. There are two inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Fe(3) and two equivalent Fe(1) atoms. In the second O site, O(2) is bonded to two equivalent Mg(1), one Fe(2), and two equivalent Fe(3) atoms to form a mixture of corner and edge-sharing OMg2Fe3 trigonal bipyramids. There are two inequivalent F sites. In the first F site, F(1) is bonded in a distorted T-shaped geometry to one Mg(1) and two equivalent Fe(2) atoms. In the second F site, F(2) is bonded in a distorted T-shaped geometry to one Fe(1) and two equivalent Fe(3) atoms.
MgFe4(OF)4 crystallizes in the monoclinic P2 space group. Mg(1) is bonded to four equivalent O(2) and two equivalent F(1) atoms to form MgO4F2 octahedra that share corners with four equivalent Fe(3)O3F2 trigonal bipyramids, edges with two equivalent Mg(1)O4F2 octahedra, edges with four equivalent Fe(2)O2F4 octahedra, and edges with two equivalent Fe(3)O3F2 trigonal bipyramids. All Mg(1)-O(2) bond lengths are 2.13 Å. Both Mg(1)-F(1) bond lengths are 1.96 Å. There are three inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to four equivalent O(1) and two equivalent F(2) atoms to form FeO4F2 octahedra that share corners with eight equivalent Fe(3)O3F2 trigonal bipyramids and edges with two equivalent Fe(1)O4F2 octahedra. All Fe(1)-O(1) bond lengths are 1.98 Å. Both Fe(1)-F(2) bond lengths are 2.20 Å. In the second Fe site, Fe(2) is bonded to two equivalent O(2) and four equivalent F(1) atoms to form FeO2F4 octahedra that share corners with four equivalent Fe(3)O3F2 trigonal bipyramids, edges with two equivalent Fe(2)O2F4 octahedra, and edges with four equivalent Mg(1)O4F2 octahedra. Both Fe(2)-O(2) bond lengths are 2.10 Å. All Fe(2)-F(1) bond lengths are 2.14 Å. In the third Fe site, Fe(3) is bonded to one O(1), two equivalent O(2), and two equivalent F(2) atoms to form distorted FeO3F2 trigonal bipyramids that share corners with two equivalent Mg(1)O4F2 octahedra, corners with two equivalent Fe(2)O2F4 octahedra, corners with four equivalent Fe(1)O4F2 octahedra,  an edgeedge with one Mg(1)O4F2 octahedra, and edges with two equivalent Fe(3)O3F2 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 31-62°. The Fe(3)-O(1) bond length is 1.89 Å. There is one shorter (2.11 Å) and one longer (2.12 Å) Fe(3)-O(2) bond length. Both Fe(3)-F(2) bond lengths are 2.09 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Fe(3) and two equivalent Fe(1) atoms. In the second O site, O(2) is bonded to two equivalent Mg(1), one Fe(2), and two equivalent Fe(3) atoms to form a mixture of corner and edge-sharing OMg2Fe3 trigonal bipyramids. There are two inequivalent F sites. In the first F site, F(1) is bonded in a distorted T-shaped geometry to one Mg(1) and two equivalent Fe(2) atoms. In the second F site, F(2) is bonded in a distorted T-shaped geometry to one Fe(1) and two equivalent Fe(3) atoms.