Fe2MgBO5 crystallizes in the orthorhombic Pbam space group. Mg(1) is bonded to one O(2), one O(4), two equivalent O(3), and two equivalent O(5) atoms to form MgO6 octahedra that share  a cornercorner with one Fe(2)O6 octahedra, corners with two equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 61-62°. There are three inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(5), two equivalent O(2), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Fe(3)O6 octahedra,  an edgeedge with one Fe(3)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 19-61°. In the second Fe site, Fe(2) is bonded to two equivalent O(4) and four equivalent O(1) atoms to form FeO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(2)O6 octahedra, and edges with four equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles are 62°. In the third Fe site, Fe(3) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form FeO6 octahedra that share corners with four equivalent Fe(1)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, edges with two equivalent Fe(3)O6 octahedra, and edges with four equivalent Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles are 19°. B(1) is bonded in a trigonal planar geometry to one O(1), one O(3), and one O(5) atom. There are five inequivalent O sites. In the first O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Mg(1), one Fe(1), and one B(1) atom. In the second O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to one Fe(1), two equivalent Fe(2), and one B(1) atom. In the third O site, O(2) is bonded to one Mg(1), two equivalent Fe(1), and two equivalent Fe(3) atoms to form OMgFe4 square pyramids that share corners with two equivalent O(2)MgFe4 square pyramids, corners with three equivalent O(4)MgFe3 tetrahedra, edges with three equivalent O(2)MgFe4 square pyramids, and  an edgeedge with one O(4)MgFe3 tetrahedra. In the fourth O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Mg(1), one Fe(3), and one B(1) atom. In the fifth O site, O(4) is bonded to one Mg(1), one Fe(2), and two equivalent Fe(1) atoms to form OMgFe3 tetrahedra that share corners with three equivalent O(2)MgFe4 square pyramids, corners with three equivalent O(4)MgFe3 tetrahedra, and  an edgeedge with one O(2)MgFe4 square pyramid.
Fe2MgBO5 crystallizes in the orthorhombic Pbam space group. Mg(1) is bonded to one O(2), one O(4), two equivalent O(3), and two equivalent O(5) atoms to form MgO6 octahedra that share  a cornercorner with one Fe(2)O6 octahedra, corners with two equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 61-62°. The Mg(1)-O(2) bond length is 2.06 Å. The Mg(1)-O(4) bond length is 1.99 Å. Both Mg(1)-O(3) bond lengths are 2.15 Å. Both Mg(1)-O(5) bond lengths are 2.17 Å. There are three inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(5), two equivalent O(2), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Fe(3)O6 octahedra,  an edgeedge with one Fe(3)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 19-61°. The Fe(1)-O(1) bond length is 2.10 Å. The Fe(1)-O(5) bond length is 2.13 Å. Both Fe(1)-O(2) bond lengths are 2.07 Å. Both Fe(1)-O(4) bond lengths are 2.02 Å. In the second Fe site, Fe(2) is bonded to two equivalent O(4) and four equivalent O(1) atoms to form FeO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, edges with two equivalent Fe(2)O6 octahedra, and edges with four equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles are 62°. Both Fe(2)-O(4) bond lengths are 2.06 Å. All Fe(2)-O(1) bond lengths are 2.23 Å. In the third Fe site, Fe(3) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form FeO6 octahedra that share corners with four equivalent Fe(1)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, edges with two equivalent Fe(3)O6 octahedra, and edges with four equivalent Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles are 19°. Both Fe(3)-O(3) bond lengths are 2.16 Å. All Fe(3)-O(2) bond lengths are 2.15 Å. B(1) is bonded in a trigonal planar geometry to one O(1), one O(3), and one O(5) atom. The B(1)-O(1) bond length is 1.39 Å. The B(1)-O(3) bond length is 1.39 Å. The B(1)-O(5) bond length is 1.40 Å. There are five inequivalent O sites. In the first O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Mg(1), one Fe(1), and one B(1) atom. In the second O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to one Fe(1), two equivalent Fe(2), and one B(1) atom. In the third O site, O(2) is bonded to one Mg(1), two equivalent Fe(1), and two equivalent Fe(3) atoms to form OMgFe4 square pyramids that share corners with two equivalent O(2)MgFe4 square pyramids, corners with three equivalent O(4)MgFe3 tetrahedra, edges with three equivalent O(2)MgFe4 square pyramids, and  an edgeedge with one O(4)MgFe3 tetrahedra. In the fourth O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Mg(1), one Fe(3), and one B(1) atom. In the fifth O site, O(4) is bonded to one Mg(1), one Fe(2), and two equivalent Fe(1) atoms to form OMgFe3 tetrahedra that share corners with three equivalent O(2)MgFe4 square pyramids, corners with three equivalent O(4)MgFe3 tetrahedra, and  an edgeedge with one O(2)MgFe4 square pyramid.