FeMg2BO5 crystallizes in the orthorhombic Pbam space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(2) and four equivalent O(4) atoms to form MgO6 octahedra that share corners with two equivalent Mg(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with four equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles are 60°. In the second Mg site, Mg(2) is bonded to one O(2), one O(5), two equivalent O(1), and two equivalent O(3) atoms to form MgO6 octahedra that share  a cornercorner with one Mg(1)O6 octahedra, corners with two equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(2)O6 octahedra, edges with two equivalent Mg(3)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 60-63°. In the third Mg site, Mg(3) is bonded to two equivalent O(3) and four equivalent O(5) atoms to form MgO6 octahedra that share corners with four equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(3)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with four equivalent Mg(2)O6 octahedra. The corner-sharing octahedral tilt angles are 14°. Fe(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(5) atoms to form FeO6 octahedra that share corners with two equivalent Mg(2)O6 octahedra, corners with two equivalent Mg(3)O6 octahedra,  an edgeedge with one Mg(3)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Mg(2)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 14-63°. B(1) is bonded in a trigonal planar geometry to one O(1), one O(3), and one O(4) atom. There are five inequivalent O sites. In the first O site, O(5) is bonded to one Mg(2), two equivalent Mg(3), and two equivalent Fe(1) atoms to form OMg3Fe2 square pyramids that share corners with two equivalent O(5)Mg3Fe2 square pyramids, corners with three equivalent O(2)Mg2Fe2 tetrahedra, edges with three equivalent O(5)Mg3Fe2 square pyramids, and  an edgeedge with one O(2)Mg2Fe2 tetrahedra. In the second O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Mg(2), one Fe(1), and one B(1) atom. In the third O site, O(2) is bonded to one Mg(1), one Mg(2), and two equivalent Fe(1) atoms to form OMg2Fe2 tetrahedra that share corners with three equivalent O(5)Mg3Fe2 square pyramids, corners with three equivalent O(2)Mg2Fe2 tetrahedra, and  an edgeedge with one O(5)Mg3Fe2 square pyramid. In the fourth O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to one Mg(3), two equivalent Mg(2), and one B(1) atom. In the fifth O site, O(4) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(1), one Fe(1), and one B(1) atom.
FeMg2BO5 crystallizes in the orthorhombic Pbam space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(2) and four equivalent O(4) atoms to form MgO6 octahedra that share corners with two equivalent Mg(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with four equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles are 60°. Both Mg(1)-O(2) bond lengths are 2.05 Å. All Mg(1)-O(4) bond lengths are 2.14 Å. In the second Mg site, Mg(2) is bonded to one O(2), one O(5), two equivalent O(1), and two equivalent O(3) atoms to form MgO6 octahedra that share  a cornercorner with one Mg(1)O6 octahedra, corners with two equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(2)O6 octahedra, edges with two equivalent Mg(3)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 60-63°. The Mg(2)-O(2) bond length is 1.99 Å. The Mg(2)-O(5) bond length is 2.08 Å. Both Mg(2)-O(1) bond lengths are 2.16 Å. Both Mg(2)-O(3) bond lengths are 2.14 Å. In the third Mg site, Mg(3) is bonded to two equivalent O(3) and four equivalent O(5) atoms to form MgO6 octahedra that share corners with four equivalent Fe(1)O6 octahedra, edges with two equivalent Mg(3)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with four equivalent Mg(2)O6 octahedra. The corner-sharing octahedral tilt angles are 14°. Both Mg(3)-O(3) bond lengths are 2.12 Å. All Mg(3)-O(5) bond lengths are 2.10 Å. Fe(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(5) atoms to form FeO6 octahedra that share corners with two equivalent Mg(2)O6 octahedra, corners with two equivalent Mg(3)O6 octahedra,  an edgeedge with one Mg(3)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Mg(2)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 14-63°. The Fe(1)-O(1) bond length is 2.11 Å. The Fe(1)-O(4) bond length is 2.14 Å. Both Fe(1)-O(2) bond lengths are 1.99 Å. Both Fe(1)-O(5) bond lengths are 2.09 Å. B(1) is bonded in a trigonal planar geometry to one O(1), one O(3), and one O(4) atom. The B(1)-O(1) bond length is 1.40 Å. The B(1)-O(3) bond length is 1.38 Å. The B(1)-O(4) bond length is 1.40 Å. There are five inequivalent O sites. In the first O site, O(5) is bonded to one Mg(2), two equivalent Mg(3), and two equivalent Fe(1) atoms to form OMg3Fe2 square pyramids that share corners with two equivalent O(5)Mg3Fe2 square pyramids, corners with three equivalent O(2)Mg2Fe2 tetrahedra, edges with three equivalent O(5)Mg3Fe2 square pyramids, and  an edgeedge with one O(2)Mg2Fe2 tetrahedra. In the second O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Mg(2), one Fe(1), and one B(1) atom. In the third O site, O(2) is bonded to one Mg(1), one Mg(2), and two equivalent Fe(1) atoms to form OMg2Fe2 tetrahedra that share corners with three equivalent O(5)Mg3Fe2 square pyramids, corners with three equivalent O(2)Mg2Fe2 tetrahedra, and  an edgeedge with one O(5)Mg3Fe2 square pyramid. In the fourth O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to one Mg(3), two equivalent Mg(2), and one B(1) atom. In the fifth O site, O(4) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(1), one Fe(1), and one B(1) atom.