MgNb2(FeO4)2 crystallizes in the monoclinic C2 space group. Mg(1) is bonded in a distorted hexagonal planar geometry to two equivalent O(2) and four equivalent O(4) atoms. Nb(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form a mixture of distorted edge and corner-sharing NbO6 octahedra. The corner-sharing octahedral tilt angles are 15°. Fe(1) is bonded in a 6-coordinate geometry to one O(2), one O(3), one O(4), and three equivalent O(1) atoms. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Nb(1) and three equivalent Fe(1) atoms to form ONbFe3 trigonal pyramids that share corners with four equivalent O(4)Mg2NbFe tetrahedra, corners with two equivalent O(1)NbFe3 trigonal pyramids, and edges with two equivalent O(1)NbFe3 trigonal pyramids. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), two equivalent Nb(1), and one Fe(1) atom. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Nb(1) and one Fe(1) atom. In the fourth O site, O(4) is bonded to two equivalent Mg(1), one Nb(1), and one Fe(1) atom to form distorted OMg2NbFe tetrahedra that share corners with four equivalent O(4)Mg2NbFe tetrahedra, corners with four equivalent O(1)NbFe3 trigonal pyramids, and  an edgeedge with one O(4)Mg2NbFe tetrahedra.
MgNb2(FeO4)2 crystallizes in the monoclinic C2 space group. Mg(1) is bonded in a distorted hexagonal planar geometry to two equivalent O(2) and four equivalent O(4) atoms. Both Mg(1)-O(2) bond lengths are 2.65 Å. There are two shorter (2.31 Å) and two longer (2.36 Å) Mg(1)-O(4) bond lengths. Nb(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form a mixture of distorted edge and corner-sharing NbO6 octahedra. The corner-sharing octahedral tilt angles are 15°. The Nb(1)-O(1) bond length is 2.05 Å. The Nb(1)-O(4) bond length is 1.96 Å. There is one shorter (1.96 Å) and one longer (1.97 Å) Nb(1)-O(2) bond length. There is one shorter (1.97 Å) and one longer (2.17 Å) Nb(1)-O(3) bond length. Fe(1) is bonded in a 6-coordinate geometry to one O(2), one O(3), one O(4), and three equivalent O(1) atoms. The Fe(1)-O(2) bond length is 2.50 Å. The Fe(1)-O(3) bond length is 2.08 Å. The Fe(1)-O(4) bond length is 2.15 Å. There are a spread of Fe(1)-O(1) bond distances ranging from 2.04-2.40 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Nb(1) and three equivalent Fe(1) atoms to form ONbFe3 trigonal pyramids that share corners with four equivalent O(4)Mg2NbFe tetrahedra, corners with two equivalent O(1)NbFe3 trigonal pyramids, and edges with two equivalent O(1)NbFe3 trigonal pyramids. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), two equivalent Nb(1), and one Fe(1) atom. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Nb(1) and one Fe(1) atom. In the fourth O site, O(4) is bonded to two equivalent Mg(1), one Nb(1), and one Fe(1) atom to form distorted OMg2NbFe tetrahedra that share corners with four equivalent O(4)Mg2NbFe tetrahedra, corners with four equivalent O(1)NbFe3 trigonal pyramids, and  an edgeedge with one O(4)Mg2NbFe tetrahedra.