MgNb2Al2O8 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded in a distorted rectangular see-saw-like geometry to four equivalent O(3) atoms. Nb(1) is bonded in a 6-coordinate geometry to one O(2), one O(3), two equivalent O(1), and two equivalent O(4) atoms. Al(1) is bonded to one O(1), one O(3), one O(4), and three equivalent O(2) atoms to form a mixture of corner and edge-sharing AlO6 octahedra. The corner-sharing octahedral tilt angles are 24°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal non-coplanar geometry to two equivalent Nb(1) and one Al(1) atom. In the second O site, O(2) is bonded to one Nb(1) and three equivalent Al(1) atoms to form distorted ONbAl3 trigonal pyramids that share corners with four equivalent O(3)Mg2NbAl tetrahedra, corners with two equivalent O(2)NbAl3 trigonal pyramids, and edges with two equivalent O(2)NbAl3 trigonal pyramids. In the third O site, O(3) is bonded to two equivalent Mg(1), one Nb(1), and one Al(1) atom to form distorted OMg2NbAl tetrahedra that share corners with four equivalent O(3)Mg2NbAl tetrahedra, corners with four equivalent O(2)NbAl3 trigonal pyramids, and  an edgeedge with one O(3)Mg2NbAl tetrahedra. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to two equivalent Nb(1) and one Al(1) atom.
MgNb2Al2O8 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded in a distorted rectangular see-saw-like geometry to four equivalent O(3) atoms. All Mg(1)-O(3) bond lengths are 2.24 Å. Nb(1) is bonded in a 6-coordinate geometry to one O(2), one O(3), two equivalent O(1), and two equivalent O(4) atoms. The Nb(1)-O(2) bond length is 2.33 Å. The Nb(1)-O(3) bond length is 2.07 Å. Both Nb(1)-O(1) bond lengths are 1.98 Å. There is one shorter (2.04 Å) and one longer (2.13 Å) Nb(1)-O(4) bond length. Al(1) is bonded to one O(1), one O(3), one O(4), and three equivalent O(2) atoms to form a mixture of corner and edge-sharing AlO6 octahedra. The corner-sharing octahedral tilt angles are 24°. The Al(1)-O(1) bond length is 1.96 Å. The Al(1)-O(3) bond length is 1.86 Å. The Al(1)-O(4) bond length is 1.93 Å. There are two shorter (1.88 Å) and one longer (2.05 Å) Al(1)-O(2) bond length. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal non-coplanar geometry to two equivalent Nb(1) and one Al(1) atom. In the second O site, O(2) is bonded to one Nb(1) and three equivalent Al(1) atoms to form distorted ONbAl3 trigonal pyramids that share corners with four equivalent O(3)Mg2NbAl tetrahedra, corners with two equivalent O(2)NbAl3 trigonal pyramids, and edges with two equivalent O(2)NbAl3 trigonal pyramids. In the third O site, O(3) is bonded to two equivalent Mg(1), one Nb(1), and one Al(1) atom to form distorted OMg2NbAl tetrahedra that share corners with four equivalent O(3)Mg2NbAl tetrahedra, corners with four equivalent O(2)NbAl3 trigonal pyramids, and  an edgeedge with one O(3)Mg2NbAl tetrahedra. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to two equivalent Nb(1) and one Al(1) atom.