MgNb4O10 crystallizes in the monoclinic Pm space group. Mg(1) is bonded to one O(3), one O(6), one O(7), and two equivalent O(4) atoms to form distorted corner-sharing MgO5 trigonal bipyramids. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a 6-coordinate geometry to one O(10), one O(2), one O(4), one O(8), and two equivalent O(1) atoms. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to one O(10), one O(2), one O(4), one O(5), and two equivalent O(3) atoms. In the third Nb site, Nb(3) is bonded in a 6-coordinate geometry to one O(1), one O(6), one O(7), one O(9), and two equivalent O(8) atoms. In the fourth Nb site, Nb(4) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(7), one O(9), and two equivalent O(10) atoms. There are ten inequivalent O sites. In the first O site, O(1) is bonded to one Nb(3), one Nb(4), and two equivalent Nb(1) atoms to form distorted ONb4 trigonal pyramids that share corners with two equivalent O(4)Mg2Nb2 tetrahedra,  a cornercorner with one O(3)MgNb3 trigonal pyramid, corners with two equivalent O(1)Nb4 trigonal pyramids, and edges with two equivalent O(10)Nb4 trigonal pyramids. In the second O site, O(2) is bonded in a linear geometry to one Nb(1) and one Nb(2) atom. In the third O site, O(3) is bonded to one Mg(1), one Nb(4), and two equivalent Nb(2) atoms to form distorted OMgNb3 trigonal pyramids that share  a cornercorner with one O(1)Nb4 trigonal pyramid, corners with two equivalent O(3)MgNb3 trigonal pyramids, edges with two equivalent O(4)Mg2Nb2 tetrahedra, and edges with two equivalent O(10)Nb4 trigonal pyramids. In the fourth O site, O(4) is bonded to two equivalent Mg(1), one Nb(1), and one Nb(2) atom to form OMg2Nb2 tetrahedra that share corners with two equivalent O(4)Mg2Nb2 tetrahedra, corners with two equivalent O(1)Nb4 trigonal pyramids,  an edgeedge with one O(10)Nb4 trigonal pyramid, and edges with two equivalent O(3)MgNb3 trigonal pyramids. In the fifth O site, O(5) is bonded in a single-bond geometry to one Nb(2) atom. In the sixth O site, O(6) is bonded in a water-like geometry to one Mg(1) and one Nb(3) atom. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Mg(1), one Nb(3), and one Nb(4) atom. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Nb(1) and two equivalent Nb(3) atoms. In the ninth O site, O(9) is bonded in a linear geometry to one Nb(3) and one Nb(4) atom. In the tenth O site, O(10) is bonded to one Nb(1), one Nb(2), and two equivalent Nb(4) atoms to form distorted ONb4 trigonal pyramids that share corners with two equivalent O(10)Nb4 trigonal pyramids,  an edgeedge with one O(4)Mg2Nb2 tetrahedra, edges with two equivalent O(3)MgNb3 trigonal pyramids, and edges with two equivalent O(1)Nb4 trigonal pyramids.
MgNb4O10 crystallizes in the monoclinic Pm space group. Mg(1) is bonded to one O(3), one O(6), one O(7), and two equivalent O(4) atoms to form distorted corner-sharing MgO5 trigonal bipyramids. The Mg(1)-O(3) bond length is 2.36 Å. The Mg(1)-O(6) bond length is 2.03 Å. The Mg(1)-O(7) bond length is 2.04 Å. Both Mg(1)-O(4) bond lengths are 2.13 Å. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a 6-coordinate geometry to one O(10), one O(2), one O(4), one O(8), and two equivalent O(1) atoms. The Nb(1)-O(10) bond length is 2.46 Å. The Nb(1)-O(2) bond length is 1.85 Å. The Nb(1)-O(4) bond length is 2.03 Å. The Nb(1)-O(8) bond length is 2.09 Å. Both Nb(1)-O(1) bond lengths are 2.04 Å. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to one O(10), one O(2), one O(4), one O(5), and two equivalent O(3) atoms. The Nb(2)-O(10) bond length is 2.44 Å. The Nb(2)-O(2) bond length is 2.00 Å. The Nb(2)-O(4) bond length is 2.18 Å. The Nb(2)-O(5) bond length is 1.76 Å. Both Nb(2)-O(3) bond lengths are 2.08 Å. In the third Nb site, Nb(3) is bonded in a 6-coordinate geometry to one O(1), one O(6), one O(7), one O(9), and two equivalent O(8) atoms. The Nb(3)-O(1) bond length is 2.45 Å. The Nb(3)-O(6) bond length is 1.80 Å. The Nb(3)-O(7) bond length is 2.19 Å. The Nb(3)-O(9) bond length is 2.00 Å. Both Nb(3)-O(8) bond lengths are 2.08 Å. In the fourth Nb site, Nb(4) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(7), one O(9), and two equivalent O(10) atoms. The Nb(4)-O(1) bond length is 2.46 Å. The Nb(4)-O(3) bond length is 2.26 Å. The Nb(4)-O(7) bond length is 1.91 Å. The Nb(4)-O(9) bond length is 1.85 Å. Both Nb(4)-O(10) bond lengths are 2.06 Å. There are ten inequivalent O sites. In the first O site, O(1) is bonded to one Nb(3), one Nb(4), and two equivalent Nb(1) atoms to form distorted ONb4 trigonal pyramids that share corners with two equivalent O(4)Mg2Nb2 tetrahedra,  a cornercorner with one O(3)MgNb3 trigonal pyramid, corners with two equivalent O(1)Nb4 trigonal pyramids, and edges with two equivalent O(10)Nb4 trigonal pyramids. In the second O site, O(2) is bonded in a linear geometry to one Nb(1) and one Nb(2) atom. In the third O site, O(3) is bonded to one Mg(1), one Nb(4), and two equivalent Nb(2) atoms to form distorted OMgNb3 trigonal pyramids that share  a cornercorner with one O(1)Nb4 trigonal pyramid, corners with two equivalent O(3)MgNb3 trigonal pyramids, edges with two equivalent O(4)Mg2Nb2 tetrahedra, and edges with two equivalent O(10)Nb4 trigonal pyramids. In the fourth O site, O(4) is bonded to two equivalent Mg(1), one Nb(1), and one Nb(2) atom to form OMg2Nb2 tetrahedra that share corners with two equivalent O(4)Mg2Nb2 tetrahedra, corners with two equivalent O(1)Nb4 trigonal pyramids,  an edgeedge with one O(10)Nb4 trigonal pyramid, and edges with two equivalent O(3)MgNb3 trigonal pyramids. In the fifth O site, O(5) is bonded in a single-bond geometry to one Nb(2) atom. In the sixth O site, O(6) is bonded in a water-like geometry to one Mg(1) and one Nb(3) atom. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Mg(1), one Nb(3), and one Nb(4) atom. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Nb(1) and two equivalent Nb(3) atoms. In the ninth O site, O(9) is bonded in a linear geometry to one Nb(3) and one Nb(4) atom. In the tenth O site, O(10) is bonded to one Nb(1), one Nb(2), and two equivalent Nb(4) atoms to form distorted ONb4 trigonal pyramids that share corners with two equivalent O(10)Nb4 trigonal pyramids,  an edgeedge with one O(4)Mg2Nb2 tetrahedra, edges with two equivalent O(3)MgNb3 trigonal pyramids, and edges with two equivalent O(1)Nb4 trigonal pyramids.