MgNb3SiO10 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(2), one O(3), one O(5), one O(6), one O(7), and one O(8) atom to form distorted MgO6 pentagonal pyramids that share  a cornercorner with one Nb(1)O6 octahedra, corners with two equivalent Si(1)O4 tetrahedra, edges with two equivalent Nb(2)O6 octahedra,  an edgeedge with one Nb(3)O5 trigonal bipyramid, and  a faceface with one Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles are 62°. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form distorted NbO6 octahedra that share corners with four equivalent Nb(2)O6 octahedra,  a cornercorner with one Mg(1)O6 pentagonal pyramid,  a cornercorner with one Si(1)O4 tetrahedra, corners with two equivalent Nb(3)O5 trigonal bipyramids, and  a faceface with one Mg(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 19-45°. In the second Nb site, Nb(2) is bonded to one O(10), one O(3), one O(4), one O(5), one O(6), and one O(7) atom to form NbO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, edges with two equivalent Mg(1)O6 pentagonal pyramids, and  an edgeedge with one Nb(3)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 19-45°. In the third Nb site, Nb(3) is bonded to one O(10), one O(2), one O(4), one O(8), and one O(9) atom to form distorted NbO5 trigonal bipyramids that share corners with two equivalent Nb(1)O6 octahedra, corners with two equivalent Si(1)O4 tetrahedra,  an edgeedge with one Nb(2)O6 octahedra, and  an edgeedge with one Mg(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 35-50°. Si(1) is bonded to one O(1), one O(2), one O(7), and one O(9) atom to form SiO4 tetrahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra, corners with two equivalent Mg(1)O6 pentagonal pyramids, and corners with two equivalent Nb(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 48-56°. There are ten inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Nb(1) and one Si(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Mg(1), one Nb(3), and one Si(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Mg(1), one Nb(1), and one Nb(2) atom. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(2), and one Nb(3) atom. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Mg(1), one Nb(1), and one Nb(2) atom. In the sixth O site, O(6) is bonded in a T-shaped geometry to one Mg(1), one Nb(1), and one Nb(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Mg(1), one Nb(2), and one Si(1) atom. In the eighth O site, O(8) is bonded in a T-shaped geometry to one Mg(1), one Nb(1), and one Nb(3) atom. In the ninth O site, O(9) is bonded in a bent 150 degrees geometry to one Nb(3) and one Si(1) atom. In the tenth O site, O(10) is bonded in a water-like geometry to one Nb(2) and one Nb(3) atom.
MgNb3SiO10 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(2), one O(3), one O(5), one O(6), one O(7), and one O(8) atom to form distorted MgO6 pentagonal pyramids that share  a cornercorner with one Nb(1)O6 octahedra, corners with two equivalent Si(1)O4 tetrahedra, edges with two equivalent Nb(2)O6 octahedra,  an edgeedge with one Nb(3)O5 trigonal bipyramid, and  a faceface with one Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles are 62°. The Mg(1)-O(2) bond length is 2.01 Å. The Mg(1)-O(3) bond length is 2.08 Å. The Mg(1)-O(5) bond length is 2.31 Å. The Mg(1)-O(6) bond length is 2.10 Å. The Mg(1)-O(7) bond length is 2.05 Å. The Mg(1)-O(8) bond length is 2.18 Å. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form distorted NbO6 octahedra that share corners with four equivalent Nb(2)O6 octahedra,  a cornercorner with one Mg(1)O6 pentagonal pyramid,  a cornercorner with one Si(1)O4 tetrahedra, corners with two equivalent Nb(3)O5 trigonal bipyramids, and  a faceface with one Mg(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 19-45°. The Nb(1)-O(1) bond length is 2.01 Å. The Nb(1)-O(3) bond length is 1.96 Å. The Nb(1)-O(4) bond length is 2.08 Å. The Nb(1)-O(5) bond length is 2.04 Å. The Nb(1)-O(6) bond length is 2.06 Å. The Nb(1)-O(8) bond length is 2.14 Å. In the second Nb site, Nb(2) is bonded to one O(10), one O(3), one O(4), one O(5), one O(6), and one O(7) atom to form NbO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, edges with two equivalent Mg(1)O6 pentagonal pyramids, and  an edgeedge with one Nb(3)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 19-45°. The Nb(2)-O(10) bond length is 1.97 Å. The Nb(2)-O(3) bond length is 2.12 Å. The Nb(2)-O(4) bond length is 2.24 Å. The Nb(2)-O(5) bond length is 1.89 Å. The Nb(2)-O(6) bond length is 1.95 Å. The Nb(2)-O(7) bond length is 2.16 Å. In the third Nb site, Nb(3) is bonded to one O(10), one O(2), one O(4), one O(8), and one O(9) atom to form distorted NbO5 trigonal bipyramids that share corners with two equivalent Nb(1)O6 octahedra, corners with two equivalent Si(1)O4 tetrahedra,  an edgeedge with one Nb(2)O6 octahedra, and  an edgeedge with one Mg(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 35-50°. The Nb(3)-O(10) bond length is 1.91 Å. The Nb(3)-O(2) bond length is 2.08 Å. The Nb(3)-O(4) bond length is 2.12 Å. The Nb(3)-O(8) bond length is 1.92 Å. The Nb(3)-O(9) bond length is 1.88 Å. Si(1) is bonded to one O(1), one O(2), one O(7), and one O(9) atom to form SiO4 tetrahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra, corners with two equivalent Mg(1)O6 pentagonal pyramids, and corners with two equivalent Nb(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 48-56°. The Si(1)-O(1) bond length is 1.63 Å. The Si(1)-O(2) bond length is 1.66 Å. The Si(1)-O(7) bond length is 1.64 Å. The Si(1)-O(9) bond length is 1.66 Å. There are ten inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Nb(1) and one Si(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Mg(1), one Nb(3), and one Si(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Mg(1), one Nb(1), and one Nb(2) atom. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(2), and one Nb(3) atom. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Mg(1), one Nb(1), and one Nb(2) atom. In the sixth O site, O(6) is bonded in a T-shaped geometry to one Mg(1), one Nb(1), and one Nb(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Mg(1), one Nb(2), and one Si(1) atom. In the eighth O site, O(8) is bonded in a T-shaped geometry to one Mg(1), one Nb(1), and one Nb(3) atom. In the ninth O site, O(9) is bonded in a bent 150 degrees geometry to one Nb(3) and one Si(1) atom. In the tenth O site, O(10) is bonded in a water-like geometry to one Nb(2) and one Nb(3) atom.