MgNb3SiO10 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(2), one O(5), one O(7), one O(8), and one O(9) atom to form distorted MgO6 pentagonal pyramids that share  a cornercorner with one Si(1)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra, edges with two equivalent Nb(3)O6 octahedra, and  a faceface with one Si(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-63°. 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 NbO6 octahedra that share corners with three equivalent Nb(3)O6 octahedra, corners with three equivalent Mg(1)O6 pentagonal pyramids, and  an edgeedge with one Si(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 41-63°. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to one O(10), one O(3), one O(4), one O(5), one O(6), and one O(7) atom. In the third Nb site, Nb(3) is bonded to one O(1), one O(10), one O(2), one O(4), one O(8), and one O(9) atom to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with four equivalent Si(1)O6 octahedra, and edges with two equivalent Mg(1)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 30-63°. Si(1) is bonded to one O(1), one O(10), one O(2), one O(6), one O(7), and one O(9) atom to form SiO6 octahedra that share corners with four equivalent Nb(3)O6 octahedra,  a cornercorner with one Mg(1)O6 pentagonal pyramid,  an edgeedge with one Nb(1)O6 octahedra, and  a faceface with one Mg(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 30-40°. There are ten inequivalent O sites. In the first O site, O(1) is bonded in a distorted see-saw-like geometry to one Mg(1), one Nb(1), one Nb(3), and one Si(1) atom. In the second O site, O(2) is bonded in a T-shaped geometry to one Mg(1), one Nb(3), and one Si(1) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Nb(1) and one Nb(2) atom. In the fourth O site, O(4) is bonded in a trigonal planar 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 trigonal planar geometry to one Mg(1), one Nb(1), and one Nb(2) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Nb(1), one Nb(2), and one Si(1) atom. In the seventh O site, O(7) is bonded in a T-shaped geometry to one Mg(1), one Nb(2), and one Si(1) atom. In the eighth O site, O(8) is bonded in a distorted trigonal planar geometry to one Mg(1), one Nb(1), and one Nb(3) atom. In the ninth O site, O(9) is bonded in a distorted trigonal planar geometry to one Mg(1), one Nb(3), and one Si(1) atom. In the tenth O site, O(10) is bonded in a distorted bent 150 degrees geometry to one Nb(2), one Nb(3), and one Si(1) atom.
MgNb3SiO10 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(2), one O(5), one O(7), one O(8), and one O(9) atom to form distorted MgO6 pentagonal pyramids that share  a cornercorner with one Si(1)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra, edges with two equivalent Nb(3)O6 octahedra, and  a faceface with one Si(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-63°. The Mg(1)-O(1) bond length is 2.26 Å. The Mg(1)-O(2) bond length is 2.05 Å. The Mg(1)-O(5) bond length is 2.04 Å. The Mg(1)-O(7) bond length is 2.22 Å. The Mg(1)-O(8) bond length is 1.99 Å. The Mg(1)-O(9) bond length is 2.01 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 NbO6 octahedra that share corners with three equivalent Nb(3)O6 octahedra, corners with three equivalent Mg(1)O6 pentagonal pyramids, and  an edgeedge with one Si(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 41-63°. The Nb(1)-O(1) bond length is 2.19 Å. The Nb(1)-O(3) bond length is 1.90 Å. The Nb(1)-O(4) bond length is 2.06 Å. The Nb(1)-O(5) bond length is 2.00 Å. The Nb(1)-O(6) bond length is 2.14 Å. The Nb(1)-O(8) bond length is 1.97 Å. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to one O(10), one O(3), one O(4), one O(5), one O(6), and one O(7) atom. The Nb(2)-O(10) bond length is 2.57 Å. The Nb(2)-O(3) bond length is 1.94 Å. The Nb(2)-O(4) bond length is 2.07 Å. The Nb(2)-O(5) bond length is 1.95 Å. The Nb(2)-O(6) bond length is 2.13 Å. The Nb(2)-O(7) bond length is 1.89 Å. In the third Nb site, Nb(3) is bonded to one O(1), one O(10), one O(2), one O(4), one O(8), and one O(9) atom to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with four equivalent Si(1)O6 octahedra, and edges with two equivalent Mg(1)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 30-63°. The Nb(3)-O(1) bond length is 2.12 Å. The Nb(3)-O(10) bond length is 2.02 Å. The Nb(3)-O(2) bond length is 1.96 Å. The Nb(3)-O(4) bond length is 2.05 Å. The Nb(3)-O(8) bond length is 2.05 Å. The Nb(3)-O(9) bond length is 2.04 Å. Si(1) is bonded to one O(1), one O(10), one O(2), one O(6), one O(7), and one O(9) atom to form SiO6 octahedra that share corners with four equivalent Nb(3)O6 octahedra,  a cornercorner with one Mg(1)O6 pentagonal pyramid,  an edgeedge with one Nb(1)O6 octahedra, and  a faceface with one Mg(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 30-40°. The Si(1)-O(1) bond length is 1.91 Å. The Si(1)-O(10) bond length is 1.81 Å. The Si(1)-O(2) bond length is 1.87 Å. The Si(1)-O(6) bond length is 1.81 Å. The Si(1)-O(7) bond length is 1.90 Å. The Si(1)-O(9) bond length is 1.74 Å. There are ten inequivalent O sites. In the first O site, O(1) is bonded in a distorted see-saw-like geometry to one Mg(1), one Nb(1), one Nb(3), and one Si(1) atom. In the second O site, O(2) is bonded in a T-shaped geometry to one Mg(1), one Nb(3), and one Si(1) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Nb(1) and one Nb(2) atom. In the fourth O site, O(4) is bonded in a trigonal planar 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 trigonal planar geometry to one Mg(1), one Nb(1), and one Nb(2) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Nb(1), one Nb(2), and one Si(1) atom. In the seventh O site, O(7) is bonded in a T-shaped geometry to one Mg(1), one Nb(2), and one Si(1) atom. In the eighth O site, O(8) is bonded in a distorted trigonal planar geometry to one Mg(1), one Nb(1), and one Nb(3) atom. In the ninth O site, O(9) is bonded in a distorted trigonal planar geometry to one Mg(1), one Nb(3), and one Si(1) atom. In the tenth O site, O(10) is bonded in a distorted bent 150 degrees geometry to one Nb(2), one Nb(3), and one Si(1) atom.