MgU2(SiO8)2 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(2) and four equivalent O(6) atoms to form MgO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra. U(1) is bonded in a 7-coordinate geometry to one O(1), one O(4), one O(5), and four equivalent O(3) atoms. Si(1) is bonded to one O(1), one O(2), and two equivalent O(3) atoms to form SiO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles are 20°. There are six inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one U(1) and one Si(1) atom. In the second O site, O(2) is bonded in a bent 150 degrees geometry to one Mg(1) and one Si(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to two equivalent U(1) and one Si(1) atom. In the fourth O site, O(4) is bonded in a single-bond geometry to one U(1) atom. In the fifth O site, O(5) is bonded in a single-bond geometry to one U(1) atom. In the sixth O site, O(6) is bonded in a single-bond geometry to one Mg(1) atom.
MgU2(SiO8)2 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(2) and four equivalent O(6) atoms to form MgO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra. Both Mg(1)-O(2) bond lengths are 1.96 Å. All Mg(1)-O(6) bond lengths are 2.07 Å. U(1) is bonded in a 7-coordinate geometry to one O(1), one O(4), one O(5), and four equivalent O(3) atoms. The U(1)-O(1) bond length is 2.18 Å. The U(1)-O(4) bond length is 1.81 Å. The U(1)-O(5) bond length is 1.81 Å. There are two shorter (2.31 Å) and two longer (2.56 Å) U(1)-O(3) bond lengths. Si(1) is bonded to one O(1), one O(2), and two equivalent O(3) atoms to form SiO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles are 20°. The Si(1)-O(1) bond length is 1.63 Å. The Si(1)-O(2) bond length is 1.63 Å. Both Si(1)-O(3) bond lengths are 1.65 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one U(1) and one Si(1) atom. In the second O site, O(2) is bonded in a bent 150 degrees geometry to one Mg(1) and one Si(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to two equivalent U(1) and one Si(1) atom. In the fourth O site, O(4) is bonded in a single-bond geometry to one U(1) atom. In the fifth O site, O(5) is bonded in a single-bond geometry to one U(1) atom. In the sixth O site, O(6) is bonded in a single-bond geometry to one Mg(1) atom.