Mg3Si4(WO7)2 crystallizes in the monoclinic C2/c space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to one O(1), one O(2), one O(4), and two equivalent O(7) atoms. In the second Mg site, Mg(2) is bonded to two equivalent O(1) and two equivalent O(3) atoms to form distorted MgO4 trigonal pyramids that share corners with two equivalent W(2)O6 octahedra, corners with four equivalent Si(2)O4 tetrahedra, and  an edgeedge with one W(1)O6 octahedra. The corner-sharing octahedral tilt angles are 61°. There are two inequivalent W sites. In the first W site, W(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted WO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent W(2)O6 octahedra, and  an edgeedge with one Mg(2)O4 trigonal pyramid. In the second W site, W(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form WO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, corners with two equivalent Mg(2)O4 trigonal pyramids, and edges with two equivalent W(1)O6 octahedra. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one W(1)O6 octahedra,  a cornercorner with one W(2)O6 octahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-50°. In the second Si site, Si(2) is bonded to one O(1), one O(3), one O(5), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one W(1)O6 octahedra,  a cornercorner with one W(2)O6 octahedra, corners with two equivalent Si(1)O4 tetrahedra, and corners with two equivalent Mg(2)O4 trigonal pyramids. The corner-sharing octahedral tilt angles are 68°. There are seven inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mg(2), one W(2), and one Si(2) atom to form distorted corner-sharing OMg2SiW tetrahedra. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Mg(1), one W(1), and one W(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Mg(2), one W(1), and one Si(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Mg(1), one W(1), one W(2), and one Si(1) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(2) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one Si(1) atom.
Mg3Si4(WO7)2 crystallizes in the monoclinic C2/c space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to one O(1), one O(2), one O(4), and two equivalent O(7) atoms. The Mg(1)-O(1) bond length is 2.24 Å. The Mg(1)-O(2) bond length is 1.91 Å. The Mg(1)-O(4) bond length is 2.52 Å. There is one shorter (2.04 Å) and one longer (2.06 Å) Mg(1)-O(7) bond length. In the second Mg site, Mg(2) is bonded to two equivalent O(1) and two equivalent O(3) atoms to form distorted MgO4 trigonal pyramids that share corners with two equivalent W(2)O6 octahedra, corners with four equivalent Si(2)O4 tetrahedra, and  an edgeedge with one W(1)O6 octahedra. The corner-sharing octahedral tilt angles are 61°. Both Mg(2)-O(1) bond lengths are 2.17 Å. Both Mg(2)-O(3) bond lengths are 2.05 Å. There are two inequivalent W sites. In the first W site, W(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted WO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent W(2)O6 octahedra, and  an edgeedge with one Mg(2)O4 trigonal pyramid. Both W(1)-O(2) bond lengths are 2.09 Å. Both W(1)-O(3) bond lengths are 2.25 Å. Both W(1)-O(4) bond lengths are 2.30 Å. In the second W site, W(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form WO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, corners with two equivalent Mg(2)O4 trigonal pyramids, and edges with two equivalent W(1)O6 octahedra. Both W(2)-O(1) bond lengths are 2.30 Å. Both W(2)-O(2) bond lengths are 2.07 Å. Both W(2)-O(4) bond lengths are 2.18 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one W(1)O6 octahedra,  a cornercorner with one W(2)O6 octahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-50°. The Si(1)-O(4) bond length is 1.65 Å. The Si(1)-O(5) bond length is 1.65 Å. The Si(1)-O(6) bond length is 1.65 Å. The Si(1)-O(7) bond length is 1.63 Å. In the second Si site, Si(2) is bonded to one O(1), one O(3), one O(5), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one W(1)O6 octahedra,  a cornercorner with one W(2)O6 octahedra, corners with two equivalent Si(1)O4 tetrahedra, and corners with two equivalent Mg(2)O4 trigonal pyramids. The corner-sharing octahedral tilt angles are 68°. The Si(2)-O(1) bond length is 1.67 Å. The Si(2)-O(3) bond length is 1.63 Å. The Si(2)-O(5) bond length is 1.65 Å. The Si(2)-O(6) bond length is 1.65 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mg(2), one W(2), and one Si(2) atom to form distorted corner-sharing OMg2SiW tetrahedra. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Mg(1), one W(1), and one W(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Mg(2), one W(1), and one Si(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Mg(1), one W(1), one W(2), and one Si(1) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(2) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one Si(1) atom.