Mg3Si4(MoO7)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 Mo(2)O6 octahedra, corners with four equivalent Si(2)O4 tetrahedra, and  an edgeedge with one Mo(1)O6 octahedra. The corner-sharing octahedral tilt angles are 62°. There are two inequivalent Mo sites. In the first Mo site, Mo(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted MoO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent Mo(2)O6 octahedra, and  an edgeedge with one Mg(2)O4 trigonal pyramid. In the second Mo site, Mo(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form MoO6 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 Mo(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 Mo(1)O6 octahedra,  a cornercorner with one Mo(2)O6 octahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 47-50°. 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 Mo(1)O6 octahedra,  a cornercorner with one Mo(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 range from 67-69°. There are seven inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mg(2), one Mo(2), and one Si(2) atom to form distorted corner-sharing OMg2SiMo tetrahedra. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Mg(1), one Mo(1), and one Mo(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Mg(2), one Mo(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 Mo(1), one Mo(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(MoO7)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 2.22 Å. The Mg(1)-O(2) bond length is 1.92 Å. The Mg(1)-O(4) bond length is 2.62 Å. Both Mg(1)-O(7) bond lengths are 2.05 Å. 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 Mo(2)O6 octahedra, corners with four equivalent Si(2)O4 tetrahedra, and  an edgeedge with one Mo(1)O6 octahedra. The corner-sharing octahedral tilt angles are 62°. Both Mg(2)-O(1) bond lengths are 2.16 Å. Both Mg(2)-O(3) bond lengths are 2.04 Å. There are two inequivalent Mo sites. In the first Mo site, Mo(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted MoO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent Mo(2)O6 octahedra, and  an edgeedge with one Mg(2)O4 trigonal pyramid. Both Mo(1)-O(2) bond lengths are 2.08 Å. Both Mo(1)-O(3) bond lengths are 2.25 Å. Both Mo(1)-O(4) bond lengths are 2.25 Å. In the second Mo site, Mo(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form MoO6 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 Mo(1)O6 octahedra. Both Mo(2)-O(1) bond lengths are 2.31 Å. Both Mo(2)-O(2) bond lengths are 2.05 Å. Both Mo(2)-O(4) bond lengths are 2.16 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 Mo(1)O6 octahedra,  a cornercorner with one Mo(2)O6 octahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 47-50°. The Si(1)-O(4) bond length is 1.64 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 Mo(1)O6 octahedra,  a cornercorner with one Mo(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 range from 67-69°. 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 1.66 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mg(2), one Mo(2), and one Si(2) atom to form distorted corner-sharing OMg2SiMo tetrahedra. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Mg(1), one Mo(1), and one Mo(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Mg(2), one Mo(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 Mo(1), one Mo(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.