Mg7(SiO3)8 crystallizes in the monoclinic C2/m space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a rectangular see-saw-like geometry to two equivalent O(5) and two equivalent O(7) atoms. In the second Mg site, Mg(2) is bonded to two equivalent O(5), two equivalent O(6), and two equivalent O(7) atoms to form MgO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with four equivalent Si(2)O4 tetrahedra,  an edgeedge with one Mg(3)O6 octahedra, and edges with two equivalent Mg(4)O6 octahedra. In the third Mg site, Mg(3) is bonded to two equivalent O(3) and four equivalent O(6) atoms to form MgO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Mg(2)O6 octahedra, and edges with four equivalent Mg(4)O6 octahedra. In the fourth Mg site, Mg(4) is bonded to two equivalent O(3), two equivalent O(6), and two equivalent O(7) atoms to form MgO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra,  an edgeedge with one Mg(4)O6 octahedra, edges with two equivalent Mg(2)O6 octahedra, and edges with two equivalent Mg(3)O6 octahedra. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(4), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Mg(2)O6 octahedra,  a cornercorner with one Mg(3)O6 octahedra,  a cornercorner with one Mg(4)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 59-62°. In the second Si site, Si(2) is bonded to one O(1), one O(4), one O(5), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Mg(4)O6 octahedra, corners with two equivalent Mg(2)O6 octahedra, and corners with two equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 39-60°. There are seven inequivalent O sites. In the first O site, O(2) is bonded in a bent 150 degrees geometry to two equivalent Si(1) atoms. In the second O site, O(3) is bonded in a trigonal non-coplanar geometry to one Mg(3) and two equivalent Mg(4) atoms. In the third O site, O(4) is bonded in a bent 150 degrees geometry to one Si(1) and one Si(2) atom. In the fourth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1), one Mg(2), and one Si(2) atom. In the fifth O site, O(6) is bonded to one Mg(2), one Mg(3), one Mg(4), and one Si(1) atom to form a mixture of distorted corner and edge-sharing OMg3Si tetrahedra. In the sixth O site, O(7) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Mg(2), one Mg(4), and one Si(2) atom. In the seventh O site, O(1) is bonded in a bent 150 degrees geometry to one Si(1) and one Si(2) atom.
Mg7(SiO3)8 crystallizes in the monoclinic C2/m space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a rectangular see-saw-like geometry to two equivalent O(5) and two equivalent O(7) atoms. Both Mg(1)-O(5) bond lengths are 2.01 Å. Both Mg(1)-O(7) bond lengths are 2.08 Å. In the second Mg site, Mg(2) is bonded to two equivalent O(5), two equivalent O(6), and two equivalent O(7) atoms to form MgO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with four equivalent Si(2)O4 tetrahedra,  an edgeedge with one Mg(3)O6 octahedra, and edges with two equivalent Mg(4)O6 octahedra. Both Mg(2)-O(5) bond lengths are 2.05 Å. Both Mg(2)-O(6) bond lengths are 2.16 Å. Both Mg(2)-O(7) bond lengths are 2.12 Å. In the third Mg site, Mg(3) is bonded to two equivalent O(3) and four equivalent O(6) atoms to form MgO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Mg(2)O6 octahedra, and edges with four equivalent Mg(4)O6 octahedra. Both Mg(3)-O(3) bond lengths are 2.08 Å. All Mg(3)-O(6) bond lengths are 2.12 Å. In the fourth Mg site, Mg(4) is bonded to two equivalent O(3), two equivalent O(6), and two equivalent O(7) atoms to form MgO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra,  an edgeedge with one Mg(4)O6 octahedra, edges with two equivalent Mg(2)O6 octahedra, and edges with two equivalent Mg(3)O6 octahedra. Both Mg(4)-O(3) bond lengths are 2.06 Å. Both Mg(4)-O(6) bond lengths are 2.09 Å. Both Mg(4)-O(7) bond lengths are 2.11 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(4), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Mg(2)O6 octahedra,  a cornercorner with one Mg(3)O6 octahedra,  a cornercorner with one Mg(4)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 59-62°. The Si(1)-O(1) bond length is 1.64 Å. The Si(1)-O(2) bond length is 1.63 Å. The Si(1)-O(4) bond length is 1.64 Å. The Si(1)-O(6) bond length is 1.63 Å. In the second Si site, Si(2) is bonded to one O(1), one O(4), one O(5), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Mg(4)O6 octahedra, corners with two equivalent Mg(2)O6 octahedra, and corners with two equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 39-60°. The Si(2)-O(1) bond length is 1.67 Å. The Si(2)-O(4) bond length is 1.65 Å. The Si(2)-O(5) bond length is 1.61 Å. The Si(2)-O(7) bond length is 1.65 Å. There are seven inequivalent O sites. In the first O site, O(2) is bonded in a bent 150 degrees geometry to two equivalent Si(1) atoms. In the second O site, O(3) is bonded in a trigonal non-coplanar geometry to one Mg(3) and two equivalent Mg(4) atoms. In the third O site, O(4) is bonded in a bent 150 degrees geometry to one Si(1) and one Si(2) atom. In the fourth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1), one Mg(2), and one Si(2) atom. In the fifth O site, O(6) is bonded to one Mg(2), one Mg(3), one Mg(4), and one Si(1) atom to form a mixture of distorted corner and edge-sharing OMg3Si tetrahedra. In the sixth O site, O(7) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Mg(2), one Mg(4), and one Si(2) atom. In the seventh O site, O(1) is bonded in a bent 150 degrees geometry to one Si(1) and one Si(2) atom.