Mg3Si4(BiO7)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 distorted rectangular see-saw-like geometry to one O(1), one O(2), 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 Bi(2)O6 octahedra, corners with four equivalent Si(2)O4 tetrahedra, and  an edgeedge with one Bi(1)O6 octahedra. The corner-sharing octahedral tilt angles are 60°. 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 Bi(1)O6 octahedra,  a cornercorner with one Bi(2)O6 octahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 41-50°. 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 Bi(1)O6 octahedra,  a cornercorner with one Bi(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 69-72°. There are two inequivalent Bi sites. In the first Bi site, Bi(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted BiO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent Bi(2)O6 octahedra, and  an edgeedge with one Mg(2)O4 trigonal pyramid. In the second Bi site, Bi(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form BiO6 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 Bi(1)O6 octahedra. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Mg(1), one Mg(2), one Si(2), and one Bi(2) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Mg(1), one Bi(1), and one Bi(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Mg(2), one Si(2), and one Bi(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Si(1), one Bi(1), and one Bi(2) 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(BiO7)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 distorted rectangular see-saw-like geometry to one O(1), one O(2), and two equivalent O(7) atoms. The Mg(1)-O(1) bond length is 2.42 Å. The Mg(1)-O(2) bond length is 1.94 Å. There is one shorter (2.01 Å) and one longer (2.02 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 Bi(2)O6 octahedra, corners with four equivalent Si(2)O4 tetrahedra, and  an edgeedge with one Bi(1)O6 octahedra. The corner-sharing octahedral tilt angles are 60°. Both Mg(2)-O(1) bond lengths are 2.08 Å. Both Mg(2)-O(3) bond lengths are 2.06 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 Bi(1)O6 octahedra,  a cornercorner with one Bi(2)O6 octahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 41-50°. The Si(1)-O(4) bond length is 1.63 Å. The Si(1)-O(5) bond length is 1.64 Å. The Si(1)-O(6) bond length is 1.67 Å. The Si(1)-O(7) bond length is 1.64 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 Bi(1)O6 octahedra,  a cornercorner with one Bi(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 69-72°. The Si(2)-O(1) bond length is 1.68 Å. The Si(2)-O(3) bond length is 1.63 Å. The Si(2)-O(5) bond length is 1.66 Å. The Si(2)-O(6) bond length is 1.68 Å. There are two inequivalent Bi sites. In the first Bi site, Bi(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted BiO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent Bi(2)O6 octahedra, and  an edgeedge with one Mg(2)O4 trigonal pyramid. Both Bi(1)-O(2) bond lengths are 2.23 Å. Both Bi(1)-O(3) bond lengths are 2.50 Å. Both Bi(1)-O(4) bond lengths are 2.41 Å. In the second Bi site, Bi(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form BiO6 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 Bi(1)O6 octahedra. Both Bi(2)-O(1) bond lengths are 2.73 Å. Both Bi(2)-O(2) bond lengths are 2.27 Å. Both Bi(2)-O(4) bond lengths are 2.30 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Mg(1), one Mg(2), one Si(2), and one Bi(2) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Mg(1), one Bi(1), and one Bi(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Mg(2), one Si(2), and one Bi(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Si(1), one Bi(1), and one Bi(2) 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.