Mg3Si4(NiO7)2 crystallizes in the monoclinic C2/c space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(2), one O(4), and two equivalent O(7) atoms to form distorted MgO5 square pyramids that share  a cornercorner with one Ni(2)O6 octahedra,  a cornercorner with one Mg(2)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, corners with three equivalent Si(1)O4 tetrahedra,  an edgeedge with one Ni(1)O6 octahedra,  an edgeedge with one Ni(2)O6 octahedra, and  an edgeedge with one Mg(1)O5 square pyramid. The corner-sharing octahedral tilt angles are 43°. In the second Mg site, Mg(2) is bonded to two equivalent O(1) and two equivalent O(3) atoms to form distorted MgO4 tetrahedra that share corners with two equivalent Ni(2)O6 octahedra, corners with two equivalent Mg(1)O5 square pyramids, corners with four equivalent Si(2)O4 tetrahedra, and  an edgeedge with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 59°. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted NiO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent Ni(2)O6 octahedra, edges with two equivalent Mg(1)O5 square pyramids, and  an edgeedge with one Mg(2)O4 tetrahedra. In the second Ni site, Ni(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form NiO6 octahedra that share corners with two equivalent Mg(1)O5 square pyramids, corners with two equivalent Mg(2)O4 tetrahedra, corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with two equivalent Mg(1)O5 square pyramids. 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 Ni(1)O6 octahedra,  a cornercorner with one Ni(2)O6 octahedra, corners with three equivalent Mg(1)O5 square pyramids, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-60°. 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 Ni(1)O6 octahedra,  a cornercorner with one Ni(2)O6 octahedra,  a cornercorner with one Mg(1)O5 square pyramid, corners with two equivalent Mg(2)O4 tetrahedra, and corners with two equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 67-73°. There are seven inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mg(2), one Ni(2), and one Si(2) atom to form corner-sharing OMg2SiNi tetrahedra. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1), one Ni(1), and one Ni(2) atom. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to one Mg(2), one Ni(1), and one Si(2) atom. In the fourth O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Ni(1), one Ni(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(NiO7)2 crystallizes in the monoclinic C2/c space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(2), one O(4), and two equivalent O(7) atoms to form distorted MgO5 square pyramids that share  a cornercorner with one Ni(2)O6 octahedra,  a cornercorner with one Mg(2)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, corners with three equivalent Si(1)O4 tetrahedra,  an edgeedge with one Ni(1)O6 octahedra,  an edgeedge with one Ni(2)O6 octahedra, and  an edgeedge with one Mg(1)O5 square pyramid. The corner-sharing octahedral tilt angles are 43°. The Mg(1)-O(1) bond length is 2.14 Å. The Mg(1)-O(2) bond length is 2.06 Å. The Mg(1)-O(4) bond length is 2.28 Å. There is one shorter (2.04 Å) and one longer (2.23 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 tetrahedra that share corners with two equivalent Ni(2)O6 octahedra, corners with two equivalent Mg(1)O5 square pyramids, corners with four equivalent Si(2)O4 tetrahedra, and  an edgeedge with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 59°. Both Mg(2)-O(1) bond lengths are 2.09 Å. Both Mg(2)-O(3) bond lengths are 2.00 Å. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted NiO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent Ni(2)O6 octahedra, edges with two equivalent Mg(1)O5 square pyramids, and  an edgeedge with one Mg(2)O4 tetrahedra. Both Ni(1)-O(2) bond lengths are 1.85 Å. Both Ni(1)-O(3) bond lengths are 1.94 Å. Both Ni(1)-O(4) bond lengths are 2.24 Å. In the second Ni site, Ni(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form NiO6 octahedra that share corners with two equivalent Mg(1)O5 square pyramids, corners with two equivalent Mg(2)O4 tetrahedra, corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with two equivalent Mg(1)O5 square pyramids. Both Ni(2)-O(1) bond lengths are 2.17 Å. Both Ni(2)-O(2) bond lengths are 1.82 Å. Both Ni(2)-O(4) bond lengths are 1.99 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 Ni(1)O6 octahedra,  a cornercorner with one Ni(2)O6 octahedra, corners with three equivalent Mg(1)O5 square pyramids, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-60°. 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 1.64 Å. The Si(1)-O(7) bond length is 1.61 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 Ni(1)O6 octahedra,  a cornercorner with one Ni(2)O6 octahedra,  a cornercorner with one Mg(1)O5 square pyramid, corners with two equivalent Mg(2)O4 tetrahedra, and corners with two equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 67-73°. The Si(2)-O(1) bond length is 1.65 Å. The Si(2)-O(3) bond length is 1.62 Å. The Si(2)-O(5) bond length is 1.64 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 Ni(2), and one Si(2) atom to form corner-sharing OMg2SiNi tetrahedra. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1), one Ni(1), and one Ni(2) atom. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to one Mg(2), one Ni(1), and one Si(2) atom. In the fourth O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Ni(1), one Ni(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.