Na2MgSi5O12 is Esseneite-like structured and crystallizes in the monoclinic P2/c space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded in a 6-coordinate geometry to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. In the second Na site, Na(2) is bonded in a 8-coordinate geometry to two equivalent O(1), two equivalent O(3), two equivalent O(5), and two equivalent O(6) atoms. Mg(1) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form distorted MgO6 octahedra that share corners with two equivalent Si(2)O4 tetrahedra, corners with four equivalent Si(1)O4 tetrahedra, and edges with two equivalent Si(3)O6 octahedra. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SiO4 tetrahedra that share  a cornercorner with one Si(3)O6 octahedra, corners with two equivalent Mg(1)O6 octahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 34-69°. 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 Mg(1)O6 octahedra, corners with two equivalent Si(3)O6 octahedra, and corners with two equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 34-64°. In the third Si site, Si(3) is bonded to two equivalent O(4), two equivalent O(5), and two equivalent O(6) atoms to form SiO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with four equivalent Si(2)O4 tetrahedra, and edges with two equivalent Mg(1)O6 octahedra. There are six inequivalent O sites. In the first O site, O(6) is bonded in a distorted T-shaped geometry to one Na(2), one Si(2), and one Si(3) atom. In the second O site, O(1) is bonded in a 3-coordinate geometry to one Na(2), one Si(1), and one Si(2) atom. In the third O site, O(2) is bonded in a distorted T-shaped geometry to one Na(1), one Mg(1), and one Si(1) atom. In the fourth O site, O(3) is bonded in a 2-coordinate geometry to one Na(1), one Na(2), one Si(1), and one Si(2) atom. In the fifth O site, O(4) is bonded in a 4-coordinate geometry to one Na(1), one Mg(1), one Si(1), and one Si(3) atom. In the sixth O site, O(5) is bonded to one Na(2), one Mg(1), one Si(2), and one Si(3) atom to form a mixture of distorted edge and corner-sharing ONaMgSi2 tetrahedra.
Na2MgSi5O12 is Esseneite-like structured and crystallizes in the monoclinic P2/c space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded in a 6-coordinate geometry to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. Both Na(1)-O(2) bond lengths are 2.46 Å. Both Na(1)-O(3) bond lengths are 2.47 Å. Both Na(1)-O(4) bond lengths are 2.42 Å. In the second Na site, Na(2) is bonded in a 8-coordinate geometry to two equivalent O(1), two equivalent O(3), two equivalent O(5), and two equivalent O(6) atoms. Both Na(2)-O(1) bond lengths are 2.38 Å. Both Na(2)-O(3) bond lengths are 2.71 Å. Both Na(2)-O(5) bond lengths are 2.36 Å. Both Na(2)-O(6) bond lengths are 2.36 Å. Mg(1) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form distorted MgO6 octahedra that share corners with two equivalent Si(2)O4 tetrahedra, corners with four equivalent Si(1)O4 tetrahedra, and edges with two equivalent Si(3)O6 octahedra. Both Mg(1)-O(2) bond lengths are 2.01 Å. Both Mg(1)-O(4) bond lengths are 2.11 Å. Both Mg(1)-O(5) bond lengths are 2.25 Å. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SiO4 tetrahedra that share  a cornercorner with one Si(3)O6 octahedra, corners with two equivalent Mg(1)O6 octahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 34-69°. The Si(1)-O(1) bond length is 1.66 Å. The Si(1)-O(2) bond length is 1.59 Å. The Si(1)-O(3) bond length is 1.67 Å. The Si(1)-O(4) bond length is 1.66 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 Mg(1)O6 octahedra, corners with two equivalent Si(3)O6 octahedra, and corners with two equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 34-64°. The Si(2)-O(1) bond length is 1.65 Å. The Si(2)-O(3) bond length is 1.64 Å. The Si(2)-O(5) bond length is 1.66 Å. The Si(2)-O(6) bond length is 1.63 Å. In the third Si site, Si(3) is bonded to two equivalent O(4), two equivalent O(5), and two equivalent O(6) atoms to form SiO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with four equivalent Si(2)O4 tetrahedra, and edges with two equivalent Mg(1)O6 octahedra. Both Si(3)-O(4) bond lengths are 1.85 Å. Both Si(3)-O(5) bond lengths are 1.84 Å. Both Si(3)-O(6) bond lengths are 1.79 Å. There are six inequivalent O sites. In the first O site, O(6) is bonded in a distorted T-shaped geometry to one Na(2), one Si(2), and one Si(3) atom. In the second O site, O(1) is bonded in a 3-coordinate geometry to one Na(2), one Si(1), and one Si(2) atom. In the third O site, O(2) is bonded in a distorted T-shaped geometry to one Na(1), one Mg(1), and one Si(1) atom. In the fourth O site, O(3) is bonded in a 2-coordinate geometry to one Na(1), one Na(2), one Si(1), and one Si(2) atom. In the fifth O site, O(4) is bonded in a 4-coordinate geometry to one Na(1), one Mg(1), one Si(1), and one Si(3) atom. In the sixth O site, O(5) is bonded to one Na(2), one Mg(1), one Si(2), and one Si(3) atom to form a mixture of distorted edge and corner-sharing ONaMgSi2 tetrahedra.