Na8Si7O18 crystallizes in the triclinic P1 space group. There are eight inequivalent Na sites. In the first Na site, Na(1) is bonded in a 6-coordinate geometry to one O(1), one O(10), one O(12), one O(13), one O(2), and one O(6) atom. In the second Na site, Na(2) is bonded to one O(16), one O(3), one O(4), and one O(9) atom to form distorted NaO4 tetrahedra that share  a cornercorner with one Si(2)O4 tetrahedra and corners with two equivalent Si(6)O4 tetrahedra. In the third Na site, Na(3) is bonded in a 5-coordinate geometry to one O(1), one O(11), one O(17), one O(2), and one O(4) atom. In the fourth Na site, Na(4) is bonded in a distorted rectangular see-saw-like geometry to one O(12), one O(2), one O(6), and one O(7) atom. In the fifth Na site, Na(5) is bonded in a 4-coordinate geometry to one O(10), one O(13), one O(15), and one O(9) atom. In the sixth Na site, Na(6) is bonded in a 4-coordinate geometry to one O(1), one O(14), one O(2), and one O(4) atom. In the seventh Na site, Na(7) is bonded in a 3-coordinate geometry to one O(12), one O(3), and one O(7) atom. In the eighth Na site, Na(8) is bonded in a 6-coordinate geometry to one O(10), one O(12), one O(13), one O(16), one O(18), and one O(9) atom. There are seven inequivalent Si sites. In the first Si site, Si(1) is bonded in a water-like geometry to one O(11) and one O(8) atom. In the second Si site, Si(2) is bonded to one O(11), one O(17), one O(18), and one O(4) atom to form SiO4 tetrahedra that share  a cornercorner with one Na(2)O4 tetrahedra. In the third Si site, Si(3) is bonded to one O(10), one O(13), one O(14), and one O(2) atom to form corner-sharing SiO4 tetrahedra. In the fourth Si site, Si(4) is bonded in a bent 120 degrees geometry to one O(15) and one O(6) atom. In the fifth Si site, Si(5) is bonded to one O(1), one O(14), one O(15), and one O(8) atom to form corner-sharing SiO4 tetrahedra. In the sixth Si site, Si(6) is bonded to one O(16), one O(5), one O(7), and one O(9) atom to form SiO4 tetrahedra that share corners with two equivalent Na(2)O4 tetrahedra. In the seventh Si site, Si(7) is bonded in a trigonal non-coplanar geometry to one O(18), one O(3), and one O(5) atom. There are eighteen inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Na(1), one Na(3), one Na(6), and one Si(5) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Na(1), one Na(3), one Na(4), one Na(6), and one Si(3) atom. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to one Na(2), one Na(7), and one Si(7) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Na(2), one Na(3), one Na(6), and one Si(2) atom. In the fifth O site, O(5) is bonded in a distorted bent 150 degrees geometry to one Si(6) and one Si(7) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Na(1), one Na(4), and one Si(4) atom. In the seventh O site, O(7) is bonded in a 3-coordinate geometry to one Na(4), one Na(7), and one Si(6) atom. In the eighth O site, O(8) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(5) atom. In the ninth O site, O(9) is bonded in a 4-coordinate geometry to one Na(2), one Na(5), one Na(8), and one Si(6) atom. In the tenth O site, O(10) is bonded in a 4-coordinate geometry to one Na(1), one Na(5), one Na(8), and one Si(3) atom. In the eleventh O site, O(11) is bonded in a distorted trigonal planar geometry to one Na(3), one Si(1), and one Si(2) atom. In the twelfth O site, O(12) is bonded in a 5-coordinate geometry to one Na(1), one Na(4), one Na(7), one Na(8), and one O(13) atom. In the thirteenth O site, O(13) is bonded in a 5-coordinate geometry to one Na(1), one Na(5), one Na(8), one Si(3), and one O(12) atom. In the fourteenth O site, O(14) is bonded in a distorted water-like geometry to one Na(6), one Si(3), and one Si(5) atom. In the fifteenth O site, O(15) is bonded in a distorted trigonal planar geometry to one Na(5), one Si(4), and one Si(5) atom. In the sixteenth O site, O(16) is bonded in a 4-coordinate geometry to one Na(2), one Na(8), one Si(6), and one O(17) atom. In the seventeenth O site, O(17) is bonded in a distorted single-bond geometry to one Na(3), one Si(2), and one O(16) atom. In the eighteenth O site, O(18) is bonded in a trigonal planar geometry to one Na(8), one Si(2), and one Si(7) atom.
Na8Si7O18 crystallizes in the triclinic P1 space group. There are eight inequivalent Na sites. In the first Na site, Na(1) is bonded in a 6-coordinate geometry to one O(1), one O(10), one O(12), one O(13), one O(2), and one O(6) atom. The Na(1)-O(1) bond length is 2.52 Å. The Na(1)-O(10) bond length is 2.28 Å. The Na(1)-O(12) bond length is 2.44 Å. The Na(1)-O(13) bond length is 2.42 Å. The Na(1)-O(2) bond length is 2.69 Å. The Na(1)-O(6) bond length is 2.29 Å. In the second Na site, Na(2) is bonded to one O(16), one O(3), one O(4), and one O(9) atom to form distorted NaO4 tetrahedra that share  a cornercorner with one Si(2)O4 tetrahedra and corners with two equivalent Si(6)O4 tetrahedra. The Na(2)-O(16) bond length is 2.36 Å. The Na(2)-O(3) bond length is 2.32 Å. The Na(2)-O(4) bond length is 2.35 Å. The Na(2)-O(9) bond length is 2.31 Å. In the third Na site, Na(3) is bonded in a 5-coordinate geometry to one O(1), one O(11), one O(17), one O(2), and one O(4) atom. The Na(3)-O(1) bond length is 2.37 Å. The Na(3)-O(11) bond length is 2.36 Å. The Na(3)-O(17) bond length is 2.79 Å. The Na(3)-O(2) bond length is 2.39 Å. The Na(3)-O(4) bond length is 2.42 Å. In the fourth Na site, Na(4) is bonded in a distorted rectangular see-saw-like geometry to one O(12), one O(2), one O(6), and one O(7) atom. The Na(4)-O(12) bond length is 2.50 Å. The Na(4)-O(2) bond length is 2.41 Å. The Na(4)-O(6) bond length is 2.25 Å. The Na(4)-O(7) bond length is 2.27 Å. In the fifth Na site, Na(5) is bonded in a 4-coordinate geometry to one O(10), one O(13), one O(15), and one O(9) atom. The Na(5)-O(10) bond length is 2.73 Å. The Na(5)-O(13) bond length is 2.29 Å. The Na(5)-O(15) bond length is 2.39 Å. The Na(5)-O(9) bond length is 2.24 Å. In the sixth Na site, Na(6) is bonded in a 4-coordinate geometry to one O(1), one O(14), one O(2), and one O(4) atom. The Na(6)-O(1) bond length is 2.24 Å. The Na(6)-O(14) bond length is 2.72 Å. The Na(6)-O(2) bond length is 2.35 Å. The Na(6)-O(4) bond length is 2.28 Å. In the seventh Na site, Na(7) is bonded in a 3-coordinate geometry to one O(12), one O(3), and one O(7) atom. The Na(7)-O(12) bond length is 2.33 Å. The Na(7)-O(3) bond length is 2.32 Å. The Na(7)-O(7) bond length is 2.33 Å. In the eighth Na site, Na(8) is bonded in a 6-coordinate geometry to one O(10), one O(12), one O(13), one O(16), one O(18), and one O(9) atom. The Na(8)-O(10) bond length is 2.32 Å. The Na(8)-O(12) bond length is 2.34 Å. The Na(8)-O(13) bond length is 2.57 Å. The Na(8)-O(16) bond length is 2.49 Å. The Na(8)-O(18) bond length is 2.43 Å. The Na(8)-O(9) bond length is 2.85 Å. There are seven inequivalent Si sites. In the first Si site, Si(1) is bonded in a water-like geometry to one O(11) and one O(8) atom. The Si(1)-O(11) bond length is 1.67 Å. The Si(1)-O(8) bond length is 1.67 Å. In the second Si site, Si(2) is bonded to one O(11), one O(17), one O(18), and one O(4) atom to form SiO4 tetrahedra that share  a cornercorner with one Na(2)O4 tetrahedra. The Si(2)-O(11) bond length is 1.70 Å. The Si(2)-O(17) bond length is 1.70 Å. The Si(2)-O(18) bond length is 1.67 Å. The Si(2)-O(4) bond length is 1.59 Å. In the third Si site, Si(3) is bonded to one O(10), one O(13), one O(14), and one O(2) atom to form corner-sharing SiO4 tetrahedra. The Si(3)-O(10) bond length is 1.61 Å. The Si(3)-O(13) bond length is 1.72 Å. The Si(3)-O(14) bond length is 1.74 Å. The Si(3)-O(2) bond length is 1.61 Å. In the fourth Si site, Si(4) is bonded in a bent 120 degrees geometry to one O(15) and one O(6) atom. The Si(4)-O(15) bond length is 1.64 Å. The Si(4)-O(6) bond length is 1.55 Å. In the fifth Si site, Si(5) is bonded to one O(1), one O(14), one O(15), and one O(8) atom to form corner-sharing SiO4 tetrahedra. The Si(5)-O(1) bond length is 1.58 Å. The Si(5)-O(14) bond length is 1.66 Å. The Si(5)-O(15) bond length is 1.70 Å. The Si(5)-O(8) bond length is 1.69 Å. In the sixth Si site, Si(6) is bonded to one O(16), one O(5), one O(7), and one O(9) atom to form SiO4 tetrahedra that share corners with two equivalent Na(2)O4 tetrahedra. The Si(6)-O(16) bond length is 1.76 Å. The Si(6)-O(5) bond length is 1.69 Å. The Si(6)-O(7) bond length is 1.60 Å. The Si(6)-O(9) bond length is 1.60 Å. In the seventh Si site, Si(7) is bonded in a trigonal non-coplanar geometry to one O(18), one O(3), and one O(5) atom. The Si(7)-O(18) bond length is 1.72 Å. The Si(7)-O(3) bond length is 1.58 Å. The Si(7)-O(5) bond length is 1.65 Å. There are eighteen inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Na(1), one Na(3), one Na(6), and one Si(5) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Na(1), one Na(3), one Na(4), one Na(6), and one Si(3) atom. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to one Na(2), one Na(7), and one Si(7) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Na(2), one Na(3), one Na(6), and one Si(2) atom. In the fifth O site, O(5) is bonded in a distorted bent 150 degrees geometry to one Si(6) and one Si(7) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Na(1), one Na(4), and one Si(4) atom. In the seventh O site, O(7) is bonded in a 3-coordinate geometry to one Na(4), one Na(7), and one Si(6) atom. In the eighth O site, O(8) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(5) atom. In the ninth O site, O(9) is bonded in a 4-coordinate geometry to one Na(2), one Na(5), one Na(8), and one Si(6) atom. In the tenth O site, O(10) is bonded in a 4-coordinate geometry to one Na(1), one Na(5), one Na(8), and one Si(3) atom. In the eleventh O site, O(11) is bonded in a distorted trigonal planar geometry to one Na(3), one Si(1), and one Si(2) atom. In the twelfth O site, O(12) is bonded in a 5-coordinate geometry to one Na(1), one Na(4), one Na(7), one Na(8), and one O(13) atom. The O(12)-O(13) bond length is 1.53 Å. In the thirteenth O site, O(13) is bonded in a 5-coordinate geometry to one Na(1), one Na(5), one Na(8), one Si(3), and one O(12) atom. In the fourteenth O site, O(14) is bonded in a distorted water-like geometry to one Na(6), one Si(3), and one Si(5) atom. In the fifteenth O site, O(15) is bonded in a distorted trigonal planar geometry to one Na(5), one Si(4), and one Si(5) atom. In the sixteenth O site, O(16) is bonded in a 4-coordinate geometry to one Na(2), one Na(8), one Si(6), and one O(17) atom. The O(16)-O(17) bond length is 1.50 Å. In the seventeenth O site, O(17) is bonded in a distorted single-bond geometry to one Na(3), one Si(2), and one O(16) atom. In the eighteenth O site, O(18) is bonded in a trigonal planar geometry to one Na(8), one Si(2), and one Si(7) atom.