Mg9Si5 crystallizes in the triclinic P1 space group. There are eighteen inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Mg(14), one Si(2), one Si(6), one Si(8), and one Si(9) atom to form distorted MgMgSi4 tetrahedra that share  a cornercorner with one Si(9)Mg8 hexagonal bipyramid, corners with two equivalent Mg(5)MgSi4 tetrahedra,  an edgeedge with one Mg(5)MgSi4 tetrahedra, and  an edgeedge with one Mg(6)Si4 tetrahedra. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one Mg(14), one Si(10), one Si(2), one Si(5), and one Si(9) atom. In the third Mg site, Mg(3) is bonded in a 3-coordinate geometry to one Si(10), one Si(2), and one Si(9) atom. In the fourth Mg site, Mg(4) is bonded in a 4-coordinate geometry to one Si(1), one Si(10), one Si(2), and one Si(5) atom. In the fifth Mg site, Mg(5) is bonded to one Mg(14), one Si(10), one Si(2), one Si(6), and one Si(9) atom to form distorted MgMgSi4 tetrahedra that share  a cornercorner with one Si(9)Mg8 hexagonal bipyramid,  a cornercorner with one Mg(6)Si4 tetrahedra, corners with two equivalent Mg(1)MgSi4 tetrahedra,  an edgeedge with one Mg(1)MgSi4 tetrahedra, and  an edgeedge with one Mg(6)Si4 tetrahedra. In the sixth Mg site, Mg(6) is bonded to one Si(10), one Si(2), one Si(5), and one Si(9) atom to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. In the seventh Mg site, Mg(7) is bonded in a distorted T-shaped geometry to one Si(1), one Si(4), and one Si(5) atom. In the eighth Mg site, Mg(8) is bonded in a 5-coordinate geometry to one Si(3), one Si(4), one Si(6), one Si(7), and one Si(8) atom. In the ninth Mg site, Mg(9) is bonded in a 3-coordinate geometry to one Si(1), one Si(4), and one Si(7) atom. In the tenth Mg site, Mg(10) is bonded in a 3-coordinate geometry to one Mg(14), one Si(4), one Si(6), and one Si(8) atom. In the eleventh Mg site, Mg(11) is bonded in a 3-coordinate geometry to one Si(1), one Si(5), and one Si(7) atom. In the twelfth Mg site, Mg(12) is bonded in a 3-coordinate geometry to one Si(4), one Si(7), and one Si(8) atom. In the thirteenth Mg site, Mg(13) is bonded in a 5-coordinate geometry to one Si(1), one Si(10), one Si(2), one Si(5), and one Si(7) atom. In the fourteenth Mg site, Mg(14) is bonded in a 10-coordinate geometry to one Mg(1), one Mg(10), one Mg(16), one Mg(2), one Mg(5), one Si(2), one Si(3), one Si(6), one Si(8), and one Si(9) atom. In the fifteenth Mg site, Mg(15) is bonded in a 3-coordinate geometry to one Si(1), one Si(10), and one Si(5) atom. In the sixteenth Mg site, Mg(16) is bonded in a 3-coordinate geometry to one Mg(14), one Si(3), one Si(6), one Si(8), and one Si(9) atom. In the seventeenth Mg site, Mg(17) is bonded in a distorted trigonal non-coplanar geometry to one Si(1), one Si(10), and one Si(7) atom. In the eighteenth Mg site, Mg(18) is bonded in a 5-coordinate geometry to one Si(3), one Si(4), one Si(6), one Si(8), and one Si(9) atom. There are ten inequivalent Si sites. In the first Si site, Si(1) is bonded in a 8-coordinate geometry to one Mg(11), one Mg(13), one Mg(15), one Mg(17), one Mg(4), one Mg(7), one Mg(9), and one Si(7) atom. In the second Si site, Si(2) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(13), one Mg(14), one Mg(2), one Mg(3), one Mg(4), one Mg(5), and one Mg(6) atom. In the third Si site, Si(3) is bonded in a 6-coordinate geometry to one Mg(14), one Mg(16), one Mg(18), one Mg(8), one Si(4), and one Si(8) atom. In the fourth Si site, Si(4) is bonded in a 8-coordinate geometry to one Mg(10), one Mg(12), one Mg(18), one Mg(7), one Mg(8), one Mg(9), one Si(3), and one Si(6) atom. In the fifth Si site, Si(5) is bonded in a 7-coordinate geometry to one Mg(11), one Mg(13), one Mg(15), one Mg(2), one Mg(4), one Mg(6), and one Mg(7) atom. In the sixth Si site, Si(6) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(10), one Mg(14), one Mg(16), one Mg(18), one Mg(5), one Mg(8), and one Si(4) atom. In the seventh Si site, Si(7) is bonded in a 7-coordinate geometry to one Mg(11), one Mg(12), one Mg(13), one Mg(17), one Mg(8), one Mg(9), and one Si(1) atom. In the eighth Si site, Si(8) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(10), one Mg(12), one Mg(14), one Mg(16), one Mg(18), one Mg(8), and one Si(3) atom. In the ninth Si site, Si(9) is bonded to one Mg(1), one Mg(14), one Mg(16), one Mg(18), one Mg(2), one Mg(3), one Mg(5), and one Mg(6) atom to form distorted SiMg8 hexagonal bipyramids that share corners with two equivalent Si(10)Mg8 hexagonal bipyramids,  a cornercorner with one Mg(1)MgSi4 tetrahedra,  a cornercorner with one Mg(5)MgSi4 tetrahedra, and  an edgeedge with one Si(10)Mg8 hexagonal bipyramid. In the tenth Si site, Si(10) is bonded to one Mg(13), one Mg(15), one Mg(17), one Mg(2), one Mg(3), one Mg(4), one Mg(5), and one Mg(6) atom to form a mixture of distorted corner and edge-sharing SiMg8 hexagonal bipyramids.
Mg9Si5 crystallizes in the triclinic P1 space group. There are eighteen inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Mg(14), one Si(2), one Si(6), one Si(8), and one Si(9) atom to form distorted MgMgSi4 tetrahedra that share  a cornercorner with one Si(9)Mg8 hexagonal bipyramid, corners with two equivalent Mg(5)MgSi4 tetrahedra,  an edgeedge with one Mg(5)MgSi4 tetrahedra, and  an edgeedge with one Mg(6)Si4 tetrahedra. The Mg(1)-Mg(14) bond length is 3.15 Å. The Mg(1)-Si(2) bond length is 2.86 Å. The Mg(1)-Si(6) bond length is 2.77 Å. The Mg(1)-Si(8) bond length is 2.83 Å. The Mg(1)-Si(9) bond length is 2.79 Å. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one Mg(14), one Si(10), one Si(2), one Si(5), and one Si(9) atom. The Mg(2)-Mg(14) bond length is 3.04 Å. The Mg(2)-Si(10) bond length is 2.80 Å. The Mg(2)-Si(2) bond length is 2.79 Å. The Mg(2)-Si(5) bond length is 2.73 Å. The Mg(2)-Si(9) bond length is 2.94 Å. In the third Mg site, Mg(3) is bonded in a 3-coordinate geometry to one Si(10), one Si(2), and one Si(9) atom. The Mg(3)-Si(10) bond length is 2.62 Å. The Mg(3)-Si(2) bond length is 2.79 Å. The Mg(3)-Si(9) bond length is 2.70 Å. In the fourth Mg site, Mg(4) is bonded in a 4-coordinate geometry to one Si(1), one Si(10), one Si(2), and one Si(5) atom. The Mg(4)-Si(1) bond length is 2.74 Å. The Mg(4)-Si(10) bond length is 2.75 Å. The Mg(4)-Si(2) bond length is 2.82 Å. The Mg(4)-Si(5) bond length is 2.69 Å. In the fifth Mg site, Mg(5) is bonded to one Mg(14), one Si(10), one Si(2), one Si(6), and one Si(9) atom to form distorted MgMgSi4 tetrahedra that share  a cornercorner with one Si(9)Mg8 hexagonal bipyramid,  a cornercorner with one Mg(6)Si4 tetrahedra, corners with two equivalent Mg(1)MgSi4 tetrahedra,  an edgeedge with one Mg(1)MgSi4 tetrahedra, and  an edgeedge with one Mg(6)Si4 tetrahedra. The Mg(5)-Mg(14) bond length is 2.96 Å. The Mg(5)-Si(10) bond length is 2.93 Å. The Mg(5)-Si(2) bond length is 2.82 Å. The Mg(5)-Si(6) bond length is 2.76 Å. The Mg(5)-Si(9) bond length is 2.72 Å. In the sixth Mg site, Mg(6) is bonded to one Si(10), one Si(2), one Si(5), and one Si(9) atom to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. The Mg(6)-Si(10) bond length is 2.71 Å. The Mg(6)-Si(2) bond length is 2.71 Å. The Mg(6)-Si(5) bond length is 2.79 Å. The Mg(6)-Si(9) bond length is 2.77 Å. In the seventh Mg site, Mg(7) is bonded in a distorted T-shaped geometry to one Si(1), one Si(4), and one Si(5) atom. The Mg(7)-Si(1) bond length is 2.79 Å. The Mg(7)-Si(4) bond length is 2.82 Å. The Mg(7)-Si(5) bond length is 2.76 Å. In the eighth Mg site, Mg(8) is bonded in a 5-coordinate geometry to one Si(3), one Si(4), one Si(6), one Si(7), and one Si(8) atom. The Mg(8)-Si(3) bond length is 2.73 Å. The Mg(8)-Si(4) bond length is 2.89 Å. The Mg(8)-Si(6) bond length is 2.94 Å. The Mg(8)-Si(7) bond length is 2.67 Å. The Mg(8)-Si(8) bond length is 2.83 Å. In the ninth Mg site, Mg(9) is bonded in a 3-coordinate geometry to one Si(1), one Si(4), and one Si(7) atom. The Mg(9)-Si(1) bond length is 2.93 Å. The Mg(9)-Si(4) bond length is 2.83 Å. The Mg(9)-Si(7) bond length is 2.79 Å. In the tenth Mg site, Mg(10) is bonded in a 3-coordinate geometry to one Mg(14), one Si(4), one Si(6), and one Si(8) atom. The Mg(10)-Mg(14) bond length is 3.03 Å. The Mg(10)-Si(4) bond length is 2.80 Å. The Mg(10)-Si(6) bond length is 2.84 Å. The Mg(10)-Si(8) bond length is 2.70 Å. In the eleventh Mg site, Mg(11) is bonded in a 3-coordinate geometry to one Si(1), one Si(5), and one Si(7) atom. The Mg(11)-Si(1) bond length is 2.87 Å. The Mg(11)-Si(5) bond length is 2.78 Å. The Mg(11)-Si(7) bond length is 2.78 Å. In the twelfth Mg site, Mg(12) is bonded in a 3-coordinate geometry to one Si(4), one Si(7), and one Si(8) atom. The Mg(12)-Si(4) bond length is 2.83 Å. The Mg(12)-Si(7) bond length is 2.73 Å. The Mg(12)-Si(8) bond length is 2.67 Å. In the thirteenth Mg site, Mg(13) is bonded in a 5-coordinate geometry to one Si(1), one Si(10), one Si(2), one Si(5), and one Si(7) atom. The Mg(13)-Si(1) bond length is 2.78 Å. The Mg(13)-Si(10) bond length is 2.82 Å. The Mg(13)-Si(2) bond length is 2.87 Å. The Mg(13)-Si(5) bond length is 2.80 Å. The Mg(13)-Si(7) bond length is 2.83 Å. In the fourteenth Mg site, Mg(14) is bonded in a 10-coordinate geometry to one Mg(1), one Mg(10), one Mg(16), one Mg(2), one Mg(5), one Si(2), one Si(3), one Si(6), one Si(8), and one Si(9) atom. The Mg(14)-Mg(16) bond length is 3.16 Å. The Mg(14)-Si(2) bond length is 2.71 Å. The Mg(14)-Si(3) bond length is 3.05 Å. The Mg(14)-Si(6) bond length is 2.98 Å. The Mg(14)-Si(8) bond length is 2.66 Å. The Mg(14)-Si(9) bond length is 2.67 Å. In the fifteenth Mg site, Mg(15) is bonded in a 3-coordinate geometry to one Si(1), one Si(10), and one Si(5) atom. The Mg(15)-Si(1) bond length is 2.89 Å. The Mg(15)-Si(10) bond length is 2.81 Å. The Mg(15)-Si(5) bond length is 2.69 Å. In the sixteenth Mg site, Mg(16) is bonded in a 3-coordinate geometry to one Mg(14), one Si(3), one Si(6), one Si(8), and one Si(9) atom. The Mg(16)-Si(3) bond length is 2.76 Å. The Mg(16)-Si(6) bond length is 2.76 Å. The Mg(16)-Si(8) bond length is 3.14 Å. The Mg(16)-Si(9) bond length is 2.90 Å. In the seventeenth Mg site, Mg(17) is bonded in a distorted trigonal non-coplanar geometry to one Si(1), one Si(10), and one Si(7) atom. The Mg(17)-Si(1) bond length is 2.83 Å. The Mg(17)-Si(10) bond length is 2.78 Å. The Mg(17)-Si(7) bond length is 2.76 Å. In the eighteenth Mg site, Mg(18) is bonded in a 5-coordinate geometry to one Si(3), one Si(4), one Si(6), one Si(8), and one Si(9) atom. The Mg(18)-Si(3) bond length is 2.78 Å. The Mg(18)-Si(4) bond length is 2.82 Å. The Mg(18)-Si(6) bond length is 2.87 Å. The Mg(18)-Si(8) bond length is 2.64 Å. The Mg(18)-Si(9) bond length is 2.76 Å. There are ten inequivalent Si sites. In the first Si site, Si(1) is bonded in a 8-coordinate geometry to one Mg(11), one Mg(13), one Mg(15), one Mg(17), one Mg(4), one Mg(7), one Mg(9), and one Si(7) atom. The Si(1)-Si(7) bond length is 2.43 Å. In the second Si site, Si(2) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(13), one Mg(14), one Mg(2), one Mg(3), one Mg(4), one Mg(5), and one Mg(6) atom. In the third Si site, Si(3) is bonded in a 6-coordinate geometry to one Mg(14), one Mg(16), one Mg(18), one Mg(8), one Si(4), and one Si(8) atom. The Si(3)-Si(4) bond length is 2.45 Å. The Si(3)-Si(8) bond length is 2.41 Å. In the fourth Si site, Si(4) is bonded in a 8-coordinate geometry to one Mg(10), one Mg(12), one Mg(18), one Mg(7), one Mg(8), one Mg(9), one Si(3), and one Si(6) atom. The Si(4)-Si(6) bond length is 2.54 Å. In the fifth Si site, Si(5) is bonded in a 7-coordinate geometry to one Mg(11), one Mg(13), one Mg(15), one Mg(2), one Mg(4), one Mg(6), and one Mg(7) atom. In the sixth Si site, Si(6) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(10), one Mg(14), one Mg(16), one Mg(18), one Mg(5), one Mg(8), and one Si(4) atom. In the seventh Si site, Si(7) is bonded in a 7-coordinate geometry to one Mg(11), one Mg(12), one Mg(13), one Mg(17), one Mg(8), one Mg(9), and one Si(1) atom. In the eighth Si site, Si(8) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(10), one Mg(12), one Mg(14), one Mg(16), one Mg(18), one Mg(8), and one Si(3) atom. In the ninth Si site, Si(9) is bonded to one Mg(1), one Mg(14), one Mg(16), one Mg(18), one Mg(2), one Mg(3), one Mg(5), and one Mg(6) atom to form distorted SiMg8 hexagonal bipyramids that share corners with two equivalent Si(10)Mg8 hexagonal bipyramids,  a cornercorner with one Mg(1)MgSi4 tetrahedra,  a cornercorner with one Mg(5)MgSi4 tetrahedra, and  an edgeedge with one Si(10)Mg8 hexagonal bipyramid. In the tenth Si site, Si(10) is bonded to one Mg(13), one Mg(15), one Mg(17), one Mg(2), one Mg(3), one Mg(4), one Mg(5), and one Mg(6) atom to form a mixture of distorted corner and edge-sharing SiMg8 hexagonal bipyramids.