Mg5Si9 crystallizes in the triclinic P1 space group. There are ten inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to one Si(10), one Si(11), one Si(14), one Si(17), one Si(2), and one Si(8) atom. In the second Mg site, Mg(2) is bonded in a 5-coordinate geometry to one Si(10), one Si(17), one Si(3), one Si(5), and one Si(7) atom. In the third Mg site, Mg(3) is bonded to one Si(10), one Si(14), one Si(17), one Si(18), and one Si(3) atom to form distorted MgSi5 trigonal bipyramids that share  a cornercorner with one Si(5)Mg4Si2 octahedra, corners with two equivalent Mg(6)Si5 tetrahedra, and  an edgeedge with one Mg(6)Si5 tetrahedra. The corner-sharing octahedral tilt angles are 71°. In the fourth Mg site, Mg(4) is bonded in a 4-coordinate geometry to one Si(13), one Si(18), one Si(3), one Si(5), and one Si(9) atom. In the fifth Mg site, Mg(5) is bonded in a 6-coordinate geometry to one Si(10), one Si(14), one Si(17), one Si(18), one Si(3), one Si(7), and one Si(8) atom. In the sixth Mg site, Mg(6) is bonded to one Si(10), one Si(13), one Si(17), one Si(18), and one Si(3) atom to form distorted MgSi5 tetrahedra that share  a cornercorner with one Si(5)Mg4Si2 octahedra, corners with two equivalent Mg(3)Si5 trigonal bipyramids, and  an edgeedge with one Mg(3)Si5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 81°. In the seventh Mg site, Mg(7) is bonded in a 8-coordinate geometry to one Si(1), one Si(12), one Si(13), one Si(15), one Si(16), one Si(5), one Si(7), and one Si(9) atom. In the eighth Mg site, Mg(8) is bonded in a 8-coordinate geometry to one Si(11), one Si(12), one Si(14), one Si(15), one Si(16), one Si(2), one Si(4), and one Si(8) atom. In the ninth Mg site, Mg(9) is bonded in a 6-coordinate geometry to one Si(13), one Si(15), one Si(3), one Si(5), one Si(7), and one Si(9) atom. In the tenth Mg site, Mg(10) is bonded in a 5-coordinate geometry to one Si(12), one Si(15), one Si(16), one Si(6), and one Si(8) atom. There are eighteen inequivalent Si sites. In the first Si site, Si(1) is bonded in a 5-coordinate geometry to one Mg(7), one Si(12), one Si(13), one Si(16), and one Si(5) atom. In the second Si site, Si(2) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(8), one Si(11), one Si(14), one Si(15), and one Si(16) atom. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to one Mg(2), one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Mg(9), one Si(13), one Si(7), and one Si(9) atom. In the fourth Si site, Si(4) is bonded in a 5-coordinate geometry to one Mg(8), one Si(10), one Si(11), one Si(14), and one Si(17) atom. In the fifth Si site, Si(5) is bonded to one Mg(2), one Mg(4), one Mg(7), one Mg(9), one Si(1), and one Si(18) atom to form distorted SiMg4Si2 octahedra that share  a cornercorner with one Si(10)Mg5Si2 pentagonal bipyramid,  a cornercorner with one Mg(6)Si5 tetrahedra, and  a cornercorner with one Mg(3)Si5 trigonal bipyramid. In the sixth Si site, Si(6) is bonded in a 5-coordinate geometry to one Mg(10), one Si(10), one Si(11), one Si(15), and one Si(8) atom. In the seventh Si site, Si(7) is bonded in a 8-coordinate geometry to one Mg(2), one Mg(5), one Mg(7), one Mg(9), one Si(13), one Si(18), one Si(3), and one Si(9) atom. In the eighth Si site, Si(8) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(10), one Mg(5), one Mg(8), one Si(14), one Si(17), and one Si(6) atom. In the ninth Si site, Si(9) is bonded in a 7-coordinate geometry to one Mg(4), one Mg(7), one Mg(9), one Si(12), one Si(16), one Si(3), and one Si(7) atom. In the tenth Si site, Si(10) is bonded to one Mg(1), one Mg(2), one Mg(3), one Mg(5), one Mg(6), one Si(4), and one Si(6) atom to form distorted corner-sharing SiMg5Si2 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 50°. In the eleventh Si site, Si(11) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(8), one Si(12), one Si(2), one Si(4), and one Si(6) atom. In the twelfth Si site, Si(12) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Si(1), one Si(11), and one Si(9) atom. In the thirteenth Si site, Si(13) is bonded in a 7-coordinate geometry to one Mg(4), one Mg(6), one Mg(7), one Mg(9), one Si(1), one Si(3), and one Si(7) atom. In the fourteenth Si site, Si(14) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(3), one Mg(5), one Mg(8), one Si(2), one Si(4), and one Si(8) atom. In the fifteenth Si site, Si(15) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Mg(9), one Si(2), and one Si(6) atom. In the sixteenth Si site, Si(16) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Si(1), one Si(2), and one Si(9) atom. In the seventeenth Si site, Si(17) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(5), one Mg(6), one Si(4), and one Si(8) atom. In the eighteenth Si site, Si(18) is bonded in a 6-coordinate geometry to one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Si(5), and one Si(7) atom.
Mg5Si9 crystallizes in the triclinic P1 space group. There are ten inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to one Si(10), one Si(11), one Si(14), one Si(17), one Si(2), and one Si(8) atom. The Mg(1)-Si(10) bond length is 2.80 Å. The Mg(1)-Si(11) bond length is 2.67 Å. The Mg(1)-Si(14) bond length is 3.15 Å. The Mg(1)-Si(17) bond length is 2.93 Å. The Mg(1)-Si(2) bond length is 3.19 Å. The Mg(1)-Si(8) bond length is 3.04 Å. In the second Mg site, Mg(2) is bonded in a 5-coordinate geometry to one Si(10), one Si(17), one Si(3), one Si(5), and one Si(7) atom. The Mg(2)-Si(10) bond length is 2.85 Å. The Mg(2)-Si(17) bond length is 2.81 Å. The Mg(2)-Si(3) bond length is 2.90 Å. The Mg(2)-Si(5) bond length is 2.78 Å. The Mg(2)-Si(7) bond length is 2.73 Å. In the third Mg site, Mg(3) is bonded to one Si(10), one Si(14), one Si(17), one Si(18), and one Si(3) atom to form distorted MgSi5 trigonal bipyramids that share  a cornercorner with one Si(5)Mg4Si2 octahedra, corners with two equivalent Mg(6)Si5 tetrahedra, and  an edgeedge with one Mg(6)Si5 tetrahedra. The corner-sharing octahedral tilt angles are 71°. The Mg(3)-Si(10) bond length is 2.90 Å. The Mg(3)-Si(14) bond length is 2.95 Å. The Mg(3)-Si(17) bond length is 2.74 Å. The Mg(3)-Si(18) bond length is 2.70 Å. The Mg(3)-Si(3) bond length is 2.93 Å. In the fourth Mg site, Mg(4) is bonded in a 4-coordinate geometry to one Si(13), one Si(18), one Si(3), one Si(5), and one Si(9) atom. The Mg(4)-Si(13) bond length is 2.92 Å. The Mg(4)-Si(18) bond length is 2.99 Å. The Mg(4)-Si(3) bond length is 2.74 Å. The Mg(4)-Si(5) bond length is 2.94 Å. The Mg(4)-Si(9) bond length is 3.08 Å. In the fifth Mg site, Mg(5) is bonded in a 6-coordinate geometry to one Si(10), one Si(14), one Si(17), one Si(18), one Si(3), one Si(7), and one Si(8) atom. The Mg(5)-Si(10) bond length is 2.69 Å. The Mg(5)-Si(14) bond length is 2.88 Å. The Mg(5)-Si(17) bond length is 2.86 Å. The Mg(5)-Si(18) bond length is 2.73 Å. The Mg(5)-Si(3) bond length is 3.23 Å. The Mg(5)-Si(7) bond length is 3.09 Å. The Mg(5)-Si(8) bond length is 2.92 Å. In the sixth Mg site, Mg(6) is bonded to one Si(10), one Si(13), one Si(17), one Si(18), and one Si(3) atom to form distorted MgSi5 tetrahedra that share  a cornercorner with one Si(5)Mg4Si2 octahedra, corners with two equivalent Mg(3)Si5 trigonal bipyramids, and  an edgeedge with one Mg(3)Si5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 81°. The Mg(6)-Si(10) bond length is 2.61 Å. The Mg(6)-Si(13) bond length is 2.67 Å. The Mg(6)-Si(17) bond length is 2.66 Å. The Mg(6)-Si(18) bond length is 2.73 Å. The Mg(6)-Si(3) bond length is 3.28 Å. In the seventh Mg site, Mg(7) is bonded in a 8-coordinate geometry to one Si(1), one Si(12), one Si(13), one Si(15), one Si(16), one Si(5), one Si(7), and one Si(9) atom. The Mg(7)-Si(1) bond length is 2.96 Å. The Mg(7)-Si(12) bond length is 2.71 Å. The Mg(7)-Si(13) bond length is 2.77 Å. The Mg(7)-Si(15) bond length is 2.90 Å. The Mg(7)-Si(16) bond length is 2.87 Å. The Mg(7)-Si(5) bond length is 3.07 Å. The Mg(7)-Si(7) bond length is 3.00 Å. The Mg(7)-Si(9) bond length is 2.91 Å. In the eighth Mg site, Mg(8) is bonded in a 8-coordinate geometry to one Si(11), one Si(12), one Si(14), one Si(15), one Si(16), one Si(2), one Si(4), and one Si(8) atom. The Mg(8)-Si(11) bond length is 3.10 Å. The Mg(8)-Si(12) bond length is 3.07 Å. The Mg(8)-Si(14) bond length is 2.94 Å. The Mg(8)-Si(15) bond length is 2.84 Å. The Mg(8)-Si(16) bond length is 2.87 Å. The Mg(8)-Si(2) bond length is 3.07 Å. The Mg(8)-Si(4) bond length is 2.83 Å. The Mg(8)-Si(8) bond length is 2.86 Å. In the ninth Mg site, Mg(9) is bonded in a 6-coordinate geometry to one Si(13), one Si(15), one Si(3), one Si(5), one Si(7), and one Si(9) atom. The Mg(9)-Si(13) bond length is 2.80 Å. The Mg(9)-Si(15) bond length is 2.75 Å. The Mg(9)-Si(3) bond length is 2.90 Å. The Mg(9)-Si(5) bond length is 2.68 Å. The Mg(9)-Si(7) bond length is 3.03 Å. The Mg(9)-Si(9) bond length is 2.88 Å. In the tenth Mg site, Mg(10) is bonded in a 5-coordinate geometry to one Si(12), one Si(15), one Si(16), one Si(6), and one Si(8) atom. The Mg(10)-Si(12) bond length is 2.72 Å. The Mg(10)-Si(15) bond length is 2.77 Å. The Mg(10)-Si(16) bond length is 2.88 Å. The Mg(10)-Si(6) bond length is 2.96 Å. The Mg(10)-Si(8) bond length is 2.81 Å. There are eighteen inequivalent Si sites. In the first Si site, Si(1) is bonded in a 5-coordinate geometry to one Mg(7), one Si(12), one Si(13), one Si(16), and one Si(5) atom. The Si(1)-Si(12) bond length is 2.40 Å. The Si(1)-Si(13) bond length is 2.49 Å. The Si(1)-Si(16) bond length is 2.42 Å. The Si(1)-Si(5) bond length is 2.40 Å. In the second Si site, Si(2) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(8), one Si(11), one Si(14), one Si(15), and one Si(16) atom. The Si(2)-Si(11) bond length is 2.47 Å. The Si(2)-Si(14) bond length is 2.39 Å. The Si(2)-Si(15) bond length is 2.36 Å. The Si(2)-Si(16) bond length is 2.40 Å. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to one Mg(2), one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Mg(9), one Si(13), one Si(7), and one Si(9) atom. The Si(3)-Si(13) bond length is 2.44 Å. The Si(3)-Si(7) bond length is 2.68 Å. The Si(3)-Si(9) bond length is 2.45 Å. In the fourth Si site, Si(4) is bonded in a 5-coordinate geometry to one Mg(8), one Si(10), one Si(11), one Si(14), and one Si(17) atom. The Si(4)-Si(10) bond length is 2.42 Å. The Si(4)-Si(11) bond length is 2.61 Å. The Si(4)-Si(14) bond length is 2.45 Å. The Si(4)-Si(17) bond length is 2.40 Å. In the fifth Si site, Si(5) is bonded to one Mg(2), one Mg(4), one Mg(7), one Mg(9), one Si(1), and one Si(18) atom to form distorted SiMg4Si2 octahedra that share  a cornercorner with one Si(10)Mg5Si2 pentagonal bipyramid,  a cornercorner with one Mg(6)Si5 tetrahedra, and  a cornercorner with one Mg(3)Si5 trigonal bipyramid. The Si(5)-Si(18) bond length is 2.39 Å. In the sixth Si site, Si(6) is bonded in a 5-coordinate geometry to one Mg(10), one Si(10), one Si(11), one Si(15), and one Si(8) atom. The Si(6)-Si(10) bond length is 2.43 Å. The Si(6)-Si(11) bond length is 2.34 Å. The Si(6)-Si(15) bond length is 2.39 Å. The Si(6)-Si(8) bond length is 2.45 Å. In the seventh Si site, Si(7) is bonded in a 8-coordinate geometry to one Mg(2), one Mg(5), one Mg(7), one Mg(9), one Si(13), one Si(18), one Si(3), and one Si(9) atom. The Si(7)-Si(13) bond length is 2.47 Å. The Si(7)-Si(18) bond length is 2.38 Å. The Si(7)-Si(9) bond length is 2.42 Å. In the eighth Si site, Si(8) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(10), one Mg(5), one Mg(8), one Si(14), one Si(17), and one Si(6) atom. The Si(8)-Si(14) bond length is 2.44 Å. The Si(8)-Si(17) bond length is 2.42 Å. In the ninth Si site, Si(9) is bonded in a 7-coordinate geometry to one Mg(4), one Mg(7), one Mg(9), one Si(12), one Si(16), one Si(3), and one Si(7) atom. The Si(9)-Si(12) bond length is 2.41 Å. The Si(9)-Si(16) bond length is 2.42 Å. In the tenth Si site, Si(10) is bonded to one Mg(1), one Mg(2), one Mg(3), one Mg(5), one Mg(6), one Si(4), and one Si(6) atom to form distorted corner-sharing SiMg5Si2 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 50°. In the eleventh Si site, Si(11) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(8), one Si(12), one Si(2), one Si(4), and one Si(6) atom. The Si(11)-Si(12) bond length is 2.35 Å. In the twelfth Si site, Si(12) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Si(1), one Si(11), and one Si(9) atom. In the thirteenth Si site, Si(13) is bonded in a 7-coordinate geometry to one Mg(4), one Mg(6), one Mg(7), one Mg(9), one Si(1), one Si(3), and one Si(7) atom. In the fourteenth Si site, Si(14) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(3), one Mg(5), one Mg(8), one Si(2), one Si(4), and one Si(8) atom. In the fifteenth Si site, Si(15) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Mg(9), one Si(2), and one Si(6) atom. In the sixteenth Si site, Si(16) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Si(1), one Si(2), and one Si(9) atom. In the seventeenth Si site, Si(17) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(5), one Mg(6), one Si(4), and one Si(8) atom. In the eighteenth Si site, Si(18) is bonded in a 6-coordinate geometry to one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Si(5), and one Si(7) atom.