Mg6Si5 crystallizes in the triclinic P1 space group. There are twelve inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to one Si(9), two equivalent Si(3), and two equivalent Si(4) atoms. In the second Mg site, Mg(2) is bonded to two equivalent Mg(7), one Si(1), one Si(6), two equivalent Si(5), and two equivalent Si(8) atoms to form distorted MgMg2Si6 tetrahedra that share corners with two equivalent Mg(9)Si4 tetrahedra and faces with two equivalent Mg(2)Mg2Si6 tetrahedra. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one Mg(7), one Si(3), one Si(7), two equivalent Si(10), and two equivalent Si(2) atoms. In the fourth Mg site, Mg(4) is bonded in a 7-coordinate geometry to one Si(9), two equivalent Si(4), two equivalent Si(5), and two equivalent Si(8) atoms. In the fifth Mg site, Mg(5) is bonded in a 5-coordinate geometry to one Si(1), two equivalent Si(4), and two equivalent Si(5) atoms. In the sixth Mg site, Mg(6) is bonded in a 5-coordinate geometry to one Si(2), one Si(6), two equivalent Si(3), and two equivalent Si(7) atoms. In the seventh Mg site, Mg(7) is bonded in a 12-coordinate geometry to one Mg(3), two equivalent Mg(2), one Si(6), one Si(7), one Si(8), two equivalent Si(1), two equivalent Si(10), and two equivalent Si(2) atoms. In the eighth Mg site, Mg(8) is bonded in a 5-coordinate geometry to one Si(5), two equivalent Si(1), two equivalent Si(2), and two equivalent Si(6) atoms. In the ninth Mg site, Mg(9) is bonded to one Si(4), one Si(5), and two equivalent Si(9) atoms to form distorted MgSi4 tetrahedra that share corners with two equivalent Mg(2)Mg2Si6 tetrahedra, corners with two equivalent Mg(10)Si4 tetrahedra, corners with two equivalent Mg(11)Si4 tetrahedra, corners with two equivalent Mg(9)Si4 tetrahedra, corners with three equivalent Mg(12)Si4 tetrahedra,  an edgeedge with one Mg(11)Si4 tetrahedra, and  an edgeedge with one Mg(12)Si4 tetrahedra. In the tenth Mg site, Mg(10) is bonded to one Si(10), one Si(9), and two equivalent Si(3) atoms to form a mixture of distorted edge and corner-sharing MgSi4 tetrahedra. In the eleventh Mg site, Mg(11) is bonded to one Si(3), one Si(7), and two equivalent Si(9) atoms to form a mixture of distorted edge and corner-sharing MgSi4 tetrahedra. In the twelfth Mg site, Mg(12) is bonded to one Si(3), one Si(4), and two equivalent Si(9) atoms to form a mixture of distorted edge and corner-sharing MgSi4 tetrahedra. There are ten inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to one Mg(2), one Mg(5), two equivalent Mg(7), two equivalent Mg(8), one Si(2), and two equivalent Si(8) atoms. In the second Si site, Si(2) is bonded in a 10-coordinate geometry to one Mg(6), two equivalent Mg(3), two equivalent Mg(7), two equivalent Mg(8), one Si(1), one Si(10), and one Si(6) atom. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to one Mg(11), one Mg(12), one Mg(3), two equivalent Mg(1), two equivalent Mg(10), and two equivalent Mg(6) atoms. In the fourth Si site, Si(4) is bonded in a 9-coordinate geometry to one Mg(12), one Mg(9), two equivalent Mg(1), two equivalent Mg(4), two equivalent Mg(5), and one Si(8) atom. In the fifth Si site, Si(5) is bonded in a 9-coordinate geometry to one Mg(8), one Mg(9), two equivalent Mg(2), two equivalent Mg(4), two equivalent Mg(5), and one Si(8) atom. In the sixth Si site, Si(6) is bonded in a 8-coordinate geometry to one Mg(2), one Mg(6), one Mg(7), two equivalent Mg(8), one Si(10), one Si(2), and one Si(7) atom. In the seventh Si site, Si(7) is bonded in a 8-coordinate geometry to one Mg(11), one Mg(3), one Mg(7), two equivalent Mg(6), one Si(6), and two equivalent Si(10) atoms. In the eighth Si site, Si(8) is bonded in a 9-coordinate geometry to one Mg(7), two equivalent Mg(2), two equivalent Mg(4), one Si(4), one Si(5), and two equivalent Si(1) atoms. In the ninth Si site, Si(9) is bonded in a 9-coordinate geometry to one Mg(1), one Mg(10), one Mg(4), two equivalent Mg(11), two equivalent Mg(12), and two equivalent Mg(9) atoms. In the tenth Si site, Si(10) is bonded in a 9-coordinate geometry to one Mg(10), two equivalent Mg(3), two equivalent Mg(7), one Si(2), one Si(6), and two equivalent Si(7) atoms.
Mg6Si5 crystallizes in the triclinic P1 space group. There are twelve inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to one Si(9), two equivalent Si(3), and two equivalent Si(4) atoms. The Mg(1)-Si(9) bond length is 2.83 Å. There is one shorter (2.94 Å) and one longer (2.97 Å) Mg(1)-Si(3) bond length. There is one shorter (3.06 Å) and one longer (3.07 Å) Mg(1)-Si(4) bond length. In the second Mg site, Mg(2) is bonded to two equivalent Mg(7), one Si(1), one Si(6), two equivalent Si(5), and two equivalent Si(8) atoms to form distorted MgMg2Si6 tetrahedra that share corners with two equivalent Mg(9)Si4 tetrahedra and faces with two equivalent Mg(2)Mg2Si6 tetrahedra. There is one shorter (2.96 Å) and one longer (3.14 Å) Mg(2)-Mg(7) bond length. The Mg(2)-Si(1) bond length is 2.76 Å. The Mg(2)-Si(6) bond length is 2.77 Å. Both Mg(2)-Si(5) bond lengths are 2.66 Å. There is one shorter (3.12 Å) and one longer (3.13 Å) Mg(2)-Si(8) bond length. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one Mg(7), one Si(3), one Si(7), two equivalent Si(10), and two equivalent Si(2) atoms. The Mg(3)-Mg(7) bond length is 2.98 Å. The Mg(3)-Si(3) bond length is 2.67 Å. The Mg(3)-Si(7) bond length is 3.22 Å. There is one shorter (2.83 Å) and one longer (2.87 Å) Mg(3)-Si(10) bond length. There is one shorter (2.71 Å) and one longer (2.75 Å) Mg(3)-Si(2) bond length. In the fourth Mg site, Mg(4) is bonded in a 7-coordinate geometry to one Si(9), two equivalent Si(4), two equivalent Si(5), and two equivalent Si(8) atoms. The Mg(4)-Si(9) bond length is 2.89 Å. There is one shorter (2.89 Å) and one longer (2.90 Å) Mg(4)-Si(4) bond length. There is one shorter (2.96 Å) and one longer (2.98 Å) Mg(4)-Si(5) bond length. There is one shorter (3.02 Å) and one longer (3.05 Å) Mg(4)-Si(8) bond length. In the fifth Mg site, Mg(5) is bonded in a 5-coordinate geometry to one Si(1), two equivalent Si(4), and two equivalent Si(5) atoms. The Mg(5)-Si(1) bond length is 2.76 Å. There is one shorter (2.86 Å) and one longer (2.90 Å) Mg(5)-Si(4) bond length. There is one shorter (2.95 Å) and one longer (3.00 Å) Mg(5)-Si(5) bond length. In the sixth Mg site, Mg(6) is bonded in a 5-coordinate geometry to one Si(2), one Si(6), two equivalent Si(3), and two equivalent Si(7) atoms. The Mg(6)-Si(2) bond length is 3.20 Å. The Mg(6)-Si(6) bond length is 3.04 Å. There is one shorter (2.89 Å) and one longer (2.90 Å) Mg(6)-Si(3) bond length. There is one shorter (2.88 Å) and one longer (2.90 Å) Mg(6)-Si(7) bond length. In the seventh Mg site, Mg(7) is bonded in a 12-coordinate geometry to one Mg(3), two equivalent Mg(2), one Si(6), one Si(7), one Si(8), two equivalent Si(1), two equivalent Si(10), and two equivalent Si(2) atoms. The Mg(7)-Si(6) bond length is 2.92 Å. The Mg(7)-Si(7) bond length is 3.06 Å. The Mg(7)-Si(8) bond length is 3.01 Å. There is one shorter (2.96 Å) and one longer (3.13 Å) Mg(7)-Si(1) bond length. There is one shorter (2.76 Å) and one longer (3.08 Å) Mg(7)-Si(10) bond length. There is one shorter (2.95 Å) and one longer (3.18 Å) Mg(7)-Si(2) bond length. In the eighth Mg site, Mg(8) is bonded in a 5-coordinate geometry to one Si(5), two equivalent Si(1), two equivalent Si(2), and two equivalent Si(6) atoms. The Mg(8)-Si(5) bond length is 2.93 Å. There is one shorter (2.95 Å) and one longer (2.96 Å) Mg(8)-Si(1) bond length. There is one shorter (2.88 Å) and one longer (3.26 Å) Mg(8)-Si(2) bond length. There is one shorter (2.81 Å) and one longer (3.18 Å) Mg(8)-Si(6) bond length. In the ninth Mg site, Mg(9) is bonded to one Si(4), one Si(5), and two equivalent Si(9) atoms to form distorted MgSi4 tetrahedra that share corners with two equivalent Mg(2)Mg2Si6 tetrahedra, corners with two equivalent Mg(10)Si4 tetrahedra, corners with two equivalent Mg(11)Si4 tetrahedra, corners with two equivalent Mg(9)Si4 tetrahedra, corners with three equivalent Mg(12)Si4 tetrahedra,  an edgeedge with one Mg(11)Si4 tetrahedra, and  an edgeedge with one Mg(12)Si4 tetrahedra. The Mg(9)-Si(4) bond length is 2.65 Å. The Mg(9)-Si(5) bond length is 2.66 Å. Both Mg(9)-Si(9) bond lengths are 2.75 Å. In the tenth Mg site, Mg(10) is bonded to one Si(10), one Si(9), and two equivalent Si(3) atoms to form a mixture of distorted edge and corner-sharing MgSi4 tetrahedra. The Mg(10)-Si(10) bond length is 2.73 Å. The Mg(10)-Si(9) bond length is 2.84 Å. There is one shorter (2.80 Å) and one longer (2.83 Å) Mg(10)-Si(3) bond length. In the eleventh Mg site, Mg(11) is bonded to one Si(3), one Si(7), and two equivalent Si(9) atoms to form a mixture of distorted edge and corner-sharing MgSi4 tetrahedra. The Mg(11)-Si(3) bond length is 2.66 Å. The Mg(11)-Si(7) bond length is 2.73 Å. Both Mg(11)-Si(9) bond lengths are 2.77 Å. In the twelfth Mg site, Mg(12) is bonded to one Si(3), one Si(4), and two equivalent Si(9) atoms to form a mixture of distorted edge and corner-sharing MgSi4 tetrahedra. The Mg(12)-Si(3) bond length is 2.66 Å. The Mg(12)-Si(4) bond length is 2.65 Å. There is one shorter (2.74 Å) and one longer (2.76 Å) Mg(12)-Si(9) bond length. There are ten inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to one Mg(2), one Mg(5), two equivalent Mg(7), two equivalent Mg(8), one Si(2), and two equivalent Si(8) atoms. The Si(1)-Si(2) bond length is 2.48 Å. Both Si(1)-Si(8) bond lengths are 2.58 Å. In the second Si site, Si(2) is bonded in a 10-coordinate geometry to one Mg(6), two equivalent Mg(3), two equivalent Mg(7), two equivalent Mg(8), one Si(1), one Si(10), and one Si(6) atom. The Si(2)-Si(10) bond length is 2.57 Å. The Si(2)-Si(6) bond length is 2.59 Å. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to one Mg(11), one Mg(12), one Mg(3), two equivalent Mg(1), two equivalent Mg(10), and two equivalent Mg(6) atoms. In the fourth Si site, Si(4) is bonded in a 9-coordinate geometry to one Mg(12), one Mg(9), two equivalent Mg(1), two equivalent Mg(4), two equivalent Mg(5), and one Si(8) atom. The Si(4)-Si(8) bond length is 2.44 Å. In the fifth Si site, Si(5) is bonded in a 9-coordinate geometry to one Mg(8), one Mg(9), two equivalent Mg(2), two equivalent Mg(4), two equivalent Mg(5), and one Si(8) atom. The Si(5)-Si(8) bond length is 2.48 Å. In the sixth Si site, Si(6) is bonded in a 8-coordinate geometry to one Mg(2), one Mg(6), one Mg(7), two equivalent Mg(8), one Si(10), one Si(2), and one Si(7) atom. The Si(6)-Si(10) bond length is 2.50 Å. The Si(6)-Si(7) bond length is 2.47 Å. In the seventh Si site, Si(7) is bonded in a 8-coordinate geometry to one Mg(11), one Mg(3), one Mg(7), two equivalent Mg(6), one Si(6), and two equivalent Si(10) atoms. There is one shorter (2.58 Å) and one longer (2.60 Å) Si(7)-Si(10) bond length. In the eighth Si site, Si(8) is bonded in a 9-coordinate geometry to one Mg(7), two equivalent Mg(2), two equivalent Mg(4), one Si(4), one Si(5), and two equivalent Si(1) atoms. In the ninth Si site, Si(9) is bonded in a 9-coordinate geometry to one Mg(1), one Mg(10), one Mg(4), two equivalent Mg(11), two equivalent Mg(12), and two equivalent Mg(9) atoms. In the tenth Si site, Si(10) is bonded in a 9-coordinate geometry to one Mg(10), two equivalent Mg(3), two equivalent Mg(7), one Si(2), one Si(6), and two equivalent Si(7) atoms.