Mg9Si5 crystallizes in the trigonal R-3c space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to one Si(3), two equivalent Si(1), and two equivalent Si(2) atoms. In the second Mg site, Mg(2) is bonded to two equivalent Si(1) and two equivalent Si(2) atoms to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to three equivalent Mg(2) and six equivalent Mg(1) atoms. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to three equivalent Mg(2) and six equivalent Mg(1) atoms. In the third Si site, Si(3) is bonded in a 6-coordinate geometry to six equivalent Mg(1) atoms.
Mg9Si5 crystallizes in the trigonal R-3c space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to one Si(3), two equivalent Si(1), and two equivalent Si(2) atoms. The Mg(1)-Si(3) bond length is 2.70 Å. There is one shorter (2.89 Å) and one longer (3.05 Å) Mg(1)-Si(1) bond length. There is one shorter (2.83 Å) and one longer (2.84 Å) Mg(1)-Si(2) bond length. In the second Mg site, Mg(2) is bonded to two equivalent Si(1) and two equivalent Si(2) atoms to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. Both Mg(2)-Si(1) bond lengths are 2.73 Å. Both Mg(2)-Si(2) bond lengths are 2.76 Å. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to three equivalent Mg(2) and six equivalent Mg(1) atoms. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to three equivalent Mg(2) and six equivalent Mg(1) atoms. In the third Si site, Si(3) is bonded in a 6-coordinate geometry to six equivalent Mg(1) atoms.