Mg6SnSi crystallizes in the orthorhombic Amm2 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(2), two equivalent Mg(4), two equivalent Sn(1), and two equivalent Si(1) atoms to form distorted MgMg4Si2Sn2 cuboctahedra that share corners with two equivalent Mg(3)Mg6Sn2 cuboctahedra, corners with six equivalent Mg(1)Mg4Si2Sn2 cuboctahedra, edges with two equivalent Mg(3)Mg6Sn2 cuboctahedra, edges with four equivalent Mg(4)Mg10Si2 cuboctahedra, edges with five equivalent Mg(1)Mg4Si2Sn2 cuboctahedra,  a faceface with one Mg(1)Mg4Si2Sn2 cuboctahedra, and faces with two equivalent Mg(3)Mg6Sn2 cuboctahedra. In the second Mg site, Mg(2) is bonded in a 12-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Sn(1), and two equivalent Si(1) atoms. In the third Mg site, Mg(3) is bonded to two equivalent Mg(4), four equivalent Mg(2), and two equivalent Sn(1) atoms to form distorted MgMg6Sn2 cuboctahedra that share corners with four equivalent Mg(1)Mg4Si2Sn2 cuboctahedra, corners with eight equivalent Mg(3)Mg6Sn2 cuboctahedra, edges with four equivalent Mg(4)Mg10Si2 cuboctahedra, edges with four equivalent Mg(1)Mg4Si2Sn2 cuboctahedra, and faces with four equivalent Mg(1)Mg4Si2Sn2 cuboctahedra. In the fourth Mg site, Mg(4) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Si(1) atoms to form distorted MgMg10Si2 cuboctahedra that share corners with six equivalent Mg(4)Mg10Si2 cuboctahedra, edges with four equivalent Mg(3)Mg6Sn2 cuboctahedra, edges with eight equivalent Mg(1)Mg4Si2Sn2 cuboctahedra, and faces with two equivalent Mg(4)Mg10Si2 cuboctahedra. Sn(1) is bonded in a 10-coordinate geometry to two equivalent Mg(3), four equivalent Mg(1), and four equivalent Mg(2) atoms. Si(1) is bonded in a 10-coordinate geometry to two equivalent Mg(4), four equivalent Mg(1), and four equivalent Mg(2) atoms.
Mg6SnSi crystallizes in the orthorhombic Amm2 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(2), two equivalent Mg(4), two equivalent Sn(1), and two equivalent Si(1) atoms to form distorted MgMg4Si2Sn2 cuboctahedra that share corners with two equivalent Mg(3)Mg6Sn2 cuboctahedra, corners with six equivalent Mg(1)Mg4Si2Sn2 cuboctahedra, edges with two equivalent Mg(3)Mg6Sn2 cuboctahedra, edges with four equivalent Mg(4)Mg10Si2 cuboctahedra, edges with five equivalent Mg(1)Mg4Si2Sn2 cuboctahedra,  a faceface with one Mg(1)Mg4Si2Sn2 cuboctahedra, and faces with two equivalent Mg(3)Mg6Sn2 cuboctahedra. Both Mg(1)-Mg(2) bond lengths are 3.12 Å. Both Mg(1)-Mg(4) bond lengths are 3.15 Å. There is one shorter (3.09 Å) and one longer (3.10 Å) Mg(1)-Sn(1) bond length. There is one shorter (3.07 Å) and one longer (3.11 Å) Mg(1)-Si(1) bond length. In the second Mg site, Mg(2) is bonded in a 12-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Sn(1), and two equivalent Si(1) atoms. There is one shorter (3.12 Å) and one longer (3.34 Å) Mg(2)-Mg(2) bond length. There is one shorter (3.08 Å) and one longer (3.11 Å) Mg(2)-Mg(3) bond length. There is one shorter (3.01 Å) and one longer (3.18 Å) Mg(2)-Mg(4) bond length. Both Mg(2)-Sn(1) bond lengths are 3.17 Å. Both Mg(2)-Si(1) bond lengths are 3.11 Å. In the third Mg site, Mg(3) is bonded to two equivalent Mg(4), four equivalent Mg(2), and two equivalent Sn(1) atoms to form distorted MgMg6Sn2 cuboctahedra that share corners with four equivalent Mg(1)Mg4Si2Sn2 cuboctahedra, corners with eight equivalent Mg(3)Mg6Sn2 cuboctahedra, edges with four equivalent Mg(4)Mg10Si2 cuboctahedra, edges with four equivalent Mg(1)Mg4Si2Sn2 cuboctahedra, and faces with four equivalent Mg(1)Mg4Si2Sn2 cuboctahedra. Both Mg(3)-Mg(4) bond lengths are 3.23 Å. Both Mg(3)-Sn(1) bond lengths are 3.10 Å. In the fourth Mg site, Mg(4) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Si(1) atoms to form distorted MgMg10Si2 cuboctahedra that share corners with six equivalent Mg(4)Mg10Si2 cuboctahedra, edges with four equivalent Mg(3)Mg6Sn2 cuboctahedra, edges with eight equivalent Mg(1)Mg4Si2Sn2 cuboctahedra, and faces with two equivalent Mg(4)Mg10Si2 cuboctahedra. Both Mg(4)-Si(1) bond lengths are 3.06 Å. Sn(1) is bonded in a 10-coordinate geometry to two equivalent Mg(3), four equivalent Mg(1), and four equivalent Mg(2) atoms. Si(1) is bonded in a 10-coordinate geometry to two equivalent Mg(4), four equivalent Mg(1), and four equivalent Mg(2) atoms.