Mg3(WAl9)2 crystallizes in the cubic Fd-3m space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 14-coordinate geometry to two equivalent Mg(2) and twelve equivalent Al(2) atoms. In the second Mg site, Mg(2) is bonded in a 16-coordinate geometry to four equivalent Mg(1) and twelve equivalent Al(2) atoms. W(1) is bonded to six equivalent Al(1) and six equivalent Al(2) atoms to form WAl12 cuboctahedra that share corners with six equivalent W(1)Al12 cuboctahedra, edges with eighteen equivalent Al(2)Mg3Al8W cuboctahedra, and faces with six equivalent Al(2)Mg3Al8W cuboctahedra. There are two inequivalent Al sites. In the first Al site, Al(2) is bonded to one Mg(2), two equivalent Mg(1), one W(1), three equivalent Al(1), and five equivalent Al(2) atoms to form distorted AlMg3Al8W cuboctahedra that share corners with fifteen equivalent Al(2)Mg3Al8W cuboctahedra, edges with two equivalent Al(2)Mg3Al8W cuboctahedra, edges with three equivalent W(1)Al12 cuboctahedra,  a faceface with one W(1)Al12 cuboctahedra, and faces with fifteen equivalent Al(2)Mg3Al8W cuboctahedra. In the second Al site, Al(1) is bonded in a distorted linear geometry to two equivalent W(1) and six equivalent Al(2) atoms.
Mg3(WAl9)2 crystallizes in the cubic Fd-3m space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 14-coordinate geometry to two equivalent Mg(2) and twelve equivalent Al(2) atoms. Both Mg(1)-Mg(2) bond lengths are 3.15 Å. All Mg(1)-Al(2) bond lengths are 3.10 Å. In the second Mg site, Mg(2) is bonded in a 16-coordinate geometry to four equivalent Mg(1) and twelve equivalent Al(2) atoms. All Mg(2)-Al(2) bond lengths are 3.19 Å. W(1) is bonded to six equivalent Al(1) and six equivalent Al(2) atoms to form WAl12 cuboctahedra that share corners with six equivalent W(1)Al12 cuboctahedra, edges with eighteen equivalent Al(2)Mg3Al8W cuboctahedra, and faces with six equivalent Al(2)Mg3Al8W cuboctahedra. All W(1)-Al(1) bond lengths are 2.59 Å. All W(1)-Al(2) bond lengths are 2.84 Å. There are two inequivalent Al sites. In the first Al site, Al(2) is bonded to one Mg(2), two equivalent Mg(1), one W(1), three equivalent Al(1), and five equivalent Al(2) atoms to form distorted AlMg3Al8W cuboctahedra that share corners with fifteen equivalent Al(2)Mg3Al8W cuboctahedra, edges with two equivalent Al(2)Mg3Al8W cuboctahedra, edges with three equivalent W(1)Al12 cuboctahedra,  a faceface with one W(1)Al12 cuboctahedra, and faces with fifteen equivalent Al(2)Mg3Al8W cuboctahedra. There is one shorter (2.68 Å) and two longer (2.85 Å) Al(2)-Al(1) bond lengths. There are three shorter (2.72 Å) and two longer (2.97 Å) Al(2)-Al(2) bond lengths. In the second Al site, Al(1) is bonded in a distorted linear geometry to two equivalent W(1) and six equivalent Al(2) atoms.