Mg3(MoAl9)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. Mo(1) is bonded to six equivalent Al(1) and six equivalent Al(2) atoms to form MoAl12 cuboctahedra that share corners with six equivalent Mo(1)Al12 cuboctahedra, edges with eighteen equivalent Al(2)Mg3Al8Mo cuboctahedra, and faces with six equivalent Al(2)Mg3Al8Mo 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 Mo(1), three equivalent Al(1), and five equivalent Al(2) atoms to form distorted AlMg3Al8Mo cuboctahedra that share corners with fifteen equivalent Al(2)Mg3Al8Mo cuboctahedra, edges with two equivalent Al(2)Mg3Al8Mo cuboctahedra, edges with three equivalent Mo(1)Al12 cuboctahedra,  a faceface with one Mo(1)Al12 cuboctahedra, and faces with fifteen equivalent Al(2)Mg3Al8Mo cuboctahedra. In the second Al site, Al(1) is bonded in a distorted linear geometry to two equivalent Mo(1) and six equivalent Al(2) atoms.
Mg3(MoAl9)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 3.18 Å. All Mg(1)-Al(2) bond lengths are 3.14 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 3.23 Å. Mo(1) is bonded to six equivalent Al(1) and six equivalent Al(2) atoms to form MoAl12 cuboctahedra that share corners with six equivalent Mo(1)Al12 cuboctahedra, edges with eighteen equivalent Al(2)Mg3Al8Mo cuboctahedra, and faces with six equivalent Al(2)Mg3Al8Mo cuboctahedra. All Mo(1)-Al(1) bond lengths are 2.61 Å. All Mo(1)-Al(2) bond lengths are 2.86 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 Mo(1), three equivalent Al(1), and five equivalent Al(2) atoms to form distorted AlMg3Al8Mo cuboctahedra that share corners with fifteen equivalent Al(2)Mg3Al8Mo cuboctahedra, edges with two equivalent Al(2)Mg3Al8Mo cuboctahedra, edges with three equivalent Mo(1)Al12 cuboctahedra,  a faceface with one Mo(1)Al12 cuboctahedra, and faces with fifteen equivalent Al(2)Mg3Al8Mo cuboctahedra. There is one shorter (2.73 Å) and two longer (2.87 Å) Al(2)-Al(1) bond lengths. There are a spread of Al(2)-Al(2) bond distances ranging from 2.75-2.98 Å. In the second Al site, Al(1) is bonded in a distorted linear geometry to two equivalent Mo(1) and six equivalent Al(2) atoms.