Ca5Mg crystallizes in the hexagonal P-6m2 space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to three equivalent Ca(2) and three equivalent Mg(1) atoms to form distorted CaCa3Mg3 cuboctahedra that share corners with six equivalent Ca(2)Ca12 cuboctahedra, corners with six equivalent Ca(3)Ca12 cuboctahedra, corners with six equivalent Ca(1)Ca3Mg3 cuboctahedra, edges with six equivalent Ca(2)Ca12 cuboctahedra, edges with six equivalent Ca(1)Ca3Mg3 cuboctahedra,  a faceface with one Ca(3)Ca12 cuboctahedra, and  a faceface with one Ca(1)Ca3Mg3 cuboctahedra. In the second Ca site, Ca(2) is bonded to three equivalent Ca(1), three equivalent Ca(3), and six equivalent Ca(2) atoms to form CaCa12 cuboctahedra that share corners with six equivalent Ca(1)Ca3Mg3 cuboctahedra, corners with twelve equivalent Ca(2)Ca12 cuboctahedra, edges with six equivalent Ca(2)Ca12 cuboctahedra, edges with six equivalent Ca(3)Ca12 cuboctahedra, edges with six equivalent Ca(1)Ca3Mg3 cuboctahedra, faces with six equivalent Ca(3)Ca12 cuboctahedra, and faces with seven equivalent Ca(2)Ca12 cuboctahedra. In the third Ca site, Ca(3) is bonded to six equivalent Ca(2) and six equivalent Ca(3) atoms to form CaCa12 cuboctahedra that share corners with six equivalent Ca(3)Ca12 cuboctahedra, corners with twelve equivalent Ca(1)Ca3Mg3 cuboctahedra, edges with six equivalent Ca(3)Ca12 cuboctahedra, edges with twelve equivalent Ca(2)Ca12 cuboctahedra, faces with two equivalent Ca(1)Ca3Mg3 cuboctahedra, faces with six equivalent Ca(3)Ca12 cuboctahedra, and faces with twelve equivalent Ca(2)Ca12 cuboctahedra. Mg(1) is bonded in a 6-coordinate geometry to six equivalent Ca(1) atoms.
Ca5Mg crystallizes in the hexagonal P-6m2 space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to three equivalent Ca(2) and three equivalent Mg(1) atoms to form distorted CaCa3Mg3 cuboctahedra that share corners with six equivalent Ca(2)Ca12 cuboctahedra, corners with six equivalent Ca(3)Ca12 cuboctahedra, corners with six equivalent Ca(1)Ca3Mg3 cuboctahedra, edges with six equivalent Ca(2)Ca12 cuboctahedra, edges with six equivalent Ca(1)Ca3Mg3 cuboctahedra,  a faceface with one Ca(3)Ca12 cuboctahedra, and  a faceface with one Ca(1)Ca3Mg3 cuboctahedra. All Ca(1)-Ca(2) bond lengths are 3.96 Å. All Ca(1)-Mg(1) bond lengths are 3.51 Å. In the second Ca site, Ca(2) is bonded to three equivalent Ca(1), three equivalent Ca(3), and six equivalent Ca(2) atoms to form CaCa12 cuboctahedra that share corners with six equivalent Ca(1)Ca3Mg3 cuboctahedra, corners with twelve equivalent Ca(2)Ca12 cuboctahedra, edges with six equivalent Ca(2)Ca12 cuboctahedra, edges with six equivalent Ca(3)Ca12 cuboctahedra, edges with six equivalent Ca(1)Ca3Mg3 cuboctahedra, faces with six equivalent Ca(3)Ca12 cuboctahedra, and faces with seven equivalent Ca(2)Ca12 cuboctahedra. All Ca(2)-Ca(3) bond lengths are 3.94 Å. All Ca(2)-Ca(2) bond lengths are 3.88 Å. In the third Ca site, Ca(3) is bonded to six equivalent Ca(2) and six equivalent Ca(3) atoms to form CaCa12 cuboctahedra that share corners with six equivalent Ca(3)Ca12 cuboctahedra, corners with twelve equivalent Ca(1)Ca3Mg3 cuboctahedra, edges with six equivalent Ca(3)Ca12 cuboctahedra, edges with twelve equivalent Ca(2)Ca12 cuboctahedra, faces with two equivalent Ca(1)Ca3Mg3 cuboctahedra, faces with six equivalent Ca(3)Ca12 cuboctahedra, and faces with twelve equivalent Ca(2)Ca12 cuboctahedra. All Ca(3)-Ca(3) bond lengths are 3.88 Å. Mg(1) is bonded in a 6-coordinate geometry to six equivalent Ca(1) atoms.