Mg2Ca crystallizes in the hexagonal P-62m space group. Ca(1) is bonded to three equivalent Ca(1), three equivalent Mg(2), and six equivalent Mg(1) atoms to form CaCa3Mg9 cuboctahedra that share corners with nine equivalent Ca(1)Ca3Mg9 cuboctahedra, corners with nine equivalent Mg(2)Ca6Mg6 cuboctahedra, edges with six equivalent Ca(1)Ca3Mg9 cuboctahedra, faces with three equivalent Mg(2)Ca6Mg6 cuboctahedra, and faces with five equivalent Ca(1)Ca3Mg9 cuboctahedra. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 8-coordinate geometry to four equivalent Ca(1), two equivalent Mg(1), and two equivalent Mg(2) atoms. In the second Mg site, Mg(2) is bonded to six equivalent Ca(1) and six equivalent Mg(1) atoms to form MgCa6Mg6 cuboctahedra that share corners with eighteen equivalent Ca(1)Ca3Mg9 cuboctahedra, edges with six equivalent Mg(2)Ca6Mg6 cuboctahedra, faces with two equivalent Mg(2)Ca6Mg6 cuboctahedra, and faces with six equivalent Ca(1)Ca3Mg9 cuboctahedra.
Mg2Ca crystallizes in the hexagonal P-62m space group. Ca(1) is bonded to three equivalent Ca(1), three equivalent Mg(2), and six equivalent Mg(1) atoms to form CaCa3Mg9 cuboctahedra that share corners with nine equivalent Ca(1)Ca3Mg9 cuboctahedra, corners with nine equivalent Mg(2)Ca6Mg6 cuboctahedra, edges with six equivalent Ca(1)Ca3Mg9 cuboctahedra, faces with three equivalent Mg(2)Ca6Mg6 cuboctahedra, and faces with five equivalent Ca(1)Ca3Mg9 cuboctahedra. All Ca(1)-Ca(1) bond lengths are 3.47 Å. All Ca(1)-Mg(2) bond lengths are 3.47 Å. All Ca(1)-Mg(1) bond lengths are 3.46 Å. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 8-coordinate geometry to four equivalent Ca(1), two equivalent Mg(1), and two equivalent Mg(2) atoms. Both Mg(1)-Mg(1) bond lengths are 3.21 Å. Both Mg(1)-Mg(2) bond lengths are 3.33 Å. In the second Mg site, Mg(2) is bonded to six equivalent Ca(1) and six equivalent Mg(1) atoms to form MgCa6Mg6 cuboctahedra that share corners with eighteen equivalent Ca(1)Ca3Mg9 cuboctahedra, edges with six equivalent Mg(2)Ca6Mg6 cuboctahedra, faces with two equivalent Mg(2)Ca6Mg6 cuboctahedra, and faces with six equivalent Ca(1)Ca3Mg9 cuboctahedra.