Mg2Ca crystallizes in the orthorhombic Cmcm space group. Ca(1) is bonded to six equivalent Ca(1), two equivalent Mg(2), and four equivalent Mg(1) atoms to form distorted CaCa6Mg6 cuboctahedra that share corners with six equivalent Ca(1)Ca6Mg6 cuboctahedra, corners with six equivalent Mg(2)Ca2Mg10 cuboctahedra, corners with six equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with three equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with four equivalent Ca(1)Ca6Mg6 cuboctahedra, edges with eleven equivalent Mg(2)Ca2Mg10 cuboctahedra, faces with four equivalent Mg(2)Ca2Mg10 cuboctahedra, faces with eight equivalent Ca(1)Ca6Mg6 cuboctahedra, and faces with eight equivalent Mg(1)Ca4Mg8 cuboctahedra. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to four equivalent Ca(1), two equivalent Mg(1), and six equivalent Mg(2) atoms to form distorted MgCa4Mg8 cuboctahedra that share corners with six equivalent Ca(1)Ca6Mg6 cuboctahedra, corners with six equivalent Mg(2)Ca2Mg10 cuboctahedra, corners with six equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with three equivalent Ca(1)Ca6Mg6 cuboctahedra, edges with seven equivalent Mg(2)Ca2Mg10 cuboctahedra, edges with eight equivalent Mg(1)Ca4Mg8 cuboctahedra, faces with six equivalent Mg(2)Ca2Mg10 cuboctahedra, faces with six equivalent Mg(1)Ca4Mg8 cuboctahedra, and faces with eight equivalent Ca(1)Ca6Mg6 cuboctahedra. In the second Mg site, Mg(2) is bonded to two equivalent Ca(1), four equivalent Mg(2), and six equivalent Mg(1) atoms to form distorted MgCa2Mg10 cuboctahedra that share corners with six equivalent Ca(1)Ca6Mg6 cuboctahedra, corners with six equivalent Mg(2)Ca2Mg10 cuboctahedra, corners with six equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with seven equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with eleven equivalent Ca(1)Ca6Mg6 cuboctahedra, faces with four equivalent Ca(1)Ca6Mg6 cuboctahedra, faces with six equivalent Mg(1)Ca4Mg8 cuboctahedra, and faces with ten equivalent Mg(2)Ca2Mg10 cuboctahedra.
Mg2Ca crystallizes in the orthorhombic Cmcm space group. Ca(1) is bonded to six equivalent Ca(1), two equivalent Mg(2), and four equivalent Mg(1) atoms to form distorted CaCa6Mg6 cuboctahedra that share corners with six equivalent Ca(1)Ca6Mg6 cuboctahedra, corners with six equivalent Mg(2)Ca2Mg10 cuboctahedra, corners with six equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with three equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with four equivalent Ca(1)Ca6Mg6 cuboctahedra, edges with eleven equivalent Mg(2)Ca2Mg10 cuboctahedra, faces with four equivalent Mg(2)Ca2Mg10 cuboctahedra, faces with eight equivalent Ca(1)Ca6Mg6 cuboctahedra, and faces with eight equivalent Mg(1)Ca4Mg8 cuboctahedra. There are two shorter (3.44 Å) and four longer (3.54 Å) Ca(1)-Ca(1) bond lengths. Both Ca(1)-Mg(2) bond lengths are 3.45 Å. There are two shorter (3.42 Å) and two longer (3.68 Å) Ca(1)-Mg(1) bond lengths. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to four equivalent Ca(1), two equivalent Mg(1), and six equivalent Mg(2) atoms to form distorted MgCa4Mg8 cuboctahedra that share corners with six equivalent Ca(1)Ca6Mg6 cuboctahedra, corners with six equivalent Mg(2)Ca2Mg10 cuboctahedra, corners with six equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with three equivalent Ca(1)Ca6Mg6 cuboctahedra, edges with seven equivalent Mg(2)Ca2Mg10 cuboctahedra, edges with eight equivalent Mg(1)Ca4Mg8 cuboctahedra, faces with six equivalent Mg(2)Ca2Mg10 cuboctahedra, faces with six equivalent Mg(1)Ca4Mg8 cuboctahedra, and faces with eight equivalent Ca(1)Ca6Mg6 cuboctahedra. Both Mg(1)-Mg(1) bond lengths are 3.44 Å. There are two shorter (3.22 Å) and four longer (3.46 Å) Mg(1)-Mg(2) bond lengths. In the second Mg site, Mg(2) is bonded to two equivalent Ca(1), four equivalent Mg(2), and six equivalent Mg(1) atoms to form distorted MgCa2Mg10 cuboctahedra that share corners with six equivalent Ca(1)Ca6Mg6 cuboctahedra, corners with six equivalent Mg(2)Ca2Mg10 cuboctahedra, corners with six equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with seven equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with eleven equivalent Ca(1)Ca6Mg6 cuboctahedra, faces with four equivalent Ca(1)Ca6Mg6 cuboctahedra, faces with six equivalent Mg(1)Ca4Mg8 cuboctahedra, and faces with ten equivalent Mg(2)Ca2Mg10 cuboctahedra. There are two shorter (3.27 Å) and two longer (3.44 Å) Mg(2)-Mg(2) bond lengths.