Mg3Ca crystallizes in the orthorhombic Imm2 space group. Ca(1) is bonded to two equivalent Ca(1), four equivalent Mg(1), and six equivalent Mg(2) atoms to form CaCa2Mg10 cuboctahedra that share corners with four equivalent Mg(1)Ca4Mg8 cuboctahedra, corners with fourteen equivalent Ca(1)Ca2Mg10 cuboctahedra, edges with two equivalent Ca(1)Ca2Mg10 cuboctahedra, edges with four equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with twelve equivalent Mg(2)Ca3Mg9 cuboctahedra, faces with two equivalent Ca(1)Ca2Mg10 cuboctahedra, faces with six equivalent Mg(1)Ca4Mg8 cuboctahedra, and faces with twelve equivalent Mg(2)Ca3Mg9 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 four equivalent Ca(1)Ca2Mg10 cuboctahedra, corners with fourteen equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with two equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with four equivalent Ca(1)Ca2Mg10 cuboctahedra, edges with twelve equivalent Mg(2)Ca3Mg9 cuboctahedra, faces with two equivalent Mg(1)Ca4Mg8 cuboctahedra, faces with six equivalent Ca(1)Ca2Mg10 cuboctahedra, and faces with twelve equivalent Mg(2)Ca3Mg9 cuboctahedra. In the second Mg site, Mg(2) is bonded to three equivalent Ca(1), three equivalent Mg(1), and six equivalent Mg(2) atoms to form distorted MgCa3Mg9 cuboctahedra that share corners with eighteen equivalent Mg(2)Ca3Mg9 cuboctahedra, edges with six equivalent Ca(1)Ca2Mg10 cuboctahedra, edges with six equivalent Mg(2)Ca3Mg9 cuboctahedra, edges with six equivalent Mg(1)Ca4Mg8 cuboctahedra, faces with six equivalent Ca(1)Ca2Mg10 cuboctahedra, faces with six equivalent Mg(1)Ca4Mg8 cuboctahedra, and faces with eight equivalent Mg(2)Ca3Mg9 cuboctahedra.
Mg3Ca crystallizes in the orthorhombic Imm2 space group. Ca(1) is bonded to two equivalent Ca(1), four equivalent Mg(1), and six equivalent Mg(2) atoms to form CaCa2Mg10 cuboctahedra that share corners with four equivalent Mg(1)Ca4Mg8 cuboctahedra, corners with fourteen equivalent Ca(1)Ca2Mg10 cuboctahedra, edges with two equivalent Ca(1)Ca2Mg10 cuboctahedra, edges with four equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with twelve equivalent Mg(2)Ca3Mg9 cuboctahedra, faces with two equivalent Ca(1)Ca2Mg10 cuboctahedra, faces with six equivalent Mg(1)Ca4Mg8 cuboctahedra, and faces with twelve equivalent Mg(2)Ca3Mg9 cuboctahedra. Both Ca(1)-Ca(1) bond lengths are 3.37 Å. There are two shorter (3.34 Å) and two longer (3.40 Å) Ca(1)-Mg(1) bond lengths. There are two shorter (3.34 Å) and four longer (3.41 Å) Ca(1)-Mg(2) 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 four equivalent Ca(1)Ca2Mg10 cuboctahedra, corners with fourteen equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with two equivalent Mg(1)Ca4Mg8 cuboctahedra, edges with four equivalent Ca(1)Ca2Mg10 cuboctahedra, edges with twelve equivalent Mg(2)Ca3Mg9 cuboctahedra, faces with two equivalent Mg(1)Ca4Mg8 cuboctahedra, faces with six equivalent Ca(1)Ca2Mg10 cuboctahedra, and faces with twelve equivalent Mg(2)Ca3Mg9 cuboctahedra. Both Mg(1)-Mg(1) bond lengths are 3.37 Å. There are four shorter (3.33 Å) and two longer (3.41 Å) Mg(1)-Mg(2) bond lengths. In the second Mg site, Mg(2) is bonded to three equivalent Ca(1), three equivalent Mg(1), and six equivalent Mg(2) atoms to form distorted MgCa3Mg9 cuboctahedra that share corners with eighteen equivalent Mg(2)Ca3Mg9 cuboctahedra, edges with six equivalent Ca(1)Ca2Mg10 cuboctahedra, edges with six equivalent Mg(2)Ca3Mg9 cuboctahedra, edges with six equivalent Mg(1)Ca4Mg8 cuboctahedra, faces with six equivalent Ca(1)Ca2Mg10 cuboctahedra, faces with six equivalent Mg(1)Ca4Mg8 cuboctahedra, and faces with eight equivalent Mg(2)Ca3Mg9 cuboctahedra. All Mg(2)-Mg(2) bond lengths are 3.37 Å.