Mg2Ca is beta-derived structured and crystallizes in the trigonal P-3m1 space group. Ca(1) is bonded to nine equivalent Ca(1) and three equivalent Mg(1) atoms to form distorted CaCa9Mg3 cuboctahedra that share corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra, corners with six equivalent Mg(1)Ca3Mg9 cuboctahedra, corners with six equivalent Mg(2)Mg12 cuboctahedra, edges with six equivalent Mg(1)Ca3Mg9 cuboctahedra, edges with twelve equivalent Ca(1)Ca9Mg3 cuboctahedra,  a faceface with one Mg(2)Mg12 cuboctahedra, faces with seven equivalent Mg(1)Ca3Mg9 cuboctahedra, and faces with twelve equivalent Ca(1)Ca9Mg3 cuboctahedra. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to three equivalent Ca(1), three equivalent Mg(2), and six equivalent Mg(1) atoms to form distorted MgCa3Mg9 cuboctahedra that share corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra; corners with six equivalent Mg(1)Ca3Mg9 cuboctahedra; corners with six equivalent Mg(2)Mg12 cuboctahedra; edges with six equivalent Ca(1)Ca9Mg3 cuboctahedra; edges with six equivalent Mg(1)Ca3Mg9 cuboctahedra; edges with six equivalent Mg(2)Mg12 cuboctahedra; faces with six equivalent Mg(1)Ca3Mg9 cuboctahedra; faces with seven equivalent Ca(1)Ca9Mg3 cuboctahedra; and faces with seven Mg(2,2)Mg12 cuboctahedra. In the second Mg site, Mg(2) is bonded to three equivalent Mg(1) and nine Mg(2,2) atoms to form MgMg12 cuboctahedra that share corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra; corners with six equivalent Mg(1)Ca3Mg9 cuboctahedra; corners with six equivalent Mg(2)Mg12 cuboctahedra; edges with six equivalent Mg(1)Ca3Mg9 cuboctahedra; edges with twelve Mg(2,2)Mg12 cuboctahedra;  a faceface with one Ca(1)Ca9Mg3 cuboctahedra; faces with seven equivalent Mg(1)Ca3Mg9 cuboctahedra; and faces with twelve Mg(2,2)Mg12 cuboctahedra. In the third Mg site, Mg(2) is bonded to three equivalent Mg(1) and nine Mg(2,2) atoms to form MgMg12 cuboctahedra that share corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra; corners with six equivalent Mg(1)Ca3Mg9 cuboctahedra; corners with six equivalent Mg(2)Mg12 cuboctahedra; edges with six equivalent Mg(1)Ca3Mg9 cuboctahedra; edges with twelve Mg(2,2)Mg12 cuboctahedra;  a faceface with one Ca(1)Ca9Mg3 cuboctahedra; faces with seven equivalent Mg(1)Ca3Mg9 cuboctahedra; and faces with twelve Mg(2,2)Mg12 cuboctahedra.
Mg2Ca is beta-derived structured and crystallizes in the trigonal P-3m1 space group. Ca(1) is bonded to nine equivalent Ca(1) and three equivalent Mg(1) atoms to form distorted CaCa9Mg3 cuboctahedra that share corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra, corners with six equivalent Mg(1)Ca3Mg9 cuboctahedra, corners with six equivalent Mg(2)Mg12 cuboctahedra, edges with six equivalent Mg(1)Ca3Mg9 cuboctahedra, edges with twelve equivalent Ca(1)Ca9Mg3 cuboctahedra,  a faceface with one Mg(2)Mg12 cuboctahedra, faces with seven equivalent Mg(1)Ca3Mg9 cuboctahedra, and faces with twelve equivalent Ca(1)Ca9Mg3 cuboctahedra. There are six shorter (3.45 Å) and three longer (3.79 Å) Ca(1)-Ca(1) bond lengths. All Ca(1)-Mg(1) bond lengths are 3.50 Å. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to three equivalent Ca(1), three equivalent Mg(2), and six equivalent Mg(1) atoms to form distorted MgCa3Mg9 cuboctahedra that share corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra; corners with six equivalent Mg(1)Ca3Mg9 cuboctahedra; corners with six equivalent Mg(2)Mg12 cuboctahedra; edges with six equivalent Ca(1)Ca9Mg3 cuboctahedra; edges with six equivalent Mg(1)Ca3Mg9 cuboctahedra; edges with six equivalent Mg(2)Mg12 cuboctahedra; faces with six equivalent Mg(1)Ca3Mg9 cuboctahedra; faces with seven equivalent Ca(1)Ca9Mg3 cuboctahedra; and faces with seven Mg(2,2)Mg12 cuboctahedra. All Mg(1)-Mg(2) bond lengths are 3.30 Å. All Mg(1)-Mg(1) bond lengths are 3.45 Å. In the second Mg site, Mg(2) is bonded to three equivalent Mg(1) and nine Mg(2,2) atoms to form MgMg12 cuboctahedra that share corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra; corners with six equivalent Mg(1)Ca3Mg9 cuboctahedra; corners with six equivalent Mg(2)Mg12 cuboctahedra; edges with six equivalent Mg(1)Ca3Mg9 cuboctahedra; edges with twelve Mg(2,2)Mg12 cuboctahedra;  a faceface with one Ca(1)Ca9Mg3 cuboctahedra; faces with seven equivalent Mg(1)Ca3Mg9 cuboctahedra; and faces with twelve Mg(2,2)Mg12 cuboctahedra. There are three shorter (3.30 Å) and six longer (3.45 Å) Mg(2)-Mg(2,2) bond lengths. In the third Mg site, Mg(2) is bonded to three equivalent Mg(1) and nine Mg(2,2) atoms to form MgMg12 cuboctahedra that share corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra; corners with six equivalent Mg(1)Ca3Mg9 cuboctahedra; corners with six equivalent Mg(2)Mg12 cuboctahedra; edges with six equivalent Mg(1)Ca3Mg9 cuboctahedra; edges with twelve Mg(2,2)Mg12 cuboctahedra;  a faceface with one Ca(1)Ca9Mg3 cuboctahedra; faces with seven equivalent Mg(1)Ca3Mg9 cuboctahedra; and faces with twelve Mg(2,2)Mg12 cuboctahedra. All Mg(2)-Mg(1) bond lengths are 3.30 Å. All Mg(2)-Mg(2) bond lengths are 3.45 Å.