MgCa crystallizes in the trigonal P3m1 space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to three equivalent Ca(2), six equivalent Ca(1), and three equivalent Mg(1) atoms to form distorted CaCa9Mg3 cuboctahedra that share corners with six equivalent Ca(3)Ca6Mg6 cuboctahedra, corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra, edges with six equivalent Ca(1)Ca9Mg3 cuboctahedra, edges with six equivalent Ca(2)Ca9Mg3 cuboctahedra,  a faceface with one Ca(3)Ca6Mg6 cuboctahedra, faces with six equivalent Ca(1)Ca9Mg3 cuboctahedra, and faces with six equivalent Ca(2)Ca9Mg3 cuboctahedra. In the second Ca site, Ca(2) is bonded to three equivalent Ca(1), six equivalent Ca(2), and three equivalent Mg(2) atoms to form a mixture of distorted edge, face, and corner-sharing CaCa9Mg3 cuboctahedra. In the third Ca site, Ca(3) is bonded to six equivalent Ca(3), three equivalent Mg(1), and three equivalent Mg(3) atoms to form distorted CaCa6Mg6 cuboctahedra that share corners with six equivalent Ca(3)Ca6Mg6 cuboctahedra, corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra, edges with six equivalent Ca(3)Ca6Mg6 cuboctahedra,  a faceface with one Ca(1)Ca9Mg3 cuboctahedra, and faces with six equivalent Ca(3)Ca6Mg6 cuboctahedra. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 6-coordinate geometry to three equivalent Ca(1) and three equivalent Ca(3) atoms. In the second Mg site, Mg(2) is bonded in a 6-coordinate geometry to three equivalent Ca(2) and three equivalent Mg(3) atoms. In the third Mg site, Mg(3) is bonded in a 6-coordinate geometry to three equivalent Ca(3) and three equivalent Mg(2) atoms.
MgCa crystallizes in the trigonal P3m1 space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to three equivalent Ca(2), six equivalent Ca(1), and three equivalent Mg(1) atoms to form distorted CaCa9Mg3 cuboctahedra that share corners with six equivalent Ca(3)Ca6Mg6 cuboctahedra, corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra, edges with six equivalent Ca(1)Ca9Mg3 cuboctahedra, edges with six equivalent Ca(2)Ca9Mg3 cuboctahedra,  a faceface with one Ca(3)Ca6Mg6 cuboctahedra, faces with six equivalent Ca(1)Ca9Mg3 cuboctahedra, and faces with six equivalent Ca(2)Ca9Mg3 cuboctahedra. All Ca(1)-Ca(2) bond lengths are 3.85 Å. All Ca(1)-Ca(1) bond lengths are 3.64 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), six equivalent Ca(2), and three equivalent Mg(2) atoms to form a mixture of distorted edge, face, and corner-sharing CaCa9Mg3 cuboctahedra. All Ca(2)-Ca(2) bond lengths are 3.64 Å. All Ca(2)-Mg(2) bond lengths are 3.52 Å. In the third Ca site, Ca(3) is bonded to six equivalent Ca(3), three equivalent Mg(1), and three equivalent Mg(3) atoms to form distorted CaCa6Mg6 cuboctahedra that share corners with six equivalent Ca(3)Ca6Mg6 cuboctahedra, corners with six equivalent Ca(1)Ca9Mg3 cuboctahedra, edges with six equivalent Ca(3)Ca6Mg6 cuboctahedra,  a faceface with one Ca(1)Ca9Mg3 cuboctahedra, and faces with six equivalent Ca(3)Ca6Mg6 cuboctahedra. All Ca(3)-Ca(3) bond lengths are 3.64 Å. All Ca(3)-Mg(1) bond lengths are 3.51 Å. All Ca(3)-Mg(3) bond lengths are 3.54 Å. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 6-coordinate geometry to three equivalent Ca(1) and three equivalent Ca(3) atoms. In the second Mg site, Mg(2) is bonded in a 6-coordinate geometry to three equivalent Ca(2) and three equivalent Mg(3) atoms. All Mg(2)-Mg(3) bond lengths are 3.30 Å. In the third Mg site, Mg(3) is bonded in a 6-coordinate geometry to three equivalent Ca(3) and three equivalent Mg(2) atoms.