MgCa3 crystallizes in the orthorhombic Imm2 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to two equivalent Ca(1), six equivalent Ca(2), and four equivalent Mg(1) atoms to form a mixture of distorted corner, face, and edge-sharing CaCa8Mg4 cuboctahedra. In the second Ca site, Ca(2) is bonded in a 2-coordinate geometry to three equivalent Ca(1) and two equivalent Mg(1) atoms. Mg(1) is bonded in a 8-coordinate geometry to four equivalent Ca(1) and four equivalent Ca(2) atoms.
MgCa3 crystallizes in the orthorhombic Imm2 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to two equivalent Ca(1), six equivalent Ca(2), and four equivalent Mg(1) atoms to form a mixture of distorted corner, face, and edge-sharing CaCa8Mg4 cuboctahedra. Both Ca(1)-Ca(1) bond lengths are 3.75 Å. There are two shorter (3.69 Å) and four longer (3.87 Å) Ca(1)-Ca(2) bond lengths. There are two shorter (3.63 Å) and two longer (3.80 Å) Ca(1)-Mg(1) bond lengths. In the second Ca site, Ca(2) is bonded in a 2-coordinate geometry to three equivalent Ca(1) and two equivalent Mg(1) atoms. Both Ca(2)-Mg(1) bond lengths are 3.59 Å. Mg(1) is bonded in a 8-coordinate geometry to four equivalent Ca(1) and four equivalent Ca(2) atoms.