MgCa crystallizes in the monoclinic Cm space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to two equivalent Ca(3), three equivalent Ca(2), one Mg(3), two equivalent Mg(1), and two equivalent Mg(2) atoms to form distorted CaCa5Mg5 cuboctahedra that share corners with four equivalent Ca(1)Ca5Mg5 cuboctahedra, corners with eight equivalent Ca(3)Ca5Mg7 cuboctahedra,  an edgeedge with one Ca(3)Ca5Mg7 cuboctahedra, faces with two equivalent Ca(1)Ca5Mg5 cuboctahedra, and faces with three equivalent Ca(3)Ca5Mg7 cuboctahedra. In the second Ca site, Ca(2) is bonded in a 12-coordinate geometry to one Ca(3), two equivalent Ca(2), three equivalent Ca(1), two equivalent Mg(1), two equivalent Mg(2), and two equivalent Mg(3) atoms. In the third Ca site, Ca(3) is bonded to one Ca(2), two equivalent Ca(1), two equivalent Ca(3), two equivalent Mg(2), two equivalent Mg(3), and three equivalent Mg(1) atoms to form distorted CaCa5Mg7 cuboctahedra that share corners with six equivalent Ca(3)Ca5Mg7 cuboctahedra, corners with eight equivalent Ca(1)Ca5Mg5 cuboctahedra,  an edgeedge with one Ca(1)Ca5Mg5 cuboctahedra, faces with two equivalent Ca(3)Ca5Mg7 cuboctahedra, and faces with three equivalent Ca(1)Ca5Mg5 cuboctahedra. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 10-coordinate geometry to two equivalent Ca(1), two equivalent Ca(2), three equivalent Ca(3), one Mg(2), and two equivalent Mg(3) atoms. In the second Mg site, Mg(2) is bonded in a 9-coordinate geometry to two equivalent Ca(1), two equivalent Ca(2), two equivalent Ca(3), one Mg(1), and two equivalent Mg(3) atoms. In the third Mg site, Mg(3) is bonded in a 9-coordinate geometry to one Ca(1), two equivalent Ca(2), two equivalent Ca(3), two equivalent Mg(1), and two equivalent Mg(2) atoms.
MgCa crystallizes in the monoclinic Cm space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to two equivalent Ca(3), three equivalent Ca(2), one Mg(3), two equivalent Mg(1), and two equivalent Mg(2) atoms to form distorted CaCa5Mg5 cuboctahedra that share corners with four equivalent Ca(1)Ca5Mg5 cuboctahedra, corners with eight equivalent Ca(3)Ca5Mg7 cuboctahedra,  an edgeedge with one Ca(3)Ca5Mg7 cuboctahedra, faces with two equivalent Ca(1)Ca5Mg5 cuboctahedra, and faces with three equivalent Ca(3)Ca5Mg7 cuboctahedra. Both Ca(1)-Ca(3) bond lengths are 3.81 Å. There is one shorter (3.58 Å) and two longer (3.85 Å) Ca(1)-Ca(2) bond lengths. The Ca(1)-Mg(3) bond length is 3.46 Å. Both Ca(1)-Mg(1) bond lengths are 3.48 Å. Both Ca(1)-Mg(2) bond lengths are 3.44 Å. In the second Ca site, Ca(2) is bonded in a 12-coordinate geometry to one Ca(3), two equivalent Ca(2), three equivalent Ca(1), two equivalent Mg(1), two equivalent Mg(2), and two equivalent Mg(3) atoms. The Ca(2)-Ca(3) bond length is 3.82 Å. Both Ca(2)-Ca(2) bond lengths are 3.58 Å. Both Ca(2)-Mg(1) bond lengths are 3.71 Å. Both Ca(2)-Mg(2) bond lengths are 3.49 Å. Both Ca(2)-Mg(3) bond lengths are 3.59 Å. In the third Ca site, Ca(3) is bonded to one Ca(2), two equivalent Ca(1), two equivalent Ca(3), two equivalent Mg(2), two equivalent Mg(3), and three equivalent Mg(1) atoms to form distorted CaCa5Mg7 cuboctahedra that share corners with six equivalent Ca(3)Ca5Mg7 cuboctahedra, corners with eight equivalent Ca(1)Ca5Mg5 cuboctahedra,  an edgeedge with one Ca(1)Ca5Mg5 cuboctahedra, faces with two equivalent Ca(3)Ca5Mg7 cuboctahedra, and faces with three equivalent Ca(1)Ca5Mg5 cuboctahedra. Both Ca(3)-Ca(3) bond lengths are 3.58 Å. Both Ca(3)-Mg(2) bond lengths are 3.40 Å. Both Ca(3)-Mg(3) bond lengths are 3.49 Å. There are two shorter (3.61 Å) and one longer (3.84 Å) Ca(3)-Mg(1) bond length. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 10-coordinate geometry to two equivalent Ca(1), two equivalent Ca(2), three equivalent Ca(3), one Mg(2), and two equivalent Mg(3) atoms. The Mg(1)-Mg(2) bond length is 3.21 Å. Both Mg(1)-Mg(3) bond lengths are 3.34 Å. In the second Mg site, Mg(2) is bonded in a 9-coordinate geometry to two equivalent Ca(1), two equivalent Ca(2), two equivalent Ca(3), one Mg(1), and two equivalent Mg(3) atoms. Both Mg(2)-Mg(3) bond lengths are 3.31 Å. In the third Mg site, Mg(3) is bonded in a 9-coordinate geometry to one Ca(1), two equivalent Ca(2), two equivalent Ca(3), two equivalent Mg(1), and two equivalent Mg(2) atoms.