Ca2Mg5(SiO3)8 crystallizes in the monoclinic C2/m space group. Ca(1) is bonded in a 6-coordinate geometry to two equivalent O(4), two equivalent O(5), and two equivalent O(6) atoms. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra and corners with two equivalent Si(2)O4 tetrahedra. In the second Mg site, Mg(2) is bonded in a distorted rectangular see-saw-like geometry to two equivalent O(3) and two equivalent O(4) atoms. In the third Mg site, Mg(3) is bonded in a 10-coordinate geometry to two equivalent O(2), four equivalent O(1), and four equivalent O(3) atoms. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(5), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 54°. In the second Si site, Si(2) is bonded to one O(2), one O(3), one O(5), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra,  a cornercorner with one Si(2)O4 tetrahedra, and corners with two equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 56°. There are seven inequivalent O sites. In the first O site, O(2) is bonded in a distorted linear geometry to one Mg(3) and two equivalent Si(2) atoms. In the second O site, O(3) is bonded in a 3-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), and one Si(2) atom. In the third O site, O(4) is bonded in a distorted T-shaped geometry to one Ca(1), one Mg(1), and one Mg(2) atom. In the fourth O site, O(5) is bonded in a 3-coordinate geometry to one Ca(1), one Si(1), and one Si(2) atom. In the fifth O site, O(6) is bonded in a distorted water-like geometry to one Ca(1), one Si(1), and one Si(2) atom. In the sixth O site, O(7) is bonded in a bent 120 degrees geometry to two equivalent Si(1) atoms. In the seventh O site, O(1) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(3), and one Si(1) atom.
Ca2Mg5(SiO3)8 crystallizes in the monoclinic C2/m space group. Ca(1) is bonded in a 6-coordinate geometry to two equivalent O(4), two equivalent O(5), and two equivalent O(6) atoms. Both Ca(1)-O(4) bond lengths are 2.16 Å. Both Ca(1)-O(5) bond lengths are 2.45 Å. Both Ca(1)-O(6) bond lengths are 2.72 Å. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra and corners with two equivalent Si(2)O4 tetrahedra. Both Mg(1)-O(1) bond lengths are 2.15 Å. Both Mg(1)-O(3) bond lengths are 2.04 Å. Both Mg(1)-O(4) bond lengths are 2.13 Å. In the second Mg site, Mg(2) is bonded in a distorted rectangular see-saw-like geometry to two equivalent O(3) and two equivalent O(4) atoms. Both Mg(2)-O(3) bond lengths are 1.99 Å. Both Mg(2)-O(4) bond lengths are 1.98 Å. In the third Mg site, Mg(3) is bonded in a 10-coordinate geometry to two equivalent O(2), four equivalent O(1), and four equivalent O(3) atoms. Both Mg(3)-O(2) bond lengths are 2.64 Å. All Mg(3)-O(1) bond lengths are 2.48 Å. All Mg(3)-O(3) bond lengths are 2.78 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(5), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 54°. The Si(1)-O(1) bond length is 1.60 Å. The Si(1)-O(5) bond length is 1.69 Å. The Si(1)-O(6) bond length is 1.69 Å. The Si(1)-O(7) bond length is 1.64 Å. In the second Si site, Si(2) is bonded to one O(2), one O(3), one O(5), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra,  a cornercorner with one Si(2)O4 tetrahedra, and corners with two equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 56°. The Si(2)-O(2) bond length is 1.67 Å. The Si(2)-O(3) bond length is 1.59 Å. The Si(2)-O(5) bond length is 1.68 Å. The Si(2)-O(6) bond length is 1.67 Å. There are seven inequivalent O sites. In the first O site, O(2) is bonded in a distorted linear geometry to one Mg(3) and two equivalent Si(2) atoms. In the second O site, O(3) is bonded in a 3-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), and one Si(2) atom. In the third O site, O(4) is bonded in a distorted T-shaped geometry to one Ca(1), one Mg(1), and one Mg(2) atom. In the fourth O site, O(5) is bonded in a 3-coordinate geometry to one Ca(1), one Si(1), and one Si(2) atom. In the fifth O site, O(6) is bonded in a distorted water-like geometry to one Ca(1), one Si(1), and one Si(2) atom. In the sixth O site, O(7) is bonded in a bent 120 degrees geometry to two equivalent Si(1) atoms. In the seventh O site, O(1) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(3), and one Si(1) atom.