Ca2Al2SiO7 crystallizes in the monoclinic P2_1 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 8-coordinate geometry to one O(1), one O(4), one O(5), one O(7), two equivalent O(3), and two equivalent O(6) atoms. In the second Ca site, Ca(2) is bonded in a 8-coordinate geometry to one O(1), one O(4), one O(5), one O(6), two equivalent O(2), and two equivalent O(7) atoms. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(2), one O(3), one O(4), and one O(5) atom to form AlO4 tetrahedra that share corners with two equivalent Al(2)O4 tetrahedra and corners with two equivalent Si(1)O4 tetrahedra. In the second Al site, Al(2) is bonded to one O(1), one O(3), one O(4), and one O(6) atom to form AlO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra and corners with two equivalent Al(1)O4 tetrahedra. Si(1) is bonded to one O(1), one O(2), one O(5), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(2)O4 tetrahedra and corners with two equivalent Al(1)O4 tetrahedra. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Ca(1), one Ca(2), one Al(2), and one Si(1) atom. In the second O site, O(2) is bonded in a 2-coordinate geometry to two equivalent Ca(2), one Al(1), and one Si(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Ca(1), one Al(1), and one Al(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Ca(1), one Ca(2), one Al(1), and one Al(2) atom. In the fifth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one Ca(1), one Ca(2), one Al(1), and one Si(1) atom. In the sixth O site, O(6) is bonded to one Ca(2), two equivalent Ca(1), and one Al(2) atom to form distorted corner-sharing OCa3Al tetrahedra. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Ca(1), two equivalent Ca(2), and one Si(1) atom.
Ca2Al2SiO7 crystallizes in the monoclinic P2_1 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 8-coordinate geometry to one O(1), one O(4), one O(5), one O(7), two equivalent O(3), and two equivalent O(6) atoms. The Ca(1)-O(1) bond length is 2.50 Å. The Ca(1)-O(4) bond length is 2.39 Å. The Ca(1)-O(5) bond length is 3.03 Å. The Ca(1)-O(7) bond length is 2.74 Å. There is one shorter (2.45 Å) and one longer (2.81 Å) Ca(1)-O(3) bond length. There is one shorter (2.41 Å) and one longer (2.52 Å) Ca(1)-O(6) bond length. In the second Ca site, Ca(2) is bonded in a 8-coordinate geometry to one O(1), one O(4), one O(5), one O(6), two equivalent O(2), and two equivalent O(7) atoms. The Ca(2)-O(1) bond length is 2.44 Å. The Ca(2)-O(4) bond length is 2.72 Å. The Ca(2)-O(5) bond length is 2.51 Å. The Ca(2)-O(6) bond length is 2.40 Å. There is one shorter (2.43 Å) and one longer (2.95 Å) Ca(2)-O(2) bond length. There is one shorter (2.53 Å) and one longer (2.63 Å) Ca(2)-O(7) bond length. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(2), one O(3), one O(4), and one O(5) atom to form AlO4 tetrahedra that share corners with two equivalent Al(2)O4 tetrahedra and corners with two equivalent Si(1)O4 tetrahedra. The Al(1)-O(2) bond length is 1.79 Å. The Al(1)-O(3) bond length is 1.76 Å. The Al(1)-O(4) bond length is 1.76 Å. The Al(1)-O(5) bond length is 1.78 Å. In the second Al site, Al(2) is bonded to one O(1), one O(3), one O(4), and one O(6) atom to form AlO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra and corners with two equivalent Al(1)O4 tetrahedra. The Al(2)-O(1) bond length is 1.79 Å. The Al(2)-O(3) bond length is 1.77 Å. The Al(2)-O(4) bond length is 1.78 Å. The Al(2)-O(6) bond length is 1.73 Å. Si(1) is bonded to one O(1), one O(2), one O(5), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(2)O4 tetrahedra and corners with two equivalent Al(1)O4 tetrahedra. The Si(1)-O(1) bond length is 1.65 Å. The Si(1)-O(2) bond length is 1.66 Å. The Si(1)-O(5) bond length is 1.66 Å. The Si(1)-O(7) bond length is 1.62 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Ca(1), one Ca(2), one Al(2), and one Si(1) atom. In the second O site, O(2) is bonded in a 2-coordinate geometry to two equivalent Ca(2), one Al(1), and one Si(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Ca(1), one Al(1), and one Al(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Ca(1), one Ca(2), one Al(1), and one Al(2) atom. In the fifth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one Ca(1), one Ca(2), one Al(1), and one Si(1) atom. In the sixth O site, O(6) is bonded to one Ca(2), two equivalent Ca(1), and one Al(2) atom to form distorted corner-sharing OCa3Al tetrahedra. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Ca(1), two equivalent Ca(2), and one Si(1) atom.