CaSe2O7 crystallizes in the triclinic P-1 space group. Ca(1) is bonded to one O(3), one O(4), one O(5), one O(6), and two equivalent O(2) atoms to form distorted CaO6 octahedra that share corners with three equivalent Se(2)O4 tetrahedra and  an edgeedge with one Ca(1)O6 octahedra. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 4-coordinate geometry to one O(1), one O(2), one O(3), and one O(7) atom. In the second Se site, Se(2) is bonded to one O(1), one O(4), one O(5), and one O(6) atom to form SeO4 tetrahedra that share corners with three equivalent Ca(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 35-55°. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Se(1) and one Se(2) atom. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Ca(1) and one Se(1) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Ca(1) and one Se(1) atom. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to one Ca(1) and one Se(2) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one Ca(1) and one Se(2) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one Ca(1) and one Se(2) atom. In the seventh O site, O(7) is bonded in a single-bond geometry to one Se(1) atom.
CaSe2O7 crystallizes in the triclinic P-1 space group. Ca(1) is bonded to one O(3), one O(4), one O(5), one O(6), and two equivalent O(2) atoms to form distorted CaO6 octahedra that share corners with three equivalent Se(2)O4 tetrahedra and  an edgeedge with one Ca(1)O6 octahedra. The Ca(1)-O(3) bond length is 2.38 Å. The Ca(1)-O(4) bond length is 2.33 Å. The Ca(1)-O(5) bond length is 2.36 Å. The Ca(1)-O(6) bond length is 2.37 Å. There is one shorter (2.42 Å) and one longer (2.49 Å) Ca(1)-O(2) bond length. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 4-coordinate geometry to one O(1), one O(2), one O(3), and one O(7) atom. The Se(1)-O(1) bond length is 2.43 Å. The Se(1)-O(2) bond length is 1.70 Å. The Se(1)-O(3) bond length is 1.69 Å. The Se(1)-O(7) bond length is 1.76 Å. In the second Se site, Se(2) is bonded to one O(1), one O(4), one O(5), and one O(6) atom to form SeO4 tetrahedra that share corners with three equivalent Ca(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 35-55°. The Se(2)-O(1) bond length is 1.67 Å. The Se(2)-O(4) bond length is 1.67 Å. The Se(2)-O(5) bond length is 1.66 Å. The Se(2)-O(6) bond length is 1.71 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Se(1) and one Se(2) atom. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Ca(1) and one Se(1) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Ca(1) and one Se(1) atom. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to one Ca(1) and one Se(2) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one Ca(1) and one Se(2) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one Ca(1) and one Se(2) atom. In the seventh O site, O(7) is bonded in a single-bond geometry to one Se(1) atom.