Ca3SiO4Cl2 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to one O(1), one O(2), one O(3), one O(4), one Cl(1), and two equivalent Cl(2) atoms to form distorted CaCl3O4 pentagonal bipyramids that share corners with two equivalent Ca(1)Cl3O4 pentagonal bipyramids, corners with two equivalent Si(1)O4 tetrahedra, and  an edgeedge with one Si(1)O4 tetrahedra. In the second Ca site, Ca(2) is bonded in a 7-coordinate geometry to one O(1), one O(2), two equivalent O(3), two equivalent O(4), and one Cl(2) atom. In the third Ca site, Ca(3) is bonded in a 6-coordinate geometry to one O(1), one O(2), one Cl(2), and three equivalent Cl(1) atoms. Si(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SiO4 tetrahedra that share corners with two equivalent Ca(1)Cl3O4 pentagonal bipyramids and  an edgeedge with one Ca(1)Cl3O4 pentagonal bipyramid. There are four inequivalent O sites. In the first O site, O(3) is bonded to one Ca(1), two equivalent Ca(2), and one Si(1) atom to form distorted OCa3Si tetrahedra that share corners with two equivalent O(3)Ca3Si tetrahedra and  a cornercorner with one Cl(1)Ca4 trigonal pyramid. In the second O site, O(4) is bonded in a distorted see-saw-like geometry to one Ca(1), two equivalent Ca(2), and one Si(1) atom. In the third O site, O(1) is bonded in a 4-coordinate geometry to one Ca(1), one Ca(2), one Ca(3), and one Si(1) atom. In the fourth O site, O(2) is bonded in a 4-coordinate geometry to one Ca(1), one Ca(2), one Ca(3), and one Si(1) atom. There are two inequivalent Cl sites. In the first Cl site, Cl(1) is bonded to one Ca(1) and three equivalent Ca(3) atoms to form distorted ClCa4 trigonal pyramids that share  a cornercorner with one O(3)Ca3Si tetrahedra, corners with four equivalent Cl(1)Ca4 trigonal pyramids, and  an edgeedge with one Cl(1)Ca4 trigonal pyramid. In the second Cl site, Cl(2) is bonded in a 4-coordinate geometry to one Ca(2), one Ca(3), and two equivalent Ca(1) atoms.
Ca3SiO4Cl2 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to one O(1), one O(2), one O(3), one O(4), one Cl(1), and two equivalent Cl(2) atoms to form distorted CaCl3O4 pentagonal bipyramids that share corners with two equivalent Ca(1)Cl3O4 pentagonal bipyramids, corners with two equivalent Si(1)O4 tetrahedra, and  an edgeedge with one Si(1)O4 tetrahedra. The Ca(1)-O(1) bond length is 2.30 Å. The Ca(1)-O(2) bond length is 2.30 Å. The Ca(1)-O(3) bond length is 2.42 Å. The Ca(1)-O(4) bond length is 2.40 Å. The Ca(1)-Cl(1) bond length is 2.99 Å. There is one shorter (2.88 Å) and one longer (2.92 Å) Ca(1)-Cl(2) bond length. In the second Ca site, Ca(2) is bonded in a 7-coordinate geometry to one O(1), one O(2), two equivalent O(3), two equivalent O(4), and one Cl(2) atom. The Ca(2)-O(1) bond length is 2.71 Å. The Ca(2)-O(2) bond length is 2.57 Å. There is one shorter (2.35 Å) and one longer (2.39 Å) Ca(2)-O(3) bond length. There is one shorter (2.36 Å) and one longer (2.43 Å) Ca(2)-O(4) bond length. The Ca(2)-Cl(2) bond length is 3.14 Å. In the third Ca site, Ca(3) is bonded in a 6-coordinate geometry to one O(1), one O(2), one Cl(2), and three equivalent Cl(1) atoms. The Ca(3)-O(1) bond length is 2.33 Å. The Ca(3)-O(2) bond length is 2.29 Å. The Ca(3)-Cl(2) bond length is 2.85 Å. There are a spread of Ca(3)-Cl(1) bond distances ranging from 2.83-2.96 Å. Si(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SiO4 tetrahedra that share corners with two equivalent Ca(1)Cl3O4 pentagonal bipyramids and  an edgeedge with one Ca(1)Cl3O4 pentagonal bipyramid. The Si(1)-O(1) bond length is 1.66 Å. The Si(1)-O(2) bond length is 1.66 Å. The Si(1)-O(3) bond length is 1.65 Å. The Si(1)-O(4) bond length is 1.65 Å. There are four inequivalent O sites. In the first O site, O(3) is bonded to one Ca(1), two equivalent Ca(2), and one Si(1) atom to form distorted OCa3Si tetrahedra that share corners with two equivalent O(3)Ca3Si tetrahedra and  a cornercorner with one Cl(1)Ca4 trigonal pyramid. In the second O site, O(4) is bonded in a distorted see-saw-like geometry to one Ca(1), two equivalent Ca(2), and one Si(1) atom. In the third O site, O(1) is bonded in a 4-coordinate geometry to one Ca(1), one Ca(2), one Ca(3), and one Si(1) atom. In the fourth O site, O(2) is bonded in a 4-coordinate geometry to one Ca(1), one Ca(2), one Ca(3), and one Si(1) atom. There are two inequivalent Cl sites. In the first Cl site, Cl(1) is bonded to one Ca(1) and three equivalent Ca(3) atoms to form distorted ClCa4 trigonal pyramids that share  a cornercorner with one O(3)Ca3Si tetrahedra, corners with four equivalent Cl(1)Ca4 trigonal pyramids, and  an edgeedge with one Cl(1)Ca4 trigonal pyramid. In the second Cl site, Cl(2) is bonded in a 4-coordinate geometry to one Ca(2), one Ca(3), and two equivalent Ca(1) atoms.