BaCa2Si3O9 is Esseneite-like structured and crystallizes in the triclinic P-1 space group. Ba(1) is bonded in a 10-coordinate geometry to one O(2), one O(3), one O(4), one O(5), one O(7), one O(9), two equivalent O(1), and two equivalent O(8) atoms. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 8-coordinate geometry to one O(3), one O(4), one O(7), one O(9), two equivalent O(5), and two equivalent O(6) atoms. In the second Ca site, Ca(2) is bonded to one O(3), one O(6), one O(7), one O(8), and two equivalent O(2) atoms to form CaO6 octahedra that share  a cornercorner with one Si(2)O4 tetrahedra, corners with two equivalent Si(3)O4 tetrahedra, corners with three equivalent Si(1)O4 tetrahedra, and  an edgeedge with one Ca(2)O6 octahedra. There are three inequivalent Si sites. In the first Si site, 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 three equivalent Ca(2)O6 octahedra,  a cornercorner with one Si(2)O4 tetrahedra, and  a cornercorner with one Si(3)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 47-58°. In the second Si site, Si(2) is bonded to one O(4), one O(5), one O(6), and one O(9) atom to form SiO4 tetrahedra that share  a cornercorner with one Ca(2)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and  a cornercorner with one Si(3)O4 tetrahedra. The corner-sharing octahedral tilt angles are 56°. In the third Si site, Si(3) is bonded to one O(1), one O(7), one O(8), and one O(9) atom to form SiO4 tetrahedra that share corners with two equivalent Ca(2)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-59°. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a bent 120 degrees geometry to two equivalent Ba(1), one Si(1), and one Si(3) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Ba(1), two equivalent Ca(2), and one Si(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Ba(1), one Ca(1), one Ca(2), and one Si(1) atom. In the fourth O site, O(4) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Ca(1), one Si(1), and one Si(2) atom. In the fifth O site, O(5) is bonded in a 3-coordinate geometry to one Ba(1), two equivalent Ca(1), and one Si(2) atom. In the sixth O site, O(6) is bonded to one Ca(2), two equivalent Ca(1), and one Si(2) atom to form distorted edge-sharing OCa3Si tetrahedra. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Ba(1), one Ca(1), one Ca(2), and one Si(3) atom. In the eighth O site, O(8) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Ca(2), and one Si(3) atom. In the ninth O site, O(9) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Ca(1), one Si(2), and one Si(3) atom.
BaCa2Si3O9 is Esseneite-like structured and crystallizes in the triclinic P-1 space group. Ba(1) is bonded in a 10-coordinate geometry to one O(2), one O(3), one O(4), one O(5), one O(7), one O(9), two equivalent O(1), and two equivalent O(8) atoms. The Ba(1)-O(2) bond length is 3.13 Å. The Ba(1)-O(3) bond length is 2.76 Å. The Ba(1)-O(4) bond length is 3.44 Å. The Ba(1)-O(5) bond length is 2.59 Å. The Ba(1)-O(7) bond length is 2.87 Å. The Ba(1)-O(9) bond length is 3.34 Å. There is one shorter (2.98 Å) and one longer (3.11 Å) Ba(1)-O(1) bond length. There is one shorter (2.79 Å) and one longer (2.86 Å) Ba(1)-O(8) bond length. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 8-coordinate geometry to one O(3), one O(4), one O(7), one O(9), two equivalent O(5), and two equivalent O(6) atoms. The Ca(1)-O(3) bond length is 2.42 Å. The Ca(1)-O(4) bond length is 2.87 Å. The Ca(1)-O(7) bond length is 2.35 Å. The Ca(1)-O(9) bond length is 2.73 Å. There is one shorter (2.45 Å) and one longer (3.05 Å) Ca(1)-O(5) bond length. There is one shorter (2.42 Å) and one longer (2.44 Å) Ca(1)-O(6) bond length. In the second Ca site, Ca(2) is bonded to one O(3), one O(6), one O(7), one O(8), and two equivalent O(2) atoms to form CaO6 octahedra that share  a cornercorner with one Si(2)O4 tetrahedra, corners with two equivalent Si(3)O4 tetrahedra, corners with three equivalent Si(1)O4 tetrahedra, and  an edgeedge with one Ca(2)O6 octahedra. The Ca(2)-O(3) bond length is 2.33 Å. The Ca(2)-O(6) bond length is 2.52 Å. The Ca(2)-O(7) bond length is 2.41 Å. The Ca(2)-O(8) bond length is 2.34 Å. There is one shorter (2.36 Å) and one longer (2.46 Å) Ca(2)-O(2) bond length. There are three inequivalent Si sites. In the first Si site, 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 three equivalent Ca(2)O6 octahedra,  a cornercorner with one Si(2)O4 tetrahedra, and  a cornercorner with one Si(3)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 47-58°. The Si(1)-O(1) bond length is 1.70 Å. The Si(1)-O(2) bond length is 1.61 Å. The Si(1)-O(3) bond length is 1.62 Å. The Si(1)-O(4) bond length is 1.69 Å. In the second Si site, Si(2) is bonded to one O(4), one O(5), one O(6), and one O(9) atom to form SiO4 tetrahedra that share  a cornercorner with one Ca(2)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and  a cornercorner with one Si(3)O4 tetrahedra. The corner-sharing octahedral tilt angles are 56°. The Si(2)-O(4) bond length is 1.69 Å. The Si(2)-O(5) bond length is 1.60 Å. The Si(2)-O(6) bond length is 1.62 Å. The Si(2)-O(9) bond length is 1.71 Å. In the third Si site, Si(3) is bonded to one O(1), one O(7), one O(8), and one O(9) atom to form SiO4 tetrahedra that share corners with two equivalent Ca(2)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-59°. The Si(3)-O(1) bond length is 1.70 Å. The Si(3)-O(7) bond length is 1.62 Å. The Si(3)-O(8) bond length is 1.61 Å. The Si(3)-O(9) bond length is 1.68 Å. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a bent 120 degrees geometry to two equivalent Ba(1), one Si(1), and one Si(3) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Ba(1), two equivalent Ca(2), and one Si(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Ba(1), one Ca(1), one Ca(2), and one Si(1) atom. In the fourth O site, O(4) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Ca(1), one Si(1), and one Si(2) atom. In the fifth O site, O(5) is bonded in a 3-coordinate geometry to one Ba(1), two equivalent Ca(1), and one Si(2) atom. In the sixth O site, O(6) is bonded to one Ca(2), two equivalent Ca(1), and one Si(2) atom to form distorted edge-sharing OCa3Si tetrahedra. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Ba(1), one Ca(1), one Ca(2), and one Si(3) atom. In the eighth O site, O(8) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Ca(2), and one Si(3) atom. In the ninth O site, O(9) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Ca(1), one Si(2), and one Si(3) atom.