Be4Si2O9 crystallizes in the orthorhombic Cmc2_1 space group. There are two inequivalent Be sites. In the first Be site, Be(1) is bonded to one O(2), one O(4), and two equivalent O(6) atoms to form BeO4 tetrahedra that share  a cornercorner with one Be(2)O4 tetrahedra,  a cornercorner with one Si(1)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, and corners with three equivalent Be(1)O4 tetrahedra. In the second Be site, Be(2) is bonded to one O(3), one O(4), and two equivalent O(5) atoms to form BeO4 tetrahedra that share  a cornercorner with one Be(1)O4 tetrahedra,  a cornercorner with one Si(1)O4 tetrahedra, corners with three equivalent Be(2)O4 tetrahedra, and corners with three equivalent Si(2)O4 tetrahedra. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to two equivalent O(1) and two equivalent O(4) atoms to form SiO4 tetrahedra that share corners with two equivalent Be(1)O4 tetrahedra, corners with two equivalent Be(2)O4 tetrahedra, and corners with two equivalent Si(1)O4 tetrahedra. In the second Si site, Si(2) is bonded to one O(2), one O(3), and two equivalent O(5) atoms to form SiO4 tetrahedra that share corners with two equivalent Be(1)O4 tetrahedra and corners with six equivalent Be(2)O4 tetrahedra. There are six inequivalent O sites. In the first O site, O(5) is bonded in a trigonal non-coplanar geometry to two equivalent Be(2) and one Si(2) atom. In the second O site, O(6) is bonded in a bent 120 degrees geometry to two equivalent Be(1) atoms. In the third O site, O(1) is bonded in a bent 120 degrees geometry to two equivalent Si(1) atoms. In the fourth O site, O(2) is bonded in a trigonal planar geometry to two equivalent Be(1) and one Si(2) atom. In the fifth O site, O(3) is bonded in a trigonal non-coplanar geometry to two equivalent Be(2) and one Si(2) atom. In the sixth O site, O(4) is bonded in a trigonal planar geometry to one Be(1), one Be(2), and one Si(1) atom.
Be4Si2O9 crystallizes in the orthorhombic Cmc2_1 space group. There are two inequivalent Be sites. In the first Be site, Be(1) is bonded to one O(2), one O(4), and two equivalent O(6) atoms to form BeO4 tetrahedra that share  a cornercorner with one Be(2)O4 tetrahedra,  a cornercorner with one Si(1)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, and corners with three equivalent Be(1)O4 tetrahedra. The Be(1)-O(2) bond length is 1.67 Å. The Be(1)-O(4) bond length is 1.65 Å. There is one shorter (1.60 Å) and one longer (1.61 Å) Be(1)-O(6) bond length. In the second Be site, Be(2) is bonded to one O(3), one O(4), and two equivalent O(5) atoms to form BeO4 tetrahedra that share  a cornercorner with one Be(1)O4 tetrahedra,  a cornercorner with one Si(1)O4 tetrahedra, corners with three equivalent Be(2)O4 tetrahedra, and corners with three equivalent Si(2)O4 tetrahedra. The Be(2)-O(3) bond length is 1.71 Å. The Be(2)-O(4) bond length is 1.65 Å. There is one shorter (1.69 Å) and one longer (1.71 Å) Be(2)-O(5) bond length. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to two equivalent O(1) and two equivalent O(4) atoms to form SiO4 tetrahedra that share corners with two equivalent Be(1)O4 tetrahedra, corners with two equivalent Be(2)O4 tetrahedra, and corners with two equivalent Si(1)O4 tetrahedra. There is one shorter (1.62 Å) and one longer (1.63 Å) Si(1)-O(1) bond length. Both Si(1)-O(4) bond lengths are 1.65 Å. In the second Si site, Si(2) is bonded to one O(2), one O(3), and two equivalent O(5) atoms to form SiO4 tetrahedra that share corners with two equivalent Be(1)O4 tetrahedra and corners with six equivalent Be(2)O4 tetrahedra. The Si(2)-O(2) bond length is 1.64 Å. The Si(2)-O(3) bond length is 1.66 Å. Both Si(2)-O(5) bond lengths are 1.67 Å. There are six inequivalent O sites. In the first O site, O(5) is bonded in a trigonal non-coplanar geometry to two equivalent Be(2) and one Si(2) atom. In the second O site, O(6) is bonded in a bent 120 degrees geometry to two equivalent Be(1) atoms. In the third O site, O(1) is bonded in a bent 120 degrees geometry to two equivalent Si(1) atoms. In the fourth O site, O(2) is bonded in a trigonal planar geometry to two equivalent Be(1) and one Si(2) atom. In the fifth O site, O(3) is bonded in a trigonal non-coplanar geometry to two equivalent Be(2) and one Si(2) atom. In the sixth O site, O(4) is bonded in a trigonal planar geometry to one Be(1), one Be(2), and one Si(1) atom.