K2Ca4U(SiO4)4 is Esseneite-derived structured and crystallizes in the triclinic P-1 space group. K(1) is bonded in a 6-coordinate geometry to one O(4), one O(8), two equivalent O(3), and two equivalent O(6) atoms. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 8-coordinate geometry to one O(4), one O(5), one O(7), one O(8), two equivalent O(1), and two equivalent O(2) atoms. In the second Ca site, Ca(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), two equivalent O(5), and two equivalent O(7) atoms. U(1) is bonded to two equivalent O(3), two equivalent O(6), and two equivalent O(8) atoms to form UO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra and corners with two equivalent Si(2)O4 tetrahedra. There are two 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  a cornercorner with one U(1)O6 octahedra and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 38°. In the second Si site, Si(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one U(1)O6 octahedra and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 44°. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Ca(2), two equivalent Ca(1), and one Si(1) atom to form a mixture of distorted corner and edge-sharing OCa3Si tetrahedra. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Ca(2), two equivalent Ca(1), and one Si(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent K(1), one U(1), and one Si(1) atom. In the fourth O site, O(4) is bonded in a distorted bent 150 degrees geometry to one K(1), one Ca(1), one Si(1), and one Si(2) atom. In the fifth O site, O(5) is bonded to one Ca(1), two equivalent Ca(2), and one Si(2) atom to form a mixture of distorted corner and edge-sharing OCa3Si trigonal pyramids. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to two equivalent K(1), one U(1), and one Si(2) atom. In the seventh O site, O(7) is bonded to one Ca(1), two equivalent Ca(2), and one Si(2) atom to form a mixture of distorted corner and edge-sharing OCa3Si trigonal pyramids. In the eighth O site, O(8) is bonded in a 3-coordinate geometry to one K(1), one Ca(1), and one U(1) atom.
K2Ca4U(SiO4)4 is Esseneite-derived structured and crystallizes in the triclinic P-1 space group. K(1) is bonded in a 6-coordinate geometry to one O(4), one O(8), two equivalent O(3), and two equivalent O(6) atoms. The K(1)-O(4) bond length is 2.80 Å. The K(1)-O(8) bond length is 2.83 Å. There is one shorter (2.83 Å) and one longer (3.01 Å) K(1)-O(3) bond length. There is one shorter (2.80 Å) and one longer (3.15 Å) K(1)-O(6) 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(4), one O(5), one O(7), one O(8), two equivalent O(1), and two equivalent O(2) atoms. The Ca(1)-O(4) bond length is 2.96 Å. The Ca(1)-O(5) bond length is 2.56 Å. The Ca(1)-O(7) bond length is 2.50 Å. The Ca(1)-O(8) bond length is 2.44 Å. There is one shorter (2.49 Å) and one longer (2.59 Å) Ca(1)-O(1) bond length. There is one shorter (2.41 Å) and one longer (2.60 Å) Ca(1)-O(2) bond length. In the second Ca site, Ca(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), two equivalent O(5), and two equivalent O(7) atoms. The Ca(2)-O(1) bond length is 2.34 Å. The Ca(2)-O(2) bond length is 2.40 Å. There is one shorter (2.32 Å) and one longer (2.47 Å) Ca(2)-O(5) bond length. There is one shorter (2.42 Å) and one longer (2.43 Å) Ca(2)-O(7) bond length. U(1) is bonded to two equivalent O(3), two equivalent O(6), and two equivalent O(8) atoms to form UO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra and corners with two equivalent Si(2)O4 tetrahedra. Both U(1)-O(3) bond lengths are 2.20 Å. Both U(1)-O(6) bond lengths are 2.22 Å. Both U(1)-O(8) bond lengths are 1.89 Å. There are two 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  a cornercorner with one U(1)O6 octahedra and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 38°. The Si(1)-O(1) bond length is 1.63 Å. The Si(1)-O(2) bond length is 1.63 Å. The Si(1)-O(3) bond length is 1.65 Å. The Si(1)-O(4) bond length is 1.66 Å. In the second Si site, Si(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one U(1)O6 octahedra and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 44°. The Si(2)-O(4) bond length is 1.66 Å. The Si(2)-O(5) bond length is 1.63 Å. The Si(2)-O(6) bond length is 1.64 Å. The Si(2)-O(7) bond length is 1.63 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Ca(2), two equivalent Ca(1), and one Si(1) atom to form a mixture of distorted corner and edge-sharing OCa3Si tetrahedra. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Ca(2), two equivalent Ca(1), and one Si(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent K(1), one U(1), and one Si(1) atom. In the fourth O site, O(4) is bonded in a distorted bent 150 degrees geometry to one K(1), one Ca(1), one Si(1), and one Si(2) atom. In the fifth O site, O(5) is bonded to one Ca(1), two equivalent Ca(2), and one Si(2) atom to form a mixture of distorted corner and edge-sharing OCa3Si trigonal pyramids. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to two equivalent K(1), one U(1), and one Si(2) atom. In the seventh O site, O(7) is bonded to one Ca(1), two equivalent Ca(2), and one Si(2) atom to form a mixture of distorted corner and edge-sharing OCa3Si trigonal pyramids. In the eighth O site, O(8) is bonded in a 3-coordinate geometry to one K(1), one Ca(1), and one U(1) atom.