Ca4Fe9O17 crystallizes in the monoclinic C2 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 7-coordinate geometry to one O(2), one O(3), one O(4), one O(5), one O(6), one O(7), and one O(9) atom. In the second Ca site, Ca(2) is bonded in a 7-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(6), one O(7), and one O(9) atom. There are six inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(3), one O(4), one O(5), and two equivalent O(1) atoms to form FeO5 trigonal bipyramids that share corners with two equivalent Fe(3)O4 tetrahedra, corners with three equivalent Fe(1)O5 trigonal bipyramids, and corners with three equivalent Fe(2)O5 trigonal bipyramids. In the second Fe site, Fe(2) is bonded to one O(5), two equivalent O(1), and two equivalent O(2) atoms to form FeO5 trigonal bipyramids that share corners with two equivalent Fe(3)O4 tetrahedra and corners with six equivalent Fe(1)O5 trigonal bipyramids. In the third Fe site, Fe(3) is bonded to one O(2), one O(3), one O(4), and one O(8) atom to form FeO4 tetrahedra that share  a cornercorner with one Fe(4)O6 octahedra,  a cornercorner with one Fe(5)O6 octahedra,  a cornercorner with one Fe(6)O6 octahedra,  a cornercorner with one Fe(2)O5 trigonal bipyramid, and corners with two equivalent Fe(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 56-60°. In the fourth Fe site, Fe(4) is bonded to one O(8), one O(9), two equivalent O(6), and two equivalent O(7) atoms to form FeO6 octahedra that share  a cornercorner with one Fe(3)O4 tetrahedra, edges with two equivalent Fe(4)O6 octahedra, edges with two equivalent Fe(5)O6 octahedra, and edges with two equivalent Fe(6)O6 octahedra. In the fifth Fe site, Fe(5) is bonded to two equivalent O(7), two equivalent O(8), and two equivalent O(9) atoms to form FeO6 octahedra that share corners with two equivalent Fe(3)O4 tetrahedra, edges with two equivalent Fe(6)O6 octahedra, and edges with four equivalent Fe(4)O6 octahedra. In the sixth Fe site, Fe(6) is bonded to two equivalent O(6), two equivalent O(8), and two equivalent O(9) atoms to form FeO6 octahedra that share corners with two equivalent Fe(3)O4 tetrahedra, edges with two equivalent Fe(5)O6 octahedra, and edges with four equivalent Fe(4)O6 octahedra. There are nine inequivalent O sites. In the first O site, O(4) is bonded to one Ca(1), one Ca(2), one Fe(1), and one Fe(3) atom to form OCa2Fe2 tetrahedra that share corners with three equivalent O(2)Ca2Fe2 tetrahedra, corners with four equivalent O(3)Ca2Fe2 tetrahedra, corners with two equivalent O(6)Ca2Fe3 trigonal bipyramids, corners with two equivalent O(7)Ca2Fe3 trigonal bipyramids,  a cornercorner with one O(1)CaFe3 trigonal pyramid,  a cornercorner with one O(8)Fe4 trigonal pyramid,  an edgeedge with one O(9)Ca2Fe3 trigonal bipyramid, and  an edgeedge with one O(1)CaFe3 trigonal pyramid. In the second O site, O(5) is bonded in a distorted trigonal planar geometry to two equivalent Ca(1), one Fe(2), and two equivalent Fe(1) atoms. In the third O site, O(6) is bonded to one Ca(1), one Ca(2), one Fe(6), and two equivalent Fe(4) atoms to form OCa2Fe3 trigonal bipyramids that share corners with two equivalent O(3)Ca2Fe2 tetrahedra, corners with two equivalent O(4)Ca2Fe2 tetrahedra, corners with three equivalent O(6)Ca2Fe3 trigonal bipyramids,  a cornercorner with one O(1)CaFe3 trigonal pyramid, corners with two equivalent O(8)Fe4 trigonal pyramids,  an edgeedge with one O(2)Ca2Fe2 tetrahedra, edges with three equivalent O(7)Ca2Fe3 trigonal bipyramids, edges with three equivalent O(9)Ca2Fe3 trigonal bipyramids, and  an edgeedge with one O(8)Fe4 trigonal pyramid. In the fourth O site, O(7) is bonded to one Ca(1), one Ca(2), one Fe(5), and two equivalent Fe(4) atoms to form OCa2Fe3 trigonal bipyramids that share corners with two equivalent O(2)Ca2Fe2 tetrahedra, corners with two equivalent O(4)Ca2Fe2 tetrahedra, corners with three equivalent O(7)Ca2Fe3 trigonal bipyramids,  a cornercorner with one O(1)CaFe3 trigonal pyramid, corners with two equivalent O(8)Fe4 trigonal pyramids,  an edgeedge with one O(3)Ca2Fe2 tetrahedra, edges with three equivalent O(6)Ca2Fe3 trigonal bipyramids, edges with three equivalent O(9)Ca2Fe3 trigonal bipyramids, and  an edgeedge with one O(8)Fe4 trigonal pyramid. In the fifth O site, O(8) is bonded to one Fe(3), one Fe(4), one Fe(5), and one Fe(6) atom to form distorted OFe4 trigonal pyramids that share  a cornercorner with one O(2)Ca2Fe2 tetrahedra,  a cornercorner with one O(3)Ca2Fe2 tetrahedra,  a cornercorner with one O(4)Ca2Fe2 tetrahedra, corners with two equivalent O(6)Ca2Fe3 trigonal bipyramids, corners with two equivalent O(7)Ca2Fe3 trigonal bipyramids, corners with five equivalent O(9)Ca2Fe3 trigonal bipyramids,  an edgeedge with one O(6)Ca2Fe3 trigonal bipyramid,  an edgeedge with one O(7)Ca2Fe3 trigonal bipyramid, and  an edgeedge with one O(8)Fe4 trigonal pyramid. In the sixth O site, O(9) is bonded to one Ca(1), one Ca(2), one Fe(4), one Fe(5), and one Fe(6) atom to form distorted OCa2Fe3 trigonal bipyramids that share corners with two equivalent O(2)Ca2Fe2 tetrahedra, corners with two equivalent O(3)Ca2Fe2 tetrahedra,  a cornercorner with one O(1)CaFe3 trigonal pyramid, corners with five equivalent O(8)Fe4 trigonal pyramids,  an edgeedge with one O(4)Ca2Fe2 tetrahedra,  an edgeedge with one O(9)Ca2Fe3 trigonal bipyramid, edges with three equivalent O(6)Ca2Fe3 trigonal bipyramids, and edges with three equivalent O(7)Ca2Fe3 trigonal bipyramids. In the seventh O site, O(1) is bonded to one Ca(2), one Fe(2), and two equivalent Fe(1) atoms to form distorted OCaFe3 trigonal pyramids that share  a cornercorner with one O(2)Ca2Fe2 tetrahedra,  a cornercorner with one O(3)Ca2Fe2 tetrahedra,  a cornercorner with one O(4)Ca2Fe2 tetrahedra,  a cornercorner with one O(6)Ca2Fe3 trigonal bipyramid,  a cornercorner with one O(7)Ca2Fe3 trigonal bipyramid,  a cornercorner with one O(9)Ca2Fe3 trigonal bipyramid, corners with three equivalent O(1)CaFe3 trigonal pyramids,  an edgeedge with one O(2)Ca2Fe2 tetrahedra,  an edgeedge with one O(3)Ca2Fe2 tetrahedra, and  an edgeedge with one O(4)Ca2Fe2 tetrahedra. In the eighth O site, O(2) is bonded to one Ca(1), one Ca(2), one Fe(2), and one Fe(3) atom to form OCa2Fe2 tetrahedra that share  a cornercorner with one O(2)Ca2Fe2 tetrahedra, corners with three equivalent O(3)Ca2Fe2 tetrahedra, corners with three equivalent O(4)Ca2Fe2 tetrahedra, corners with two equivalent O(7)Ca2Fe3 trigonal bipyramids, corners with two equivalent O(9)Ca2Fe3 trigonal bipyramids,  a cornercorner with one O(1)CaFe3 trigonal pyramid,  a cornercorner with one O(8)Fe4 trigonal pyramid,  an edgeedge with one O(6)Ca2Fe3 trigonal bipyramid, and  an edgeedge with one O(1)CaFe3 trigonal pyramid. In the ninth O site, O(3) is bonded to one Ca(1), one Ca(2), one Fe(1), and one Fe(3) atom to form OCa2Fe2 tetrahedra that share corners with three equivalent O(2)Ca2Fe2 tetrahedra, corners with four equivalent O(4)Ca2Fe2 tetrahedra, corners with two equivalent O(6)Ca2Fe3 trigonal bipyramids, corners with two equivalent O(9)Ca2Fe3 trigonal bipyramids,  a cornercorner with one O(1)CaFe3 trigonal pyramid,  a cornercorner with one O(8)Fe4 trigonal pyramid,  an edgeedge with one O(7)Ca2Fe3 trigonal bipyramid, and  an edgeedge with one O(1)CaFe3 trigonal pyramid.
Ca4Fe9O17 crystallizes in the monoclinic C2 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 7-coordinate geometry to one O(2), one O(3), one O(4), one O(5), one O(6), one O(7), and one O(9) atom. The Ca(1)-O(2) bond length is 2.28 Å. The Ca(1)-O(3) bond length is 2.28 Å. The Ca(1)-O(4) bond length is 2.27 Å. The Ca(1)-O(5) bond length is 2.85 Å. The Ca(1)-O(6) bond length is 2.51 Å. The Ca(1)-O(7) bond length is 2.53 Å. The Ca(1)-O(9) bond length is 2.51 Å. In the second Ca site, Ca(2) is bonded in a 7-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(6), one O(7), and one O(9) atom. The Ca(2)-O(1) bond length is 2.52 Å. The Ca(2)-O(2) bond length is 2.30 Å. The Ca(2)-O(3) bond length is 2.30 Å. The Ca(2)-O(4) bond length is 2.31 Å. The Ca(2)-O(6) bond length is 2.57 Å. The Ca(2)-O(7) bond length is 2.57 Å. The Ca(2)-O(9) bond length is 2.59 Å. There are six inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(3), one O(4), one O(5), and two equivalent O(1) atoms to form FeO5 trigonal bipyramids that share corners with two equivalent Fe(3)O4 tetrahedra, corners with three equivalent Fe(1)O5 trigonal bipyramids, and corners with three equivalent Fe(2)O5 trigonal bipyramids. The Fe(1)-O(3) bond length is 2.05 Å. The Fe(1)-O(4) bond length is 2.05 Å. The Fe(1)-O(5) bond length is 2.05 Å. There is one shorter (2.06 Å) and one longer (2.07 Å) Fe(1)-O(1) bond length. In the second Fe site, Fe(2) is bonded to one O(5), two equivalent O(1), and two equivalent O(2) atoms to form FeO5 trigonal bipyramids that share corners with two equivalent Fe(3)O4 tetrahedra and corners with six equivalent Fe(1)O5 trigonal bipyramids. The Fe(2)-O(5) bond length is 2.02 Å. Both Fe(2)-O(1) bond lengths are 2.00 Å. Both Fe(2)-O(2) bond lengths are 2.04 Å. In the third Fe site, Fe(3) is bonded to one O(2), one O(3), one O(4), and one O(8) atom to form FeO4 tetrahedra that share  a cornercorner with one Fe(4)O6 octahedra,  a cornercorner with one Fe(5)O6 octahedra,  a cornercorner with one Fe(6)O6 octahedra,  a cornercorner with one Fe(2)O5 trigonal bipyramid, and corners with two equivalent Fe(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 56-60°. The Fe(3)-O(2) bond length is 1.88 Å. The Fe(3)-O(3) bond length is 1.88 Å. The Fe(3)-O(4) bond length is 1.88 Å. The Fe(3)-O(8) bond length is 2.02 Å. In the fourth Fe site, Fe(4) is bonded to one O(8), one O(9), two equivalent O(6), and two equivalent O(7) atoms to form FeO6 octahedra that share  a cornercorner with one Fe(3)O4 tetrahedra, edges with two equivalent Fe(4)O6 octahedra, edges with two equivalent Fe(5)O6 octahedra, and edges with two equivalent Fe(6)O6 octahedra. The Fe(4)-O(8) bond length is 2.06 Å. The Fe(4)-O(9) bond length is 2.10 Å. There is one shorter (2.02 Å) and one longer (2.09 Å) Fe(4)-O(6) bond length. There is one shorter (2.05 Å) and one longer (2.11 Å) Fe(4)-O(7) bond length. In the fifth Fe site, Fe(5) is bonded to two equivalent O(7), two equivalent O(8), and two equivalent O(9) atoms to form FeO6 octahedra that share corners with two equivalent Fe(3)O4 tetrahedra, edges with two equivalent Fe(6)O6 octahedra, and edges with four equivalent Fe(4)O6 octahedra. Both Fe(5)-O(7) bond lengths are 2.03 Å. Both Fe(5)-O(8) bond lengths are 2.10 Å. Both Fe(5)-O(9) bond lengths are 2.07 Å. In the sixth Fe site, Fe(6) is bonded to two equivalent O(6), two equivalent O(8), and two equivalent O(9) atoms to form FeO6 octahedra that share corners with two equivalent Fe(3)O4 tetrahedra, edges with two equivalent Fe(5)O6 octahedra, and edges with four equivalent Fe(4)O6 octahedra. Both Fe(6)-O(6) bond lengths are 2.09 Å. Both Fe(6)-O(8) bond lengths are 2.08 Å. Both Fe(6)-O(9) bond lengths are 2.03 Å. There are nine inequivalent O sites. In the first O site, O(4) is bonded to one Ca(1), one Ca(2), one Fe(1), and one Fe(3) atom to form OCa2Fe2 tetrahedra that share corners with three equivalent O(2)Ca2Fe2 tetrahedra, corners with four equivalent O(3)Ca2Fe2 tetrahedra, corners with two equivalent O(6)Ca2Fe3 trigonal bipyramids, corners with two equivalent O(7)Ca2Fe3 trigonal bipyramids,  a cornercorner with one O(1)CaFe3 trigonal pyramid,  a cornercorner with one O(8)Fe4 trigonal pyramid,  an edgeedge with one O(9)Ca2Fe3 trigonal bipyramid, and  an edgeedge with one O(1)CaFe3 trigonal pyramid. In the second O site, O(5) is bonded in a distorted trigonal planar geometry to two equivalent Ca(1), one Fe(2), and two equivalent Fe(1) atoms. In the third O site, O(6) is bonded to one Ca(1), one Ca(2), one Fe(6), and two equivalent Fe(4) atoms to form OCa2Fe3 trigonal bipyramids that share corners with two equivalent O(3)Ca2Fe2 tetrahedra, corners with two equivalent O(4)Ca2Fe2 tetrahedra, corners with three equivalent O(6)Ca2Fe3 trigonal bipyramids,  a cornercorner with one O(1)CaFe3 trigonal pyramid, corners with two equivalent O(8)Fe4 trigonal pyramids,  an edgeedge with one O(2)Ca2Fe2 tetrahedra, edges with three equivalent O(7)Ca2Fe3 trigonal bipyramids, edges with three equivalent O(9)Ca2Fe3 trigonal bipyramids, and  an edgeedge with one O(8)Fe4 trigonal pyramid. In the fourth O site, O(7) is bonded to one Ca(1), one Ca(2), one Fe(5), and two equivalent Fe(4) atoms to form OCa2Fe3 trigonal bipyramids that share corners with two equivalent O(2)Ca2Fe2 tetrahedra, corners with two equivalent O(4)Ca2Fe2 tetrahedra, corners with three equivalent O(7)Ca2Fe3 trigonal bipyramids,  a cornercorner with one O(1)CaFe3 trigonal pyramid, corners with two equivalent O(8)Fe4 trigonal pyramids,  an edgeedge with one O(3)Ca2Fe2 tetrahedra, edges with three equivalent O(6)Ca2Fe3 trigonal bipyramids, edges with three equivalent O(9)Ca2Fe3 trigonal bipyramids, and  an edgeedge with one O(8)Fe4 trigonal pyramid. In the fifth O site, O(8) is bonded to one Fe(3), one Fe(4), one Fe(5), and one Fe(6) atom to form distorted OFe4 trigonal pyramids that share  a cornercorner with one O(2)Ca2Fe2 tetrahedra,  a cornercorner with one O(3)Ca2Fe2 tetrahedra,  a cornercorner with one O(4)Ca2Fe2 tetrahedra, corners with two equivalent O(6)Ca2Fe3 trigonal bipyramids, corners with two equivalent O(7)Ca2Fe3 trigonal bipyramids, corners with five equivalent O(9)Ca2Fe3 trigonal bipyramids,  an edgeedge with one O(6)Ca2Fe3 trigonal bipyramid,  an edgeedge with one O(7)Ca2Fe3 trigonal bipyramid, and  an edgeedge with one O(8)Fe4 trigonal pyramid. In the sixth O site, O(9) is bonded to one Ca(1), one Ca(2), one Fe(4), one Fe(5), and one Fe(6) atom to form distorted OCa2Fe3 trigonal bipyramids that share corners with two equivalent O(2)Ca2Fe2 tetrahedra, corners with two equivalent O(3)Ca2Fe2 tetrahedra,  a cornercorner with one O(1)CaFe3 trigonal pyramid, corners with five equivalent O(8)Fe4 trigonal pyramids,  an edgeedge with one O(4)Ca2Fe2 tetrahedra,  an edgeedge with one O(9)Ca2Fe3 trigonal bipyramid, edges with three equivalent O(6)Ca2Fe3 trigonal bipyramids, and edges with three equivalent O(7)Ca2Fe3 trigonal bipyramids. In the seventh O site, O(1) is bonded to one Ca(2), one Fe(2), and two equivalent Fe(1) atoms to form distorted OCaFe3 trigonal pyramids that share  a cornercorner with one O(2)Ca2Fe2 tetrahedra,  a cornercorner with one O(3)Ca2Fe2 tetrahedra,  a cornercorner with one O(4)Ca2Fe2 tetrahedra,  a cornercorner with one O(6)Ca2Fe3 trigonal bipyramid,  a cornercorner with one O(7)Ca2Fe3 trigonal bipyramid,  a cornercorner with one O(9)Ca2Fe3 trigonal bipyramid, corners with three equivalent O(1)CaFe3 trigonal pyramids,  an edgeedge with one O(2)Ca2Fe2 tetrahedra,  an edgeedge with one O(3)Ca2Fe2 tetrahedra, and  an edgeedge with one O(4)Ca2Fe2 tetrahedra. In the eighth O site, O(2) is bonded to one Ca(1), one Ca(2), one Fe(2), and one Fe(3) atom to form OCa2Fe2 tetrahedra that share  a cornercorner with one O(2)Ca2Fe2 tetrahedra, corners with three equivalent O(3)Ca2Fe2 tetrahedra, corners with three equivalent O(4)Ca2Fe2 tetrahedra, corners with two equivalent O(7)Ca2Fe3 trigonal bipyramids, corners with two equivalent O(9)Ca2Fe3 trigonal bipyramids,  a cornercorner with one O(1)CaFe3 trigonal pyramid,  a cornercorner with one O(8)Fe4 trigonal pyramid,  an edgeedge with one O(6)Ca2Fe3 trigonal bipyramid, and  an edgeedge with one O(1)CaFe3 trigonal pyramid. In the ninth O site, O(3) is bonded to one Ca(1), one Ca(2), one Fe(1), and one Fe(3) atom to form OCa2Fe2 tetrahedra that share corners with three equivalent O(2)Ca2Fe2 tetrahedra, corners with four equivalent O(4)Ca2Fe2 tetrahedra, corners with two equivalent O(6)Ca2Fe3 trigonal bipyramids, corners with two equivalent O(9)Ca2Fe3 trigonal bipyramids,  a cornercorner with one O(1)CaFe3 trigonal pyramid,  a cornercorner with one O(8)Fe4 trigonal pyramid,  an edgeedge with one O(7)Ca2Fe3 trigonal bipyramid, and  an edgeedge with one O(1)CaFe3 trigonal pyramid.