FeOOH crystallizes in the monoclinic C2/m space group. Fe(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form a mixture of corner and edge-sharing FeO6 octahedra. The corner-sharing octahedral tilt angles are 24°. H(1,2) is bonded in a linear geometry to two equivalent O(2) atoms. There are two inequivalent O sites. In the first O site, O(1) is bonded to four equivalent Fe(1) atoms to form OFe4 trigonal pyramids that share corners with six equivalent O(2)Fe2H2 tetrahedra, corners with four equivalent O(1)Fe4 trigonal pyramids,  an edgeedge with one O(2)Fe2H2 tetrahedra, and edges with four equivalent O(1)Fe4 trigonal pyramids. In the second O site, O(2) is bonded to two equivalent Fe(1) and two equivalent H(1,2) atoms to form distorted OFe2H2 tetrahedra that share corners with four equivalent O(2)Fe2H2 tetrahedra, corners with six equivalent O(1)Fe4 trigonal pyramids, and  an edgeedge with one O(1)Fe4 trigonal pyramid.
FeOOH crystallizes in the monoclinic C2/m space group. Fe(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form a mixture of corner and edge-sharing FeO6 octahedra. The corner-sharing octahedral tilt angles are 24°. Both Fe(1)-O(2) bond lengths are 2.05 Å. There are two shorter (2.00 Å) and two longer (2.11 Å) Fe(1)-O(1) bond lengths. H(1,2) is bonded in a linear geometry to two equivalent O(2) atoms. Both H(1,2)-O(2) bond lengths are 1.22 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded to four equivalent Fe(1) atoms to form OFe4 trigonal pyramids that share corners with six equivalent O(2)Fe2H2 tetrahedra, corners with four equivalent O(1)Fe4 trigonal pyramids,  an edgeedge with one O(2)Fe2H2 tetrahedra, and edges with four equivalent O(1)Fe4 trigonal pyramids. In the second O site, O(2) is bonded to two equivalent Fe(1) and two equivalent H(1,2) atoms to form distorted OFe2H2 tetrahedra that share corners with four equivalent O(2)Fe2H2 tetrahedra, corners with six equivalent O(1)Fe4 trigonal pyramids, and  an edgeedge with one O(1)Fe4 trigonal pyramid.