We report the crystal structure of a dehydration product of the mineral kaňkite (FeAsO4 · 3.5H2O). The structure was solved and refined by precession electron diffraction tomography. Initially, we believed that we solved the structure of kaňkite; this mineral, however, decomposes in vacuum to Fe2(AsO4)(HAsO4)(OH)(H2O)3 (=FeAsO4 · 2H2O). The crystal structure was solved in the space group Cc and the model was refined by the full-matrix least-squares method by Jana2006. The model converged to R(obs) = 12.02 %, wR(obs) = 12.49 % (with GOF = 6.39) for 1139 observed reflections with [Iobs > 3σ(I)]. The structure of the dehydration product of kaňkite consists of corrugated heteropolyhedral sheets. Pairs of Feϕ6 octahedra, flanked by five adjacent arsenate tetrahedra, could be seen as the building units of the corrugated sheets. Variable-temperature powder X-ray diffraction showed that kaňkite dehydrates to FeAsO4 · 2H2O at 55–56°C. Based on the similar topology of FeAsO4 · 2H2O and the mineral lausenite [Fe2(SO4)3 · 5H2O], and the structural relationship between lausenite and kornelite [Fe2(SO4)3 · 7.5H2O], we conjecture that the structure of kaňkite could be also built by corrugated sheets. One polyhedral linkage between an Feϕ6 octahedron and an Asϕ4 tetrahedron in the sheet of the dehydrated kaňkite needs to be broken to allow for stretching of the sheets, the introduction of an additional H2O molecule into the sheet and perhaps also additional H2O molecules in between the sheets.