Crystal structure of Fe2(AsO4)(HAsO4)(OH)(H2O)3, a dehydration product of kankite

We report the crystal structure of a dehydration product of the mineral kankite (FeAsO4 center dot 3.5H(2)O). The structure was solved and refined by precession electron diffraction tomography. Initially, we believed that we solved the structure of kankite; this mineral, however, decomposes in vacuum to Fe-2(AsO4)(HAsO4)(OH)(H2O)(3) (-FeAsO4 center dot 2H(2)O). 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 [I-obs > 3 sigma(I)]. The structure of the dehydration product of kankite consists of corrugated heteropolyhedral sheets. Pairs of Fe phi(6) 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 kan. kite dehydrates to FeAsO4 center dot 2H(2)O at 55-56 degrees C. Based on the similar topology of FeAsO4 center dot 2H(2)O and the mineral lausenite [Fe-2(SO4)(3)center dot 5H(2)O], and the structural relationship between lausenite and kornelite [Fe-2(SO4)(3)center dot 7.5H(2)O], we conjecture that the structure of kan. kite could be also built by corrugated sheets. One polyhedral linkage between an Fe phi(6) octahedron and an As phi(4) tetrahedron in the sheet of the dehydrated kan. kite 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.