Crystal structure of Fe (sub 2) (AsO (sub 4) )(HAsO (sub 4) )(OH)(H (sub 2) O) (sub 3) , a dehydration product of kankite

We report the crystal structure of a dehydration product of the mineral kankite (FeAsO (sub 4) .3.5H (sub 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 (sub 2) (AsO (sub 4) )(HAsO (sub 4) )(OH)(H (sub 2) O) (sub 3) (=FeAsO (sub 4) .2H (sub 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 (sub (obs)) = 12.02 %, wR (sub (obs)) = 12.49 % (with GOF = 6.39) for 1139 observed reflections with [I (sub obs) > 3sigma (I)]. The structure of the dehydration product of kankite consists of corrugated heteropolyhedral sheets. Pairs of Fephi (sub 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 kankite dehydrates to FeAsO (sub 4) .2H (sub 2) O at 55-56 degrees C. Based on the similar topology of FeAsO (sub 4) .2H (sub 2) O and the mineral lausenite [Fe (sub 2) (SO (sub 4) ) (sub 3) .5H (sub 2) O], and the structural relationship between lausenite and kornelite [Fe (sub 2) (SO (sub 4) ) (sub 3) .7.5H (sub 2) O], we conjecture that the structure of kankite could be also built by corrugated sheets. One polyhedral linkage between an Fephi (sub 6) octahedron and an Asphi (sub 4) tetrahedron in the sheet of the dehydrated kankite needs to be broken to allow for stretching of the sheets, the introduction of an additional H (sub 2) O molecule into the sheet and perhaps also additional H (sub 2) O molecules in between the sheets.