DEHYDRATION BEHAVIOR OF KAňKITE AND THE CRYSTAL STRUCTURE OF THE DEHYDRATED KAňKITE

Kaňkite is an iron arsenate with a nominal formula FeAsO₄×3.5H₂O. It occurs in arsenic-rich environments generated by decomposition of arsenopyrite, löllingite, and pyrite in underground spaces and weathering mining waste. Since we encounter this phase often in our work on As mineralogy in mining waste, we attempted to solve its crystal structure.

Kaňkite forms masses of minute (< 2 mm) platy crystals intergrown with each other and standard methods (e.g., single-crystal X-ray diffraction, XRD) were deemed inadequate for this problem. Crystal structure was solved by precession electron diffraction in a transmission electron microscope (TEM). The data were processed to yield the unit cell parameters and integrated reflection intensities. The structure was solved in the space group Cc with lattice parameters a = 5.6825 Å, b = 21.0912 Å, c= 9.119 Å, and b = 92.793°. The structure consists of corrugated heteropolyhedral layers in which arsenate tetrahedra attach to Fe-centered octahedra.

Surprisingly, this structural model failed to describe the synchrotron powder XRD data collected on the same kaňkite sample. After some ruminations, we arrived at a hypothesis that the solved structure belongs to a dehydration product of kaňkite because the TEM work is carried out in high vacuum. A series of in-house, powder XRD heating experiments confirmed this suspicion. Upon heating, kaňkite breaks down to its dehydration product at approximately 50 °C. The high-temperature phase possesses indeed the structure solved by precession electron diffraction and persists up to ~170 °C.

We assume that the crystal structure of kaňkite is closely related to that of its dehydration product but remains unknown. The reason for this assumption is a similar topology of the corrugated layers in the dehydrated kaňkite and lausenite [Fe₂(SO₄)₃×5H₂O], noting that lausenite reversibly hydrates to kornelite [Fe₂(SO₄)₃×7.5H₂O]. The structure of kornelite also consists of sheets, not as corrugated as in lausenite. It is possible that the structure of kaňkite could be captured by considering this analogy; we were not able to do so until now.