DEHYDRATION BEHAVIOR OF KAINITE: CRYSTAL STRUCTURE OF KCL*MG(SO4)*2H2O

Hydration cycles of minerals on the martian surface are of great interest to planetary geology, as they are thought to be a potential source for bioavailable and episodic water. Several salt systems are known to change hydration state with changes in relative humidity (RH) and temperature (T) and this work adds to the inventory of minerals that can hydrate and dehydrate under changing conditions. The KCl∙Mg(SO₄)∙nH₂O system occurs as kainite (n=3) at STP on Earth and is found in potash deposits around the world. Kainite was examined with thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD) under controlled-RH and -T conditions (from -50° to 300° C), and single-crystal X-ray diffraction (SCXRD) at elevated temperatures. TGA was used to observe the dehydration products of kainite, although this method provides no structural information about the products; PXRD provided companion crystallographic information. The PXRD pattern of kainite abruptly changed when exposed to T > 90° C, revealing a structural transformation. No other structural changes were observed from -50° to 300° C. The new PXRD pattern could not be explained by any known phases, and SCXRD data were then obtained. These data yielded a crystal structure with two H₂O molecules per formula unit, loosely consistent with the TGA data. The surface of Mars is not likely to reach the T required for this dehydration reaction, making this phase unlikely to participate in the martian H₂O cycle. However, Earth’s surface is thought to reach 90-100° C (Mildrexler et al. 2011) in low-P_H2O desert environments where these hydration cycles are likely to influence P_H2O in the dry season. The n=2 phase rehydrated at room T and RH on the scale of hours, showing that it can participate in a terrestrial desert H₂O cycle.