Hydrated sodium-magnesium sulfate minerals associated with inland saline systems Atomic structure, hydrogen bonding and phase stability.

摘要

Hydrated sodium-magnesium sulfate minerals are common in many continental evaporite settings around the world. The crystallization sequence of these minerals depends on such parameters as the composition of the parent brine, the temperature, the evaporation rate of the brine, and the differences in the atomic structure and water content of the minerals. The atomic structures of konyaite [Na2Mg(SO4)2˙5H2O] and sodium-magnesium decahydrate [Na2Mg(SO4)2˙10H 2O], a newly described sulfate salt, have been determined from single-crystal X-ray diffraction experiments. The refined structures are discussed and compared to that of blodite [Na2Mg(SO 4)2˙4H2O]. The arrangement and importance of hydrogen bonds within all three structures are also discussed, and have been further investigated by infrared spectroscopy. Loweite [Na12Mg 7(SO4)13˙15H2O] was included in this experiment to provide a low-hydration end-member. Differences in water content and the importance of hydrogen bonds in the respective structures were clearly reflected in the generated infrared spectra. The growth conditions of the decahydrate, konyaite, blodite, loweite, and other phases of the Na2O-MgO-H2O system, as well as their stability relationships, were studied in a temperature-controlled crystal-growth experiment. Konyaite and the decahydrate phase were found as first precipitates over a range of temperatures and brine compositions where they are not considered to be the thermodynamically stable phase. The importance of evaporation rate in the formation of these, and other metastable phases, is discussed in relation to inland saline systems. Possible localities where the decahydrate could exist in nature are discussed, and challenges for future are presented.