Deciphering the Gypsum – Anhydrite Phase Transition in the Atacama Desert

Present as gypsum (CaSO4∙2H2O), bassanite (CaSO4∙0.5H2O) and anhydrite (CaSO4), calcium sulphates comprise a significant fraction of the surface and soil in the Atacama Desert.Field observation suggest a secondary formation process for the dehydrated sulphates.These thermodynamically predicted phase transitions were ruled out by previous studies to occur by heating under ambient conditions of the Atacama Desert or by suppressing the water activity with simplified brine-solutions[1]. Furthermore, high induction times for nucleation hampers anhydrite growth.Recent studies[2,3] have shown that precipitation of anhydrite can be achieved in flow-through reactors at room temperature even with high water-activity (aw = 0.98).Results from our own flow-through experiments with gypsum seeded with anhydrite under a variety of temperatures and salinities contradict the notion that the production of anhydrite is a consequence of simple hydrological factors. It appears essential to gain a deeper understanding of catalytic processes that stabilize anhydrite nuclei.Possible responsible catalysts could be of ionic, nano-particulate or organic nature. The effects of these catalysts have been studied in the literature extensively[4] and especially nanoparticles and ions from soluble salts are abundant in Atacama Desert sediments.We show results from experiments performed on natural and synthetic samples that systematically explore the domains of solution flow, ionic contamination and nanoparticles.

[1] Ossorio, M., et al., Chemical Geology 386 (2014): 16-21.[2] Dixon, E., et al., Journal of Geophysical Research: Planets 120.4 (2015): 625-642.[3] Miller, Kayla., (2017).[4] Cody, R. D., and A. B. Hull., Geology 8.10 (1980): 505-509.