Micro- and nanoscale observations on anhydrite formation at surface conditions

The evolution of hyper-arid surface environments on Earth and Mars is strongly linked to the occurrence of three calcium sulfates minerals: gypsum (CaSO₄∙2H₂O), bassanite (CaSO₄∙0.5H₂O), and anhydrite (CaSO₄) [1,2]. While the formation mechanism of the first, gypsum, is well understood, though do the crystallisation pathway and the required (paleo-)environmental conditions for the formation of the water-free sulfate, anhydrite, still depict a conundrum.

To date, there exists no model that can reliably predict how anhydrite forms at earth’s surface conditions. While thermodynamics favour its occurrence, it is hardly achieved on laboratory time scales at conditions fitting either the Atacama Desert on Earth or the surface of Mars.

Considering recent transmission electron microscopy findings [3] advocating for a complex, non-classical nucleation mechanism for all calcium sulfates, we present analyses of natural samples from different facies of the Atacama Desert, i.e. aeolian deposits, (sub-)surface nodules and selenites, to identify key features that promote the nucleation and growth of anhydrite under planetary surface conditions.

Additionally, alternative pathways for anhydrite nucleation facilitated by metabolic products of biological organisms (e.g. fungi, lichen, cyanobacteria) found in soils of the Atacama [4] will be discussed.

References:

[1] Voigt et al. (2020) Glob Planet Change 148:103077

[2] Vaniman et al. (2018) Am Min 103(7):1011–1020

[3] Stawski et al. (2016) J Phys Chem C 124(15):8411-8422

[4] Knief et al. (2020) Glob Planet Change 148:103077