Low-temperature mechanism for formation of coarse crystalline hematite through nanoparticle aggregation

We have discovered a low-temperature mechanism for the formation of coarse specular hematite grains. Either freezing and subsequent thawing or cryodesiccation of aqueous nanoparticle suspensions under confinement leads to aggregation of hematite platelets with initial diameters of ~ 10 nm into domains < 100 nm in diameter. Crystallographic alignment of particles in the (001) basal plane only occurs on the scale of the 50–100 nm domains, is absent for air-dried grains, and increases for freeze–thawed and freeze-dried samples. Stacking of the domains accommodates curvature at the microscale, leading to final grain sizes greater than 1 mm with curved but smooth surfaces reflective to visible light. Confinement of freezing suspensions increases ordering of aggregates. No spherules were produced; however, thermal emission spectra of the nanoparticle aggregates are consistent with coarse crystalline spectra from Mars. TIR spectra of air-dried, freeze–thawed, and cryodesiccated samples are similar, suggesting that the crystallographic alignment observed at the nanoscale is not responsible for the lack of 390 cm−1 feature. Aqueous alteration of Fe-bearing sulfates has been demonstrated to form hematite nanoparticles in systems analogous to Meridiani Planum. Evaporation and aggregation of resulting nanoparticles is consistent with the context of low-temperature playa-groundwater models, while freezing and/or cryodesiccation are consistent with models of low-temperature ice-hosted weathering. Additionally, this new mechanism for specular hematite formation may lead to re-evaluation and re-interpretation of coarse hematite formation throughout Earth history.