Low temperature mechanism for formation

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 b100 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.