Characterization of sodium chloride crystals grown in microgravity

NaCl crystals grown by the evaporation of an aqueous salt solution in microgravity on the International Space Station (ISS) were characterized and compared to salt crystals grown on earth. NaCl crystallized as thin wafers in a supersaturated film of 200–700 μm thickness and 50 mm diameter, or as hopper cubes in 10 mm diameter supersaturated spheres. Neutron diffraction shows no change in crystal structure and in cell parameters compared to earth-grown crystals. However, the morphology can be different, frequently showing circular, disk-like shapes of single crystals with 〈1 1 1〉 perpendicular to the disks, an unusual morphology for salt crystals. In contrast to the growth on earth the lateral faces of the microgravity tabular hopper crystals are symmetrical because they are free floating during the crystallization process. Hopper cubes were produced without the need to suspend the growing crystals by an ongoing stirring. “Fleur de Sel” is shown as an example of two-dimensional growth of salt on earth and compared to the space grown crystals. It is shown that in microgravity conditions brine fluid inclusions form within the salt crystals. ► In microgravity, sodium chloride (NaCl) has the same crystal structure with the same lattice constant a. ► Unusual disk-like morphology observed in “two-dimensional” crystallizer in microgravity is a single crystal of salt (NaCl) with 〈1 1 1〉 perpendicular to the disk. ► Morphology of lateral faces of the tabular hopper crystals are different in microgravity due to the presence of free floating crystals without convection, resulting in different mass transfer to the faces. ► In microgravity it is possible to form brine fluid inclusions.