XPS of Fast-Frozen Hematite Colloids in NaCl Aqueous Solutions:  I. Evidence for the Formation of Multiple Layers of Hydrated Sodium and Chloride Ions Induced by the {001} Basal Plane

The composition of fast-frozen wet pastes of hematite particles of different morphologies equilibrated in NaCl aqueous solutions was investigated by X-ray photoelectron spectroscopy. Two hematite preparations consisted of micrometer-sized platelets with 42% (HEM-1) and 82% (HEM-8) of the surface terminated by the {001} basal plane and a third of spheroids with no recognizable crystal plane (HEM-control). All hematite samples responded to changes in pH (4 and 9) and ionic strength (0, 10, 100 mM), showing that acid/base reactions of surface hydroxyl groups impact the composition of the paste. The HEM-1 and HEM-8 sample exhibited unusually large Na, Cl, and water contents at the highest ionic strength (100 mM) compared to HEM-control and all other minerals studied with this technique previously. The Na 1s and Cl 2p spectra occurred at binding energies typical of hydrated Na + and Cl - ions and possessed energy-loss features, suggesting a three-dimensional distribution of these ions in the paste. An approximate stochiometric Na/Cl/H 2 O ratio of 1:1:2 was obtained in all samples in 100 mM NaCl as well as a strong correlation between the these compounds and the fraction of the {001} basal plane present in the hematite particles. The basal plane of hematite is proposed to induce the formation of a hydrated NaCl structure in the fast-frozen pastes, one that is compositionally reminiscent of hydrohalite (NaCl·2H 2 O), by stabilizing multiple layers of hydrated Na + and Cl - ions prior to freezing.