| Abstract
| - The composition of fast-frozen wet pastes of hematite particles of different morphologies equilibrated inNaCl aqueous solutions was investigated by X-ray photoelectron spectroscopy. Two hematite preparationsconsisted of micrometer-sized platelets with 42% (HEM-1) and 82% (HEM-8) of the surface terminated bythe {001} basal plane and a third of spheroids with no recognizable crystal plane (HEM-control). All hematitesamples responded to changes in pH (4 and 9) and ionic strength (0, 10, 100 mM), showing that acid/basereactions of surface hydroxyl groups impact the composition of the paste. The HEM-1 and HEM-8 sampleexhibited unusually large Na, Cl, and water contents at the highest ionic strength (100 mM) compared toHEM-control and all other minerals studied with this technique previously. The Na 1s and Cl 2p spectraoccurred at binding energies typical of hydrated Na+ and Cl- ions and possessed energy-loss features, suggestinga three-dimensional distribution of these ions in the paste. An approximate stochiometric Na/Cl/H2O ratio of1:1:2 was obtained in all samples in 100 mM NaCl as well as a strong correlation between the these compoundsand the fraction of the {001} basal plane present in the hematite particles. The basal plane of hematite isproposed to induce the formation of a hydrated NaCl structure in the fast-frozen pastes, one that iscompositionally reminiscent of hydrohalite (NaCl·2H2O), by stabilizing multiple layers of hydrated Na+ andCl- ions prior to freezing.
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