Hydrated clay minerals that are common to Earth’s atmosphere and terrestrial and aquatic environments can form gels that host saline solutions. Using cryogenic electron microscopy and vibration spectroscopy, we show that saline gels of montmorillonite frozen at < –90 °C host elongated hexagonal ice (I_h) microcrystals embedded in a network of honeycomb micropores. Freezing segregates salts into walls of aggregated clay nanoparticles sharing face-to-face contacts. Above ∼ –50 °C, clay gels that are sufficiently dense (≫10 g/L) and flexible (Na-exchanged montmorillonite) also host the cryosalt mineral hydrohalite (NaCl·2H₂O), either co-existing or entirely replacing I_h in the gels. Hydrohalite does not form in gels of low-density (<10 g/L) or rigid (Ca-exchange montmorillonite) clay particles. These results suggest that hydrohalite forms in expandable clay gels that are sufficiently dense and flexible to retain saline solutions within their walls, possibly through interparticle capillary and hydration forces. These forces effectively oppose water diffusion to growing ice microcrystals within micropores, thus prolonging the lifetime of hydrohalite within these hydrated clay gels. Our findings tie the fate of ice and cryosalt nucleation and growth to the water-retention capability of expandable clay gels.

中文翻译：

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盐微孔粘土凝胶中的冰和冰晶形成

地球大气以及陆地和水生环境中常见的水合粘土矿物可以形成容纳盐溶液的凝胶。使用低温电子显微镜和振动光谱法，我们发现冻结在<–90°C的蒙脱石的盐凝胶具有嵌入蜂窝状微孔网络中的细长六角形冰（I _h）微晶。冷冻将盐分离到聚集的粘土纳米颗粒的壁中，它们共享面对面的接触。在〜–50°C以上，足够稠密（≫10 g / L）和柔性（钠交换蒙脱石）的粘土凝胶也可以容纳冰晶石矿物氢卤石（NaCl·2H ₂ O），可以共存或完全代替I _H在凝胶中。低密度（<10 g / L）或硬质（钙交换蒙脱土）粘土颗粒的凝胶中不会形成卤水石。这些结果表明水卤石在可膨胀的粘土凝胶中形成，该凝胶具有足够的密度和柔韧性以可能通过颗粒间的毛细管作用和水合力将盐溶液保留在其壁内。这些力有效地阻止了水扩散到微孔内正在生长的冰微晶上，从而延长了这些水合粘土凝胶中卤化氢的寿命。我们的发现将冰和冷冻盐成核和生长的命运与可膨胀粘土凝胶的保水能力联系在一起。