The position of the vapor‐liquid dome and of the critical point determine the evolution of the outermost parts of the protolunar disk during cooling and condensation after the Giant Impact. The parts of the disk in supercritical or liquid state evolve as a single thermodynamic phase; when the thermal trajectory of the disk reaches the liquid‐vapor dome, gas and melt separate leading to heterogeneous convection and phase separation due to friction. Different layers of the proto‐Earth behaved differently during the Giant Impact depending on their constituent materials and initial thermodynamic conditions. Here we use first‐principles molecular dynamics to determine the position of the critical point for NaAlSi₃O₈ and KAlSi₃O₈ feldspars, major minerals of the Earth and Moon crusts. The variations of the pressure calculated at various volumes along isotherms yield the position of the critical points: 0.5–0.8 g cm⁻³ and 5500–6000 K range for the Na‐feldspar, 0.5–0.9 g cm⁻³ and 5000–5500 K range for the K‐feldspar. The simulations suggest that the vaporization is incongruent, with a degassing of O₂ starting at 4000 K and gas component made mostly of free Na and K cations, O₂, SiO and SiO₂ species for densities below 1.5 g cm⁻³. The Hugoniot equations of state imply that low‐velocity impactors (<8.3 km s⁻¹) would at most melt a cold feldspathic crust, whereas large impacts in molten crust would see temperatures raise up to 30000 K.

中文翻译：

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碱长石的临界点和超临界状态：对撞击期间地壳行为的影响


汽液穹顶和临界点的位置决定了大撞击后冷却和凝结过程中原月盘最外层部分的演化。盘中处于超临界或液态的部分演化为单一热力学相；当圆盘的热轨迹到达液-汽圆顶时，气体和熔体分离，由于摩擦而导致不均匀对流和相分离。原地球的不同层在大撞击期间表现不同，这取决于它们的组成材料和初始热力学条件。在这里，我们使用第一原理分子动力学来确定 NaAlSi ₃ O ₈和 KAlSi ₃ O ₈长石（地球和月壳的主要矿物）的临界点位置。沿着等温线在不同体积下计算的压力变化产生了临界点的位置：钠长石的0.5-0.8 g cm ^-3和5500-6000 K范围，0.5-0.9 g cm ^-3和5000-5500 K范围钾长石的范围。模拟表明，蒸发是不一致的，O ₂的脱气在4000 K 开始，气体成分主要由游离Na 和K 阳离子、O ₂ 、SiO 和SiO ₂物质组成，密度低于1.5 g cm ^-3 。于格尼奥状态方程表明低速撞击器 ( < 8.3 km s ^-1 ）至多会融化冷的长石地壳，而熔地壳中的大撞击会导致温度升高至 30000 K。