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The Critical Point and the Supercritical State of Alkali Feldspars: Implications for the Behavior of the Crust During Impacts
Author(s) -
Kobsch Anaïs,
Caracas Razvan
Publication year - 2020
Publication title -
journal of geophysical research: planets
Language(s) - English
Resource type - Journals
eISSN - 2169-9100
pISSN - 2169-9097
DOI - 10.1029/2020je006412
Subject(s) - supercritical fluid , vaporization , critical point (mathematics) , feldspar , eutectic system , thermodynamics , mineralogy , phase (matter) , crust , condensation , materials science , chemistry , geology , composite material , geophysics , physics , mathematical analysis , quartz , mathematics , organic chemistry , alloy
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 NaAlSi 3 O 8 and KAlSi 3 O 8 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 O 2 starting at 4000 K and gas component made mostly of free Na and K cations, O 2 , SiO and SiO 2 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.