Density of Sodium Aluminosilicate Melts Along the NaAlSiO ₄ ‐NaAlSi ₃ O ₈ Join at High Pressure: In‐Situ Measurements and Re‐Calibration of a Modified Hard‐Sphere Equation of State For Silicate Melts

Silicate melts play a crucial role in planetary differentiation. The density contrast between silicate melts and the surrounding solid residue exerts a primary control on many magmatic processes. However, direct measurements of the density of silicate melts at high pressure ( P ) and temperature ( T ) conditions remain challenging, particularly for the highly viscous and reactive silica‐ and alkali‐rich melts. Here we determined the high P ‐ T densities of three sodium aluminosilicate melts with nepheline (NaAlSiO 4 ), jadeite (NaAlSi 2 O 6 ), and albite (NaAlSi 3 O 8 ) compositions, using the high‐ P X‐ray microtomography technique up to 4.1 GPa and 2020 K. Our results suggest that the substitution of (NaAl) 4+ for Si 4+ along the NaAlSiO 4 ‐NaAlSi 3 O 8 join leads to higher melt density and lower melt compressibility. In addition, our new data, combined with a wide range of literature data, were employed to re‐calibrate a modified hard‐sphere equation of state (HS‐EOS) for silicate melts, which provides a unified framework for calculating the density and other compressional properties of multi‐component silicate melts in the CaO‐MgO‐Al 2 O 3 ‐SiO 2 ‐FeO‐Na 2 O‐K 2 O (CMASFNK) system up to ∼25 GPa. The calibration also reveals that SiO 2 , alkalis, and CaO are the major components contributing to the compositional dependence of melt elastic properties. The HS‐EOS was then applied to alkali basaltic melts at cratonic mantle conditions and silica‐ and alkali‐rich melts at early planetesimal melting conditions, with implications for the gravitational stability and extraction of melts in Earth's mantle and planetesimal settings.