The compression behavior of blödite at low and high temperature up to ∼10 GPa: Implications for the stability of hydrous sulfates on icy planetary bodies

Abstract

Recent satellite inferences of hydrous sulfates as recurrent minerals on the surface of icy planetary bodies link with the potential mineral composition of their interior. Blödite, a mixed Mg-Na sulfate, is here taken as representative mineral of icy satellites surface to investigate its crystal structure and stability at conditions of the interior of icy bodies. To this aim we performed in situ synchrotron angle-dispersive X-ray powder diffraction experiments on natural blödite at pressures up to ∼10.4 GPa and temperatures from ∼118.8 K to ∼490.0 K using diamond anvil cell technique to investigate the compression behavior and establish a low-to-high temperature equation of state that can be used as reference when modeling the interior of sulfate-rich icy satellites such as Ganymede.

The experimentally determined volume expansivity, α, varies from 7.6 (7) 10⁻⁵ K⁻¹ at 0.0001 GPa (from 118.8 to 413.15 K) to 2.6 (3) 10⁻⁵ K⁻¹ at 10 GPa (from 313.0 to 453.0 K) with a δα/δ_P coefficient = −5.6(9)10⁻⁶ GPa⁻¹ K⁻¹.

The bulk modulus calculated from the least squares fitting of P-V data on the isotherm at 413 K using a second-order Birch - Murnaghan equation of state is 38(5) GPa, which gives the value of δK/δ_T equal to 0.01(5) GPa K⁻¹. The thermo-baric behavior of blödite appears strongly anisotropic with c lattice parameter being more deformed with respect to a and b.

Thermogravimetric analyses performed at ambient pressure showed three endotherms at 413 K, 533 K and 973 K with weight losses of approximately 11%, 11% and 43% caused by partial dehydration, full dehydration and sulfate decomposition respectively. Interestingly, no clear evidence of dehydration was observed up to ∼453 K and ∼10.4 GPa, suggesting that pressure acts to stabilize the crystalline structure of blödite.

The data collected allow to write the following equation of state,

V(P, T) = V₀[1 + 7.6(7)10 ^− 5ΔT − 0.026(3)P − 5.6(9)10 ^− 6PΔT−6.6(9)10 ^− 6PΔT)]

from which the density of blödite can be determined at conditions of the mantle of the large icy satellites of Jupiter.

Blödite has higher density, bulk modulus and thermal stability than similar hydrous sulfates (e.g. mirabilite and epsomite) implying, therefore, a different contribution of these minerals to the extent of deep oceans in icy planets and their distribution over the local geotherms.