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 mineral representative 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 GPa and temperatures from ~100 K to ~570 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 modelling the interior of sulfate-rich icy satellites such as Ganymede.The experimentally determined volume expansivity, , varies from 7.6 (6) 10 -5 K -1 at 0.0001 GPa (from 199 to 413 K) to 2.6 (3) 10 -5 K -1 at 10 GPa (from 293 to 453 K) with aδ/δP coefficient = -5.6(8)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 GPa, which gives the value of K/δT equal to 0.01(5) GPa -1 K -1 . The thermobaric 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 410 K, 500 K and 1000 K, with weight losses of approximately 11%, 11% and 43% caused by partial dehydration, full dehydration and sulfate decomposition. Interestingly, no clear evidence of dehydration was observed up to ~470 K and ~10 GPa, suggesting that pressure would act to stabilize the crystalline structure of blödite.The data collected allow to write the following equation of state, V(P,T) = V 0 [ 1+ 7.6(6) 10 -5 Δ T -0.026(3)P -5.6(8) 10 -6 PΔT -6.6(9) 10 -6 PT ) from which the density of blödite can be determined at conditions of the ice mantle of the large icy satellites of Jupiter. Based on current thermal models for the interior of Ganymede, for instance, blödite is predicted to remain stable with the density almost unchanged to about 1000 km in depth in equilibrium with a deep saline ocean.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 Keywords Blödite, high pressure, icy satellite, sulfate, in situ angle-dispersive X-ray diffraction, synchrotron. V between 119 and 433 K of 7.6 (7)10 -5 K -1 .The c lattice parameter remains the most deformable with respect to a and b that, therefore, show very similar thermal expansion values. Because the monoclinic symmetry of blödite with β angle significantly different from 90°, the principal directions of the expansion tensor should be investigated. The symmetry requires a direction parallel to b axis, and ...

The Phase Transition and Dehydration in Epsomite under High Temperature and High Pressure

“…As shown in Figure 7, it is verified that the dehydration temperature of epsomite gradually increased with the rise in pressure. This positive relation between dehydration temperature and pressure was coincident with the result of other similar hydrated sulfates (gypsum, chalcanthite and blödite) [11][12][13]. In addition, we also found that the epsomite undergoes three-step dehydration reactions in the heating process: from epsomite (MgSO4•7H2O) to hexahydrite (MgSO4•6H2O) to magnesium sulfate trihydrate (MgSO4•3H2O) to anhydrous magnesium sulfate (MgSO4).…”

“…It is a common phenomenon for hydrated sulfates to undergo dehydration reactions to form lower hydrates under a high temperature and pressure. A great number of previous high-pressure studies were undertaken, mainly concerned with the dehydration process of gypsum, chalcanthite and blödite, and all of them were found to have a positive relationship between the dehydration temperature and pressure [9][10][11][12][13]. For example, Yang et al found that the dehydration temperature of gypsum gradually increased with the rise in pressure, and the dehydration boundary between gypsum and bassanite was determined to be P (GPa) = -23.708 + 0.050T (K) [13].…”

High Pressure Behavior of Mascagnite from Single Crystal Synchrotron X-ray Diffraction Data

“…The comparison of high-pressure structural data shows that the volume and average S-O bond distances of sulfate tetrahedron remain almost unchanged, with the average distance <S-O> of 1.466 (1)−1.468 (1) Å, and polyhedral volume changing from 1.623 Å 3 to 1.610 Å 3 , in the pressure range 0.0001-17.6 GPa (Table 3). The values are very near to those measured in other sulfate minerals, for example, in bloedite [24] or gypsum [25].…”

High-Pressure Behavior of Nickel Sulfate Monohydrate: Isothermal Compressibility, Structural Polymorphism, and Transition Pathway

“…Polymorphism of M 2+ SO 4 ·H 2 O is an important topic in astrophysics and planetary geology with respect to the monohydrate sulfates being an abundant component on the surface of icy moons (cf. Comodi et al and McCord et al). α-MgSO 4 ·H 2 O and β-MgSO 4 ·H 2 O have recently been considered for the interior of Callisto, where the maximum pressure corresponds to around 5 GPa.…”