IR-Raman Study on Hexahydrite Amorphization Under Vacuum and Low Temperatures

Abstract

Hexahydrite (MgSO₄·6H₂O) is among the possible candidates of the hydrated salts composing the “non-icy” materials on the surface of Europa. However, given the conditions of extreme vacuum characterizing the surfaces of icy and other airless Solar System bodies, a crucial aspect is to determine the conditions of stability of such hydrated salts in a wide range of pressures from ambient to high vacuum. The effect of varying temperatures in conjunction with variable vacuum is another important aspect to be considered. Studying the stability field of such compounds is crucial to establish the conditions of their presence on icy moons. In this work, we conducted an infrared (1–12 μm) and Raman spectral study on hexahydrite samples from ambient pressure to high vacuum, at ambient and low temperatures. We observe two main effects occurring upon lowering the pressure at which the sample is exposed, that is (a) de-hydration of the sample and consequent (b) loss of crystallinity and increase of structural disorder of the material. While the de-hydration is clear from the notable modification of H₂O absorption bands in the near-infrared, the effect of loss of crystallinity is observable in the mid-IR spectral region and also confirmed by Raman analyses. The transition is marked by the progressive loss of fine structure in the 1.5 and 2 μm hydration bands and by the collapse of the ∼9 μm Reststrahlen peak. Our measurements provide important laboratory constraints concerning the stability conditions of such hydrated materials that should provide aid in interpretation of mission data.