2019A&A...624A.136T

Context. Laboratory analogues can provide physical constraints to the interpretation of astronomical observations of cosmic dust but clearly do not experience the same formation conditions. To distinguish between properties intrinsic to the material and properties imprinted by their means of formation requires extensive characterisation.
Aims. Sol-gel methods can produce amorphous silicates with potentially high reproducibility, but often require long drying times (24+h) at elevated temperatures in air, controlled atmosphere, or vacuum. We investigate the possibility that microwave drying can be used to form amorphous silicate on a timescale of ∼10 min and characterise their structural and spectroscopic properties relative to silicates produced by other drying methods.
Methods. Microwave-dried amorphous MgSiO₃, Fe_0.1Mg_0.9SiO₃ and Mg₂SiO₄ are characterised using X-ray powder diffraction, total X-ray scattering, small angle X-ray scattering and mid-IR FTIR spectroscopy, and compared to samples produced from the same gels but dried in-air and under vacuum. The development of crystalline structure in the microwave-dried silicates via thermal annealing up to 999°C is also investigated using in situ X-ray powder diffraction.
Results. At the inter-atomic level the silicate structures are largely independent of drying method, however larger-scale structured domains, ranging from a ∼ few x10Å to ∼100'sÅ in size, are observed. These are ordered as mass fractals with discernible variation caused by the drying processes. The mid-IR 10µm band profile is also found to be influenced by the drying process, likely due to the way removal of water and bonded OH influences the distribution of tetrahedral species. However, microwave drying also allows Fe to be easily incorporated into the silicate structure. In situ annealing shows that for amorphous MgSiO₃ crystalline forsterite, enstatite and cristobalite are high temperature phases, while for Mg₂SiO₄ forsterite crystallises at lower temperatures followed by cristobalite at high temperature. For Fe_0.1Mg_0.9SiO₃ the crystallisation temperature is significantly increased and only forsterite is observed. Crystalline SiO₂ may be diagnostic of Mg-rich, Fe-poor grain mineralogies. The results are discussed in relation to the different thermal conditions required for dust to crystallise within protoplanetary disk lifetimes.
Conclusions. Sol-gel microwave drying provides a fast and easy method of producing amorphous Mg- and Fe,Mg-silicates of both pyroxene and olivine compositions. Their structure and spectroscopic characteristics although similar to silicates produced using other drying methods, exhibit subtle variations which are particularly manifest spectroscopically in the mid-IR, and structurally over medium- and long-range length scales.