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Abstract: 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+ hours) 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 minutes and characterise their structural and spectroscopic properties relative to silicates produced by other drying methods. Methods. Microwave-dried amorphous MgSiO3, Fe 0.1Mg0.9SiO3 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 largerscale structured domains, ranging from a ∼few×10 ˚A to∼100’s ˚A in size, are observed. These are ordered as mass fractals with discernible variation caused by the drying processes. The mid-IR 10 μm 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 Fe0.1Mg0.9SiO3 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.

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Abstract: Facilities for studying gas-solid interactions on beamline I11 at the Diamond Light Source are described. Sample evolution in low and high gas pressure capillary cells (1 × 10-7 to 100 bar) with non-contact cooling and heating (80 to 1273 K) can be monitored structurally (X-rays) and spectroscopically (Raman). First results on the dehydration of MgSO4.7H2O, the formation of CO2 clathrate hydrate and the reaction of amorphous CaSiO3 grains with CO2 gas to form CaCO3 are presented to demonstrate the application of these cells to laboratory investigations involving the processing of cosmic dust simulants and planetary materials analogues.

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Abstract: Context. Variations in the 10 μm amorphous silicate band profile between different environments have been variously attributed to differences in composition, grain processing or macroscopic constitution (e.g. compact versus aggregate grains). However the relationship with mineralogical structure has remained poorly defined. Aims. The relationship between the 10 μm band and medium-range (~2–20 Å) structure is investigated. Methods. Synchrotron X-ray scattering, Fourier transform infrared and Raman spectroscopy are used to relate changes in medium-range structure to changes in the 10 μm band profile for amorphous MgSiO3 annealed at temperatures leading up to crystallisation. Results. Raman and X-ray data show a build up of strain within the silicate network, which is released between ~400–550 °C, causing a relaxation of both the Si-O-Si bond angle and Si-O bond length. Decomposing the 10 μm band at each temperature step shows SiO3 is the initially dominant component and increases in proportion as other species become incorporated into the silicate structure. However at ~400 °C the proportion of SiO3 decreases as species with greater numbers of non-bridging oxygens form. The coincidence with strain release implies a breakup of larger tetrahedral structures. Conclusions. We identify a correspondence between the spectral response at 10 μm of amorphous MgSiO3 and the evolution of its structural state at medium-range distances. The dependence of the 10 μm band on medium-range structure may account for the hitherto poor correlation between composition and band behaviour, variations in the 10 μm band observed in other silicates of similar compositions, or, between observed bands in different astronomical settings.

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Abstract: The production of amorphous calcium silicate, CaSiO3, by vacuum drying a sol–gel is described. The material exhibits a highly porous, irregular extended aggregate morphology. Its amorphous nature is confirmed by X-ray powder diffractometry and its silicate nature by vibrational spectroscopy. The latter is used to characterise the bridging/non-bridging oxygen distribution and hydration state, which comprises SiO3 bonded hydroxyl and interstitial water. No indicators of silicon hydride defects were observed. From Raman/FTIR spectroscopy, the Si-O tetrahedral species are predominantly peaked around SiO32?, as predicted by the stoichiometry of the starting materials. Variable temperature X-ray diffraction was used to measure the silicate's thermal expansion behaviour up to the point of crystallisation, where the mineral phase wollastonite was observed. Refined lattice parameters were in good agreement with standard values for wollastonite. The procedure as implemented does result in the formation of a small amount of poorly ordered carbonate, which can be removed by post-drying annealing. The carbonation in the presence of CO2 of the dried amorphous silicate was also investigated, showing the conversion to CaCO3 calcite and SiO2.

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Abstract: We present a laboratory study of crystallisation in three annealed amorphous Mg-silicates with decreasing Mg:Si, using IR spectroscopy and X-ray diffraction. Crystallisation of high Mg:Si silicate was accompanied by the development of features in both 10 and 20 ?m bands. Crystallisation was not observed for low Mg:Si, but features developed at 10 ?m while the 20 ?m band remained largely amorphous. At intermediate Mg:Si, amorphous structural rearrangement occurred prior to crystallisation, with features in the 10 ?m band developing ahead of the 20 ?m band. The spectroscopy results imply short-range ordering precedes long-range ordering, however X-ray diffraction on annealed samples showed crystallisation occurring first over long-ranges, with each silicate retaining shorter-range disorder. The discrepancy is resolved if the notion of 10 ?m band features being solely attributable to macroscopic crystallinity is abandoned. We argue such features, although present in crystalline silicates, arise initially from Si–O bond ordering, which precedes the formation of bulk crystalline structure. Astronomical observations at 10 ?m, in isolation, may therefore not be reliable indicators of the physical state of grains.