Structural signatures of medium-range order in annealed laboratory silicates

Context. Silicate dust grains exist in a wide range of astronomical environments and understanding the effect of these on grain structure is of great interest, particularly the effect of thermal annealing on amorphous silicates as a possible route to the formation of crystalline grains. Although laboratory simulations have largely focussed on IR-spectroscopic measurements, since these relate directly to observational data, laboratory synchrotron techniques such as X-ray absorption spectroscopy (XAS) and X-ray scattering are becoming increasingly routine in the analysis of recovered materials. With the increasing prospect of performing astronomical XAS observations, there is much to be gained from applying these techniques to laboratory analogues. Aims. Diagnostic markers for medium-range order and the effect of thermal annealing on these in amorphous silicates of differing Mg content are characterised using synchrotron X-ray methods. Methods. Three synthetic amorphous silicates with high, medium and low Mg:Si ratios were annealed at varying temperatures up to 1300 K. X-ray Absorption Near Edge Structure (XANES) spectra at the Si and Mg K-shell absorption edges and X-ray scattering patterns for low values of the X-ray scattering wavevector were recorded for these along with comparative data for commercially produced amorphous SiO2 and mineral samples of forsterite, enstatite and quartz. Results. XANES features due to short- and medium-range structure are identified at both Mg and Si edges and a new temperature dependent behaviour observed in the medium-range structure surrounding Mg and Si. Based on changes to the morphological details of the XANES spectra, the medium-range structure changes between an enstatite-like and forsterite-like coordination with increasing temperature and appears to correlate directly with Mg content. Low wavevector X-ray scattering features were also found to be diagnostic of the type of medium-range structural ordering. However, these features depend on whether the relative arrangement of clusters of medium-range structure exhibit semi-periodic ordering over the longer-range, which can vary with annealing