Structural signatures of medium-range order in annealed laboratory silicates

Abstract

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 SiO 2 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.

XANES spectra of forsterite in crystal, surface, and amorphous states

“…The observed spectrum is consistent with those reported by Wu et al 9,10 and Thompson. 11 In the crystal phase, four characteristic peaks, denoted as A to D, 11 and a shoulder at 1308 eV are observed. The spectra of the amorphous phase are smeared, in particular, above 1320 eV.…”

Catalytic conversion of acetylene to polycyclic aromatic hydrocarbons over particles of pyroxene and alumina

“…Owing to the difference in structure of the samples used, we have not considered the size effect in this work. Prior to the Infrared Space Observatory, it was reported that silicates formed as disordered or amorphous grains in the outflows of O-rich stars at various stages of their evolution [32,33] and very likely that they are a precursor to the crystalline silicates [34]. Therefore, it is of interest to investigate the catalytic conversion of C 2 H 2 on the surface of amorphous enstatite grains.…”

The 10 μm band in amorphous MgSiO3: the influence of medium-range structure, defects and thermal processing

“…The annealing time at peak temperature for each sample was ∼17.5 ± 1 h. This method of sample preparation while obviously not mimicking the formation conditions of cosmic grains, has been found by us to consistently produce an amorphous silicate with reproducible properties providing good parity between different investigations. As such, the sample prepared for the present work is equivelent to the Product I silicate refered to in Thompson (2008), Thompson et al (2007) and the MgSiO 3 silicates in earlier works (Thompson et al 2002(Thompson et al , 2003Thompson & Tang 2001).…”

“…Previously (Thompson 2008), the presence and extent of medium-range structure in three amorphous Mg-silicates annealed over coarse temperature intervals was investigated using X-ray absorption near edge structure (XANES) spectroscopy and by X-ray scattering. The XANES data showed the growth of medium-range structure from 5 to 10 Å; while certain X-ray scattering features at low values of the X-ray scattering vector were identified with correlations between medium-range structures with differing tetrahedral connectivity.…”

“…above ∼1 Å −1 in Fig. 3) originates from the sample, while the high intensity undulator source allows scattering data to be collected with a much higher statistical quality than was available in the earlier Thompson (2008) study. Furthermore the low background, low noise signal means that the I11 instrument is much more sensitive to the presence of weakly scattering, trace level or crystallising phases (Thompson et al 2009).…”