Crystallisation processes in cosmic silicates: Laboratory progress towards understanding structural–spectral relationships

Thompson, Stephen P.; Verrienti, C.; Fonti, S.; Orofino, V.; Blanco, A.

doi:10.1016/j.asr.2005.05.009

Cited by 10 publications

(15 citation statements)

References 70 publications

(74 reference statements)

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“…Making comparisons between published measurements of analogue samples produced by different methods has been a long-standing issue (Jäger et al 1994(Jäger et al , 2003Nuth et al 2002). The results presented here however have a high degree of parity with previously reported investigations (Thompson et al 1996(Thompson et al , 2002a(Thompson et al ,b, 2003(Thompson et al , 2007Thompson & Tang 2001). Sol-gel based silicates have also been used as analogues for circumstellar silicates in AGB stars by Jäger et al (2003) who found the gel method produces hydrogenated materials with chemical properties similar to those expected of cosmic silicates.…”

Section: Sample Manufacturecontrasting

confidence: 51%

“…Average Mg:Si and Si:O ratios for the three silicates are given in Table 1, based on previous EDX analysis (Thompson et al 2007) and are consistent, for example, with estimates of Mg:Si ratios for Comet Halley dust (Jessberger et al 1988;Lawler et al 1989). The average Mg:Si ratios for Products I and II are generally consistent with the enstatite endproduct predicted by reactions (11) and (12).…”

Section: Sample Manufacturesupporting

confidence: 48%

“…1) Product I crystallizes into a well formed phase identified as forsterite (Thompson & Tang 2001). At 1300 K additional diffraction features attributable to crystalline enstatite have also been found to form (Thompson et al 2007). Figure 2 shows powder diffraction patterns for Products II and III, both of which remain amorphous throughout the entire annealing range, although product II does show a change in the morphology of its powder pattern when annealed.…”

Section: Sample Manufacturementioning

confidence: 99%

“…Hallenbeck et al 1998;Nuth et al 2002) attempt to simulate, to any degree, the astrophysical conditions of grain formation. For the present work amorphous silicates were manufactured using three variations of a sol-gel process due to Sabatier (1950) and Day (1974), which has been described in detail elsewhere (Thompson et al 2007 …”

Section: Sample Manufacturementioning

confidence: 99%

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Structural signatures of medium-range order in annealed laboratory silicates

Thompson¹

2008

A&A

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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.

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Section: Sample Manufacturecontrasting

confidence: 51%

Section: Sample Manufacturesupporting

confidence: 48%

Section: Sample Manufacturementioning

confidence: 99%

Section: Sample Manufacturementioning

confidence: 99%

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Structural signatures of medium-range order in annealed laboratory silicates

Thompson¹

2008

A&A

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“…Furthermore, annealing experiments on fixed compositions (e.g. Brucato et al 1999Brucato et al , 2002Thompson et al 2002Thompson et al , 2007 show obvious changes to the 10 μm band profile, width and peak position, indicating changes to internal properties are also driven by external environmental conditions. In this paper we present laboratory results that relate the behaviour of the 10 μm band of a sample of amorphous MgSiO 3 , annealed at different temperatures, with changes occurring in its medium-range structure.…”

Section: The 10 μM Silicate Bandmentioning

confidence: 99%

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

Thompson

Parker

Tang

2012

A&A

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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 mediumrange structure to changes in the 10 μm band profile for amorphous MgSiO 3 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 SiO 3 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 SiO 3 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 MgSiO 3 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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Crystallization of amorphous silicates far from equilibrium part I: A versatile nitrate-based sol–gel synthesis of amorphous porous Ca,Mg-rich silicates

Gillot¹,

Roskosz²,

Leroux³

et al. 2011

Journal of Non-Crystalline Solids

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Crystallisation processes in cosmic silicates: Laboratory progress towards understanding structural–spectral relationships

Cited by 10 publications

References 70 publications

Structural signatures of medium-range order in annealed laboratory silicates

Structural signatures of medium-range order in annealed laboratory silicates

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

Crystallization of amorphous silicates far from equilibrium part I: A versatile nitrate-based sol–gel synthesis of amorphous porous Ca,Mg-rich silicates

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Crystallisation processes in cosmic silicates: Laboratory progress towards understanding structural–spectral relationships

Cited by 10 publications

References 70 publications

Structural signatures of medium-range order in annealed laboratory silicates

Structural signatures of medium-range order in annealed laboratory silicates

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

Crystallization of amorphous silicates far from equilibrium part I: A versatile nitrate-based sol–gel synthesis of amorphous porous Ca,Mg-rich silicates

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Product

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