Laboratory investigation of crystallisation in annealed amorphous MgSiO$\mathsf{_{3}}$

Thompson, Stephen P.; Tang, Chiu C.

doi:10.1051/0004-6361:20000576

Cited by 29 publications

(32 citation statements)

References 39 publications

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“…Our amorphous starting material was produced by gel desiccation (Sabatier 1950;Day 1974Day , 1976a, which has the advantage of yielding a silicate with an evenly dispersed composition and a well established Si-O tetrahedral environment. Samples produced by this method were also among the earliest laboratory silicates studied from an astronomical view point (Day 1974(Day , 1976aSteel andDuley 1987, Thompson, Evans, andJones 1996;Thompson and Tang 2001). A hydrated solid precipitate is formed in solution according to the following basic reaction,…”

Section: Experimental Details Sample Manufacture and Preparationmentioning

confidence: 99%

Crystalline comet dust: Laboratory experiments on a simple silicate system

Thompson

Fonti

Verrienti

et al. 2003

Meteorit & Planetary Scien

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Abstract-Spectra for certain comets show the presence of crystalline silicate dust grains believed to have been incorporated during comet formation. While grain crystallization is widely assumed to result from the thermal annealing of precursor amorphous grains, the physical processes behind the silicate amorphous-to-crystalline transition are poorly understood. This makes it difficult to place constraints on the evolutionary histories of both grains and comets, and consequently, on the nebular conditions in which they formed. It has, therefore, become necessary to study this process in the laboratory using simulated grain materials. In this paper, we discuss recent results from laboratory investigations into a basic amorphous MgSiO 3 silicate annealed in the region of 1000 K. Our object is not to model the behavior of dust grains per se, but to study the underlying process of crystallization and separate the physics of the material from the astrophysics of dust grains. In our experiments, we bring together spectroscopic measurements made in the infrared with the high resolution structural probing capabilities of synchrotron X-ray powder diffraction. The combined use of these complementary techniques provides insights into the crystallization process that would not be easily obtained if each was used in isolation. In particular, we focus on the extent to which the identification of certain spectral features attributed to crystalline phases extends to the physical structure of the grain material itself. Specifically, we have identified several key features in the way amorphous MgSiO 3 behaves when annealed. Rather than crystallize directly to enstatite (MgSiO 3 ) structures, in crystallographic terms, amorphous MgSiO 3 can enter a mixed phase of crystalline forsterite (Mg 2 SiO 4 ) and SiO 2 -rich amorphous silicate where structural evolution appears to stall. Spectroscopically, the evolution of the 10 µm band does not appear to correlate directly with structural evolution, and therefore, may be a poor indicator of the degree of crystallinity. Indeed, certain features in this band may not be indicators of crystal type. However, the 20 µm band is found to be a good indicator of crystal structure. We suggest that forsterite forms from the ordering of pre-existing regions rich in SiO 4 and that this phase separation is aided by a dehydrogenation processes that results in the evolutionary stall. The implications of this work regarding future observations of comets are discussed.

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Section: Experimental Details Sample Manufacture and Preparationmentioning

confidence: 99%

Crystalline comet dust: Laboratory experiments on a simple silicate system

Thompson

Fonti

Verrienti

et al. 2003

Meteorit & Planetary Scien

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“…Laser vaporization can produce similarly amorphous materials (quenched on a microsecond timescale), but also produces mineral grain fragments and glasses via shock interactions with the target. Finally, gel dessication techniques (Thompson and Tang, 2001) start with tetrahedrally coordinated silicate ions in solution, and precipitate a gel with no longer range order. Each of these materials might be expected to behave somewhat differently.…”

Section: Introductionmentioning

confidence: 99%

Condensation processes in astrophysical environments: The composition and structure of cometary grains

Nuth

Rietmeijer

Hill

2002

Meteorit & Planetary Scien

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Abstract-We review the results ofour recent experimental studies ofastrophysical dust analogs. We discuss the condensation of amorphous silicates from mixed metal vapors, including evidence that such condensates form with metastable eutectic compositions. We consider the spectral evolution of amorphous magnesium silicate condensates as a function oftime and temperature. Magnesium silicate smokes anneal readily at temperatures of about 1000-1100 K. In contrast we find that iron silicates require much higher temperatures (--1300 K) to bring about similar changes on the same timescale (days to months). We first apply these results to infrared space observatory observations ofcrystalline magnesium silicate grains around high-mass-outflow asymptotic giant branch stars in order to demonstrate their general utility in a rather simple environment. Finally, we apply these experimental results to infrared observations ofcomets and proto stars in order to derive some interesting conclusions regarding large-scale nebular dynamics, the natural production of organic molecules in protostellar nebulae, and the use of crystalline magnesium silicates as a relative indicator of a comet's formation age.

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“…These differences of production can ultimately lead to a difference in physical behaviour. As noted previously (Thompson & Tang 2001), samples such as vapour condensed silicate may require substatially higher annealing temperatures in order to crystallise. However cometary grains are likely to have suffered post-formation processing in stellar atmospheres, the interstellar medium and pre-solar nebular prior to incorporation into comet bodies and thus may not be best modelled by samples more representative of freshly nucleated dust grains.…”

Section: Structural Change and The 10 µM Bandmentioning

confidence: 83%

“…Previously reported XRD annealing results (Thompson & Tang 2001) show the forsterite structure formed in this MgSiO 3 powder persists as the prominent crystal phase at least up to 1173 K, whether further annealing at this, or higher, temperatures would eventually produce a phase change resulting in the expected enstatite pyroxene structure is currently unknown. The formation of macroscopic crystalline pyroxene structures from amorphous grains could in fact be harder to realise than their composition alone suggests.…”

Section: Structural Change and The 10 µM Bandmentioning

confidence: 85%

“…arrangements of SiO n 4 ). This was suggested (Thompson & Tang 2001) as being due to the non-forsteritic amorphous arrangements being strengthened by the annealing process itself via an increase in the polymerisation of the amorphous component. One proposed mechanism by which this could occur is through annealing induced dehydrogenation of the silicate.…”

Section: X-ray Diffractionmentioning

confidence: 99%

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Laboratory study of annealed amorphous MgSiO$_{\sf 3}$ silicate using IR spectroscopy and synchrotron X-ray diffraction

Thompson

Fonti

Verrienti

et al. 2002

A&A

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Abstract. We present the results of combining in situ high resolution synchrotron X-ray powder diffraction and infrared spectroscopic measurements on an amorphous pyroxene powder sample annealed in the region of 1000 K. We find using both techniques that the crystalline structure formed during annealing is Mg 2 SiO 4 (forsterite), but that the presence of certain features in the 10 µm band normally attributed to crystalline enstatite (MgSiO 3 ) is contradicted by spectroscopy in the 20 µm region (along with certain other 10 µm band features) and X-ray diffraction. Both indicate crystalline forsterite as the only crystalline phase formed at this temperature. We discuss the possible mechanism of forsterite formation from amorphous pyroxene and identify the presence of proto-forsteritic structures in the amorphous starting material. We suggest the likely origin of the 10 µm band "crystalline enstatite" features as being due to short-range improvements in the amorphous MgSiO 3 network ordering which are not necessarily accompanied by the formation of crystalline enstatite structure. These results not only suggest that the formation of crystalline enstatite dust grains via annealing may be difficult to realise at this temperature, but also highlight the possibility, in the absence of additional corroborating evidence, of misidentifying the nature of the carrier of the 10 µm "enstatite" features when observed in the spectra of objects such as comets. We also discuss the evolution of fine structure in the region of 15 to 16 µm, which may serve as an observational indicator of grain processing in stellar sources.

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Laboratory investigation of crystallisation in annealed amorphous MgSiO$\mathsf{_{3}}$

Cited by 29 publications

References 39 publications

Crystalline comet dust: Laboratory experiments on a simple silicate system

Crystalline comet dust: Laboratory experiments on a simple silicate system

Condensation processes in astrophysical environments: The composition and structure of cometary grains

Laboratory study of annealed amorphous MgSiO$_{\sf 3}$ silicate using IR spectroscopy and synchrotron X-ray diffraction

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