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Crystallization processes in amorphous MgSi03 S.P. Thompson and C.C. Tang

Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK.

Structural changes during crystallization of annealed MgSiC>3 are investigated. The most important changes occur in the initial stages of annealing. Significant amounts of amorphous structure survive up to high temperatures limiting crystal development. Grains initially processed at lower temperatures will have had their amorphous component strengthened by the annealing process and could retain some structural signature of their pre-processed form.

1. introduction

Many main sequence stars show the Vega phenomenon whereby large amounts of refractory materials persist long after star formation. The disk once possessed by our Sun survives today as interplanetary debris (comets, dust, etc.), while the best known extrasolar debris disk is P Pic, whose lO^im band is similar to certain solar system comets in that both show a subfeature at 11.2|am attributed to crystalline olivine [1], This feature is also seen in a subclass of IDP's believed to have suffered extensive cosmic ray exposure in the ISM or proto-stellar nebula [2]. Crystalline olivine is perplexing as it is seen only in some comets and in the outflows of certain evolved stars where dust is condensing [3] but not in star forming regions or diffuse interstellar clouds [4], Crystalline cometary dust is also troublesome as comets are believed to be frozen reservoirs of the most primitive pre-solar materials. As crystalline olivine condenses at >1200K (typical of the inner solar nebula) it is possible that it could have been transported to the outer comet forming zones and mixed with ice grains. How this could have been accomplished is not known as crystalline features are not seen in star forming regions where inner nebula condensation would occur. Observations of evolved AGB stars show crystallites to be confined to systems with high mass loss rates, only constituting -10% of the total dust outflow [5], contrary to the expectation that more crystalline material should be observed in higher temperature shells. Alternatively, post-condensation annealing of amorphous grains could have occurred. However the annealing properties of silicates are poorly defined, with few laboratory results available [6,7], We present here a basic experimental study of annealing-induced crystallization. We have concentrated on Mg silicate because: no evidence for Fe rich dust around AGB stars exists [8]; laboratory spectra for Mg-rich silicates give good fits to cometary 11,2|am features [6,9] and in condensation experiments mixed Fe-Mg solids do not readily form [8],

2. experiments

An amorphous sample of MgSi03 was continuously annealed at various increasing temperatures (see §3 below). Simultaneous to this, X-ray powder diffraction (XRD) data were collected. As a long-range structural probe XRD is ideally suited to measuring structural modification during crystallization. The annealing furnace was mounted at the centre of rotation of the 2.3 diffractometer at Daresbury Laboratory's Synchrotron Radiation Source.

The furnace enclosure has kapton entrance/exit windows allowing data collection during annealing. Heating is via 2kW RF Cu coil and temperature is measured by thermocouple at the sample crucible. The operational range is 290K - 2000K, with heating response times of ~30s, while thermal stability (±1K) is achieved in under a few minutes (see [10,11]). Standard 0-20 XRD geometry was used, but the need to cover a wide angular range (~55000 points, Is per point) limits temporal resolution to ~1.6hrs per scan.

3. RESULTS

The sample was scanned at room temperature, then raised to 1000K and repeatedly scanned for 19.5hrs. During this time the XRD patterns showed the evolution of a crystallite. At the end of this period a developed crystalline diffraction pattern was visible, superposed upon an amorphous background that had changed little throughout. The crystalline component had formed after only ~2hrs and after ~5hrs had developed to its full extent. Neither component changed from then on. The temperature was increased to 103IK and the sample annealed for 24.5hrs. During this time no major changes in either phase occurred. Further processing for 24hrs at 1043K again produced no major change. The temperature was then increased in steps through 1083K, 1113K and 1143K over 5hrs. Again, no changes were observed. Finally raising the temperature to 1173K, annealing continued for a further 14.5hrs. After 5-6hrs at this temperature, the crystalline phase underwent further growth while the amorphous features diminished. Figure 1 shows XRD data for selected temperatures and times.

4. DISCUSSION

The evolution of our sample matches published results for the lOpim band [6] where initial annealing gave a band profile characteristic of astronomical silicate, followed by the development of a 1 l|im sub-feature. After this is a stall period with no further development. On exiting the stall full crystalline features develop. Changes in the 10}Am band originate in structural changes induced by annealing and are mirrored in our structural data. Our sample quickly entered a structural stall, only exiting after substantial annealing. The crystalline XRD pattern is matched by forsterite (Mg2Si04) [10]. If all available Mg is in the crystal phase then, given an MgSi03 composition, the amorphous phase will be Si02-rich. The permanency of this component is shown by the widths of the broad features at ~30°and -50° 20. At room temperature these were ~8.5° and ~6° 20. After ~2hrs at 1000K they shrank to ~4° each and remained constant until 1173K when the sample began to change. XRD feature widths reflect the disorder in the reflecting lattice layers. Some alteration to the amorphous phase thus occurred as the crystallite formed, but then showed strong resilience to higher temperatures.

A likely explanation is that, as well as promoting crystallization, annealing strengthens the amorphous phase. Laboratory and cosmic silicates will have significant hydroxyl content [12,13,14]: molecular hydrogen dissolves into silicate structures over a wide temperature range, reacting with Si02 to form hydroxyl, or hydroxyl/hydride pairs [15,16]. Network bonds can also break by ionization or direct displacement and react with void space H to form Si-OH or Si-H [17], Hydroxyl and hydride are removed from amorphous Si02 at >973K and completely removed over a period of hours [18], Plotting the percentage of hydroxyl/hydride removed as a function of time, the data for each fall on the same curve implying that the effective diffusion coefficients are the same for both. The recombination process is [18]

Thermal Evolution of MgSi03

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