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Astron. Astrophys. 364, 282-292 (2000)

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6. Conclusions

As a result, rapid heating to temperatures exceeding 1100 K would be sufficient to recrystallize micrometre-sized glassy particles in a few minutes. In the history of the solar nebulae, a flash-heating event might have happened to the pre-chondrules. Because rapid cooling hindered complete crystallization, silicates both in the crystalline but also in the glassy state are observed in chondrules.

At a temperature of 1000 K, differently-sized amorphous magnesium silicates can be transformed into the polycrystalline state in the timescale of hours to days. According to the grain size and the kind of the amorphous material (smoke or glass), forsterite, tridymite and silica or enstatite have been formed. In detail, bulk and micrometre-sized [FORMULA] glass evolved into orthoenstatite; nanometre-sized smokes changed into forsterite and silica.

For smoke-like nanoparticles, annealing times and activation energies have been determined (see Table 5). We found that [FORMULA] strongly depends on the Mg/Si ratio. It varies from 39100 K for [FORMULA] smoke and 42040 K for [FORMULA] smoke to 49190 K for pure silica particles and determines the annealing times of the compositional different nanoparticles. For [FORMULA] glass particles, a mean value of the activation energy could not be obtained. We indicated that the constant [FORMULA] that defines the value of [FORMULA] in Eq. (1) has most probably to be determined independently.


[TABLE]

Table 5. Activation energies


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© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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