5 Conclusions

Both the XRD data and corresponding IR spectra at 20 $\mu$m confirmed the formation of forsterite in the annealed pyroxene samples. Both sets of data also showed that crystallisation was incomplete. The evolution of the 20 $\mu$m band, even at 1000 K, tended to show more variation than the XRD data, suggesting that IR spectroscopy is perhaps more sensitive to small scale variations than can be determined from XRD, which gives a bulk snap-shot of the average sample structure, whereby small amounts of variation can be averaged out by the rest of the sample. This difference can be understood by considering the nature of the two techniques. XRD is a probe of long-range configurational correlations, whilst IR bands are sensitive to changes in the lattice dynamics of the individual SiO_n units. Although XRD is invaluable as a structural fingerprint, its apparent insensitivity relative to the 10 $\mu$m band suggests the changes revealed by IR spectroscopy in this band are not structurally correlated over long length scales and indeed may even be to some degree insensitive to the specific crystallographic silicate structure. However, even if IR spectroscopy is in some instances less indicative of the overall structural arrangement, we must bear in mind that so far this is the only astronomical tool available with which to probe the in situ changes apparent in cometary grains. We conclude therefore that in the laboratory such combined techniques are particularly useful in studying the possible processes to which grains are subjected, since experiments designed to explicitly reproduce only the astronomical data are unlikely to reveal the full picture behind grain histories as the specific parameter space explored by them is unlikely to be enough on its own to allow unique fits or constraints to be found.

In conclusion, we have presented the results of annealing experiments on an initially amorphous MgSiO₃ silicate system as its structure changes from amorphous to crystalline. Using the combined techniques of synchrotron X-ray powder diffraction and IR spectroscopy we have shown that this particular system can crystallise on a local scale to a forsterite (Mg₂SiO₄) structure. However, we have also identified the presence in the crystalline IR spectra of our sample certain 10 $\mu$m band features usually attributed to the presence of crystalline enstatite (MgSiO₃), a phase that appears to be absent from our annealed sample when viewed using XRD. This raises the possibility that, in the absence of corroborating features at other wavelengths, the detection of 10 $\mu$m band "crystalline enstatite'' features in comets and other objects may not be enough to definitively identify the presence of crystalline enstatite grains. Such features may originate in improvements to the short-range ordering of the amorphous MgSiO₃ grain components as a result of the grains having been annealed and does not necessarily require the formation of bulk crystalline enstatite. These findings could have potentially significant consequences for the reconstruction of the thermal history of comets along with implications for our understanding of the evolution of the solar system.

Acknowledgements

This work has been partially supported by the Italian Space Agency (ASI) and the Italian Ministry of University and Research (MIUR). The authors would also like to express their thanks to J. A. Nuth for his helpfull and constructive comments during the refereeing of this paper.