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

We present a laboratory study of crystallisation in three annealed amorphous Mg-silicates with decreasing Mg:Si, using IR spectroscopy and X-ray diffraction. Crystallisation of high Mg:Si silicate was accompanied by the development of features in both 10 and 20 μm bands. Crystallisation was not observed for low Mg:Si, but features developed at 10 μm while the 20 μm band remained largely amorphous. At intermediate Mg:Si, amorphous structural rearrangement occurred prior to crystallisation, with features in the 10 μm band developing ahead of the 20 μm band. The spectroscopy results imply short-range ordering precedes long-range ordering, however X-ray diffraction on annealed samples showed crystallisation occurring first over long-ranges, with each silicate retaining shorter-range disorder. The discrepancy is resolved if the notion of 10 μm band features being solely attributable to macroscopic crystallinity is abandoned. We argue such features, although present in crystalline silicates, arise initially from Si-O bond ordering, which precedes the formation of bulk crystalline structure. Astronomical observations at 10 μm, in isolation, may therefore not be reliable indicators of the physical state of grains.