2012A&A...545A..60T

Variations in the 10 µm amorphous silicate band profile between different environments have been variously attributed to differences in composition, grain processing or macroscopic constitution (e.g. compact versus aggregate grains). However the relationship with mineralogical structure has remained poorly defined. The relationship between the 10µm band and medium-range (∼2-20Å) structure is investigated. Synchrotron X-ray scattering, Fourier transform infrared and Raman spectroscopy are used to relate changes in medium-range structure to changes in the 10µm band profile for amorphous MgSiO₃ annealed at temperatures leading up to crystallisation. Raman and X-ray data show a build up of strain within the silicate network, which is released between ∼400-550°C, causing a relaxation of both the Si-O-Si bond angle and Si-O bond length. Decomposing the 10µm band at each temperature step shows SiO₃ is the initially dominant component and increases in proportion as other species become incorporated into the silicate structure. However at ∼400°C the proportion of SiO₃ decreases as species with greater numbers of non-bridging oxygens form. The coincidence with strain release implies a breakup of larger tetrahedral structures. We identify a correspondence between the spectral response at 10µm of amorphous MgSiO₃ and the evolution of its structural state at medium-range distances. The dependence of the 10µm band on medium-range structure may account for the hitherto poor correlation between composition and band behaviour, variations in the 10µm band observed in other silicates of similar compositions, or, between observed bands in different astronomical settings.