Laboratory study of annealed amorphous MgSiO$_{\sf 3}$ silicate using IR spectroscopy and synchrotron X-ray diffraction

Abstract

We present the results of combining in situ high resolution synchrotron X-ray powder diffraction and infrared spectroscopic measurements on an amorphous pyroxene powder sample annealed in the region of 1000 K. We find using both techniques that the crystalline structure formed during annealing is Mg2SiO4 (forsterite), but that the presence of certain features in the 10 μm band normally attributed to crystalline enstatite (MgSiO3) is contradicted by spectroscopy in the 20 μm region (along with certain other 10 μm band features) and X-ray diffraction. Both indicate crystalline forsterite as the only crystalline phase formed at this temperature. We discuss the possible mechanism of forsterite formation from amorphous pyroxene and identify the presence of proto-forsteritic structures in the amorphous starting material. We suggest the likely origin of the 10 μm band “crystalline enstatite” features as being due to short-range improvements in the amorphous MgSiO3 network ordering which are not necessarily accompanied by the formation of crystalline enstatite structure. These results not only suggest that the formation of crystalline enstatite dust grains via annealing may be difficult to realise at this temperature, but also highlight the possibility, in the absence of additional corroborating evidence, of misidentifying the nature of the carrier of the 10 μm “enstatite” features when observed in the spectra of objects such as comets. We also discuss the evolution of fine structure in the region of 15 to 16 μm, which may serve as an observational indicator of grain processing in stellar sources.