2002ApJ...568L.131G

We have obtained infrared spectra of amorphous forsterite, Mg₂SiO₄, prepared in the presence of gaseous methane and find that hydrocarbon groups such as CH₂ and CH₃ can be incorporated in the silicate lattice. These chemical groups have little effect on the 10 µm silicate SiO stretching band but introduce a 3.4 µm band similar, but not identical, to that seen in pure carbonaceous deposits including hydrogenated amorphous carbon. Absorption in this band is enhanced in the 2958 and 2873 cm^–1 (3.38 and 3.48 µm) transitions of the CH₃ asymmetric and symmetric stretching vibrations, respectively. Additional absorption features identified with Si(CH₃)_1,2,3 and Si(CH₂)Si groups have also been observed, and a band corresponding to the Si-phenyl stretch has been detected. We discuss the possibility that hydrocarbon-silicate grains may be a component of interstellar matter, particularly in the winds from asymptotic giant branch stars with mixed carbon-silicate chemistry.