A 3.4 micron band in magnesium silicates: Preparation of hydrocarbon-silicate condensates

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 mum silicate Si-O stretching band but introduce a 3.4 mum 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 mum) 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.