As part of an investigation of cosmic grains, we have measured the critical partial pressure of SiO necessary to initiate nucleation from SiO-H2 and Mg-SiO-H2 vapors as a function of the temperature of the ambient gas (750 ¿K2 system, it does not effect the onset of nucleation at higher temperatures. The infrared spectra of particles collected from our system, and of those particles which have subsequently been annealed, are quite similar to spectra of the circumstellar shells of oxygen rich stars. To a lesser extent, the spectra of these particles also resemble the spectra of some individual chondritic stratospheric dust particles and the matrix material of some carbonaceous chondrites. Determination of the temperature dependent rate of recrystallization in our synthetic materials could elucidate the temperature histories of similar 'cosmic' grains.