Aerodynamic levitation and CO2 laser melting have been used to synthesize the yttrium aluminosilicate glasses zY(2)O(3)-yAl(2)O(3)-xSiO(2) with z/y = 3/5 corresponding to the YAG (Y3Al5O12) composition and x between similar to 5 and similar to 45 mol%. The low-and high-density (LDA inclusion and HDA matrix) polyamorphic phases in glasses with less than similar to 14 mol% SiO2 were identified with backscattering electron imaging. Polarized and depolarized Raman spectra show the formation of various Q(n) SiO4 species whose relative populations change smoothly as the SiO2 content is altered. The AlOs (s = 4-6) and YOz (z = 6-9) polyhedra formed in the YAG glass are preserved upon silica additions while the terminal oxygens of the Q(2)AlO(4) tetrahedra are gradually bridged to the Q(n)-SiO4 species. The low-frequency Boson Peak overlaps with the vibrational spectrum and its maximum is redshifted with increasing silica content. Micro-Raman spectra measured for the LDA and HDA amorphous phases are found to be similar to the spectra of the bulk glass indicating common structural characteristics. The stability of the LDA phase against crystallization appears to be lower than that of the HDA phase. The crystallinity on certain inclusions consisted of YAG microcrystals and a new unidentified microcrystalline phase within Y4Al2(1-x)Si2xO(9+x) solid solution.