WHAT ARE THE TRUE ASTRONOMICAL SILICATES

We redefine the term ''astronomical silicate'' by interpreting the 9.7 mu m and 18 mu m interstellar features not in terms of-absorption by a pure silicate but rather in terms of silicate core-organic refractor y mantle particles. The silicate core optical properties used are thos e of a glassy (amorphous) olivine produced in the laboratory. The opti cal constants of the organic refractory mantle material are derived fr om a combination of astronomical and laboratory spectra of residues of ultraviolet photoprocessed ices, which we define as ''astronomical or ganic refractory''. The silicate core-organic refractory mantle elonga ted dust model provides an excellent match to the observation of the B ecklin-Neugebauer object polarization features in shape, width, and in relative strength. An additional ice mantle is included on a fraction of the dust in the line of sight to the BN object which is consistent with the 3.1 mu m absorption strength. This further improves the matc h to observations.