The chemical composition of the silicate dust around RAFGL7009S and IRAS 19110+1045

We have studied the chemical composition of the silicates around the two ma ssive protostellar objects RAFGL7009S (IRAS 18316-0602) and IRAS 19110+1045 . The silicate features of both sources are similar. The position and shape of the 9.6 and 18 mu m silicate bands coincide with those of amorphous pyr oxene-type silicates. We find that the silicate features cannot be reproduc ed with compact pyroxene grains containing magnesium and iron. An admixture of iron, aluminium and some degree of porosity is necessary to reproduce r espectively the red wing of the 18 mu m band, the plateau between the two s ilicate bands and the widths of the bands. The iron is to be found in the f orm of Fe-rich pyroxenes, also containing small amounts of calcium, and in iron-oxides. Better fits are obtained with aluminium incorporated into the silicate structure (in aluminosilicates) than for aluminium in oxides such as Al2O3 The compositions we propose are in agreement with cosmic elemental abundance constraints. No crystalline material is observed in either sourc e and we find that at most 1-2% of the silicate mass could be crystalline i n these sources.