Sketch showing the region where (sub-) micron-sized silicate grains have a temperature (T ~ 200–400 K) where they produce strong silicate features. Transitional group Ia sources (top) show silicate features, because the inner disk and/or the outer disk wall have a sufficient amount of silicate dust in this region. In the group Ib objects of our sample (bottom), the silicate dust is severely depleted in this region, and thus, no silicate features are produced.