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*Context The evolution of young massive protoplanetary disks toward planetarysystems is expected to include the formation of gaps and the depletion of dustand gas. *Aims A special group of flaring disks around Herbig Ae/Be stars donot show prominent silicate emission features. We focus our attention on fourkey Herbig Ae/Be stars to understand the structural properties responsible forthe absence of silicate feature emission. *Methods We investigate Q- and N-bandimages taken with Subaru/COMICS, Gemini South/T-ReCS and VLT/VISIR. Ourradiative transfer modeling solutions require a separation of inner- and outer-disks by a large gap. From this we characterize the radial density structure ofdust and PAHs in the disk. *Results The inner edge of the outer disk has a highsurface brightness and a typical temperature between ~100-150 K and thereforedominates the emission in the Q-band. We derive radii of the inner edge of theouter disk of 34, 23, 30 and 63 AU for HD97048, HD169142, HD135344B and Oph IRS48 respectively. For HD97048 this is the first detection of a disk gap. Thecontinuum emission in the N-band is not due to emission in the wings of PAHs.This continuum emission can be due to VSGs or to thermal emission from theinner disk. We find that PAH emission is not always dominated by PAHs on thesurface of the outer disk. *Conclusions. The absence of silicate emissionfeatures is due to the presence of large gaps in the critical temperatureregime. Many, if not all Herbig disks with Spectral Energy Distribution (SED)classification `group I' are disks with large gaps and can be characterized as(pre-) transitional. An evolutionary path from the observed group I to theobserved group II sources seems no longer likely. Instead, both might derivefrom a common ancestor.

Identifying gaps in flaring Herbig Ae/Be disks using spatially resolved mid-infrared imaging. Are all group I disks transitional?