  Context: We have analysed Herschel-PACS spectra of 32 circumstellar disks
around Herbig Ae/Be and T-Tauri stars obtained within the Herschel key
programme DIGIT. In this paper we focus on the 69mu emission band of the
crystalline silicate forsterite.
  Aims: This work provides an overview of the 69mu forsterite bands in the
DIGIT sample. We aim to derive the temperature and composition of the
forsterite grains. With this information, constraints can be placed on the
spatial distribution of the forsterite in the disk and its formation history.
  Methods: Position and shape of the 69mu band are used to derive the
temperature and composition of the dust by comparison to laboratory spectra of
that band. We combine our data with existing Spitzer IRS spectra to compare the
presence and strength of the 69mu band to the forsterite bands at shorter
wavelengths.
  Results: A total of 32 sources have been observed, 8 of them show a 69mu
emission band that can be attributed to forsterite. With the exception of the
T-Tauri star AS205, all of the detections are for disks associated with Herbig
Ae/Be stars. Most of the forsterite grains that give rise to the 69mu bands are
warm (~100-200 K) and iron-poor (less than ~2% iron). Only AB-Aur requires
approximately 3-4% of iron.
  Conclusions: Our findings support the hypothesis that the forsterite grains
form through an equilibrium condensation process at high temperatures. The
connection between the strength of the 69 and 33mu bands shows that at least
part of the emission in these bands originates from the same dust grains.
Further, any model that explains the PACS and the Spitzer IRS observations must
take the effects of a wavelength dependent optical depth into account. We find
indications of a correlation of the detection rate of the 69mu band with the
spectral type of the host stars. However, our sample is too small to obtain a
definitive result.
