  We study the dust present in 56 Galactic planetary nebulae (PNe) through
their iron depletion factors, their C/O abundance ratios (in 51 objects), and
the dust features that appear in their infrared spectra (for 33 objects). Our
sample objects have deep optical spectra of good quality, and most of them also
have ultraviolet observations. We use these observations to derive the iron
abundances and the C/O abundance ratios in a homogeneous way for all the
objects. We compile detections of infrared dust features from the literature
and we analyze the available Spitzer/IRS spectra. Most of the PNe have C/O
ratios below one and show crystalline silicates in their infrared spectra. The
PNe with silicates have C/O < 1, with the exception of Cn 1-5. Most of the PNe
with dust features related to C-rich environments (SiC or the 30 {\mu}m feature
usually associated to MgS) have C/O $\gtrsim$ 0.8. PAHs are detected over the
full range of C/O values, including 6 objects that also show silicates. Iron
abundances are low in all the objects, implying that more than 90% of their
iron atoms are deposited into dust grains. The range of iron depletions in the
sample covers about two orders of magnitude, and we find that the highest
depletion factors are found in C-rich objects with SiC or the 30 {\mu}m feature
in their infrared spectra, whereas some of the O-rich objects with silicates
show the lowest depletion factors.
