are located in grains. We also found that the range of iron depletions covers about two orders of magnitude, which may be a consequence of differences in the formation and evolution of the grains in the different nebulae. We did not find any obvious correlation between iron abundances and parameters related to the nebular age (such as the nebular radius or the surface brightness) or to the dominant chemistry in the nebulae (C-rich or O-rich) that could provide clues on the reasons behind the different depletions. However, the information available for the studied sample was scarce.

On the other hand, the dust features that appear in the infrared spectra of PNe provide information on the composition of their dust grains. In our galaxy, PNe with crystalline silicates, many of them also with PAHs, are the most numerous [@Gutenkunst_08; @PC_09; @Stanghellini_12]. Besides, a general correlation has been found between infrared features and C/O abundance ratios: Magellanic Clouds PNe with silicates have C/O $<1$, and those with carbonaceous dust have C/O $>1$, although for Galactic PNe the situation is less clear [@Casassus_01a; @Cohen_05; @Stanghellini_07; @Waters_98].

The relative abundances of carbon and oxygen in the envelope of an AGB star depends on the 