oxygen is more abundant than carbon, all the carbon atoms will be trapped by CO and the remaining oxygen atoms will form O-rich grains like silicates and oxides.

Oxygen rich nebulae could have iron deposited in metallic iron grains, silicates and oxides, whereas C-rich nebulae are expected to have their iron atoms in the form of metallic grains, Fe$_3$C, FeSi, FeS, and FeS$_2$ [@Whittet_03]. Therefore, iron depletion factors could be different in C-rich and O-rich PNe. The C-rich or O-rich chemistry may be studied from the C/O abundance ratios or from the infrared dust features, and we expect both to be related. In principle, C-rich features such as SiC are expected in PNe with C/O $>1$ [but see @Bond_10] whereas O-rich features like silicates are expected in PNe with C/O $<1$. [*ISO*]{} and [*Spitzer*]{} telescopes allow us to detect and identify several dust species in the spectra of PNe and therefore, to classify PNe as C-rich or O-rich according to the observed features.

Here, we extend the work performed in @DelgadoInglada_09 by studying a sample of Galactic PNe that nearly doubles the previous one and also by analyzing possible correlations between their iron depletions, C/O abundance ratios, and infrared dust 