  We investigate the dual-dust chemistry (DDC) phenomenon in PNe and discuss
reasons for its occurrence, by analyzing Spitzer/IRS spectra of a sample of 40
Galactic PNe among which 26 belong to the Galactic Bulge (GB). The mixed
chemistry is derived from the simultaneous detection of PAH features in the
6-14 micron range and crystalline silicates (CS) beyond 20 microns in the
Spitzer/IRS spectra. Out of the 26 PNe observed in the GB, 21 show signatures
of DDC. Our observations reveal that the simultaneous presence of O- and C-rich
dust features in the IR spectra of [WC]-type PNe is not restricted to late/cool
[WC]-type stars, as previously suggested in the literature, but is a common
feature associated with all [WC]-type PNe. Surprisingly, we found that the DDC
is seen also in all observed wels, as well as in other PNe with central stars
being neither [WC] nor wels. Most sources observed display CS features in their
spectra, with only a few PNe exhibiting, in addition, amorphous silicate bands.
We appear to detect a recent change of chemistry at the end of the AGB
evolution in the low-mass, high-metallicity population of GB PNe observed. The
deficit of C-rich AGB stars in this environment suggests that the process of
PAH formation in PNe occurs at the very end of the AGB phase. In addition, the
population of low-mass, O-rich AGB stars in the GB, do not exhibit CS features
in their spectra. Thus, the high detection rate of DDC that we find cannot be
explained by long-lived O-rich (primordial or circumbinary) disks. Our most
plausible scenario is a final thermal pulse on the AGB (or just after), which
could produce enhanced mass loss, capable of removing/mixing (sometimes
completely) the remaining H-rich envelope and exposing the internal C-rich
layers, and generating shocks responsible for the silicate crystallization.
