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We investigate Galactic bulge planetary nebulae without emission-line centralstars for which peculiar infrared spectra have been obtained with the SpitzerSpace Telescope, including the simultaneous signs of oxygen and carbon baseddust. Three separate sub-groups can be defined characterized by the differentchemical composition of the dust and the presence of crystalline and amorphoussilicates.  We find that the classification based on the dust properties is reflected inthe more general properties of these planetary nebulae. However, some observedproperties are difficult to relate to the common view of planetary nebulae. Inparticular, it is challenging to interpret the peculiar gas chemicalcomposition of many analyzed objects in the standard picture of the evolutionof planetary nebulae progenitors.  We confirm that the dual-dust chemistry phenomenon is not limited toplanetary nebulae with emission-line central stars.

New groups of planetary nebulae with peculiar dust chemistry towards the Galactic bulge