  We present a new diagnostic diagram for mid-infrared spectra of infrared
galaxies based on the equivalent width of the 6.2 micron PAH emission feature
and the strength of the 9.7 micron silicate feature. Based on the position in
this diagram we classify galaxies into 9 classes ranging from
continuum-dominated AGN hot dust spectra and PAH-dominated starburst spectra to
absorption-dominated spectra of deeply obscured galactic nuclei. We find that
galaxies are systematically distributed along two distinct branches: one of AGN
and starburst-dominated spectra and one of deeply obscured nuclei and
starburst-dominated spectra. The separation into two branches likely reflects a
fundamental difference in the dust geometry in the two sets of sources: clumpy
versus non-clumpy obscuration. Spectra of ULIRGs are found along the full
length of both branches, reflecting the diverse nature of the ULIRG family.
