The high-resolution (R=600) Spitzer/IRS spectrum of the bipolar proto planetary nebula (PN) IRAS 17423-1755 is presented in order to clarify the dominant chemistry (C-rich versus O-rich) of its circumstellar envelope as well as to
constrain its evolutionary stage. The high quality Spitzer/IRS spectrum shows
weak 9.7 um absorption from amorphous silicates. This confirms for the first
time the O-rich nature of IRAS 17423-1755 in contradiction to a previous C-rich
classification, which was based on the wrong identification of the strong 3.1
um absorption feature seen in the Infrared Space Observatory (ISO) spectrum as
due to acetylene (C2H2). The high-resolution Spitzer/IRS spectrum displays a
complete lack of C-rich mid-IR features such as molecular absorption features
(e.g., 13.7 um C2H2, 14.0 um HCN, etc.) or the classical polycyclic aromatic
hydrocarbon infrared emission bands. Thus, the strong 3.1 um absorption band
toward IRAS 17423-1755 has to be identified as water ice. In addition, an [NeII]
nebular emission line at 12.8 um is clearly detected, indicating that the
ionization of its central region may be already started. The spectral energy
distribution in the infrared (2-200 um) and other observational properties of
IRAS 17423-1755 are discussed in comparison with the similar post-asymptotic
giant branch (AGB) objects IRAS 19343+2926 and IRAS 17393-2727. We conclude that
IRAS 17423-1755 is an O-rich high-mass post-AGB object that represents a link
between OH/IR stars with extreme outflows and highly bipolar PN.