  The origin and depletion of iron differ from all other abundant refractory
elements that make up the composition of the interstellar dust. Iron is
primarily synthesized in Type Ia supernovae (SNe Ia) and in core collapse
supernovae (CCSN), and is present in the outflows from AGB stars. Only the
latter two are observed to be sources of interstellar dust, since searches for
dust in SN~Ia have provided strong evidence for the absence of any significant
mass of dust in their ejecta. Consequently, more than 65% of the iron is
injected into the ISM in gaseous form. Yet, ultraviolet and X-ray observations
along many lines of sight in the ISM show that iron is severely depleted in the
gas phase compared to expected solar abundances. The missing iron, comprising
about 90% of the total, is believed to be locked up in interstellar dust. This
suggests that most of the missing iron must have precipitated from the ISM gas
by cold accretion onto preexisting silicate, carbon, or composite grains. Iron
is thus the only element that requires most of its growth to occur outside the
traditional stellar condensation sources. This is a robust statement that does
not depend on our evolving understanding of the dust destruction efficiency in
the ISM. Reconciling the physical, optical, and chemical properties of such
composite grains with their many observational manifestations is a major
challenge for understanding the nature and origin of interstellar dust.
