  Iron, the Universe's most abundant refractory element, is highly depleted in
both circumstellar and interstellar environments, meaning it exists in solid
form. The nature of this solid is unknown. In this Letter, we provide evidence
that metallic iron grains are present around oxygen-rich AGB stars, where it is
observationally manifest as a featureless mid-infrared excess. This
identification is made using Spitzer Space Telescope observations of evolved
globular cluster stars, where iron dust production appears ubiquitous and in
some cases can be modelled as the only observed dust product. In this context,
FeO is examined as the likely carrier for the 20-micron feature observed in
some of these stars. Metallic iron appears to be an important part of the dust
condensation sequence at low metallicity, and subsequently plays an influential
role in the interstellar medium. We explore the stellar metallicities and
luminosities at which iron formation is observed, and how the presence of iron
affects the outflow and its chemistry. The conditions under which iron can
provide sufficient opacity to drive a wind remain unclear.
