  Observational evidence seems to indicate that the depletion of interstellar
carbon into dust shows rather wide variations and that carbon undergoes rather
rapid recycling in the interstellar medium (ISM). Small hydrocarbon grains are
processed in photo-dissociation regions by UV photons, by ion and electron
collisions in interstellar shock waves and by cosmic rays. A significant
fraction of hydrocarbon dust must therefore be re-formed by accretion in the
dense, molecular ISM. A new dust model (Jones et al., Astron. Astrophys., 2013,
558, A62) shows that variations in the dust observables in the diffuse
interstellar medium (nH = 1000 cm^3), can be explained by systematic and
environmentally-driven changes in the small hydrocarbon grain population. Here
we explore the consequences of gas-phase carbon accretion onto the surfaces of
grains in the transition regions between the diffuse ISM and molecular clouds
(e.g., Jones, Astron. Astrophys., 2013, 555, A39). We find that significant
carbonaceous dust re-processing and/or mantle accretion can occur in the outer
regions of molecular clouds and that this dust will have significantly
different optical properties from the dust in the adjacent diffuse ISM. We
conclude that the (re-)processing and cycling of carbon into and out of dust is
perhaps the key to advancing our understanding of dust evolution in the ISM.
