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The interstellar abundances of refractory elements indicate a substantialdepletion from the gas phase, that increases with gas density. Our recent modelof dust evolution, based on hydrodynamic simulations of the lifecycle of giantmolecular clouds (GMCs) proves that the observed trend for [Si$_{gas}$/H] isdriven by a combination of dust growth by accretion in the cold diffuseinterstellar medium (ISM) and efficient destruction by supernova (SN) shocks(Zhukovska et al. 2016). With an analytic model of dust evolution, wedemonstrate that even with optimistic assumptions for the dust input from starsand without destruction of grains by SNe it is impossible to match the observed[Si$_{gas}$/H]$-n_H$ relation without growth in the ISM. We extend theframework developed in our previous work for silicates to include the evolutionof iron grains and address a long-standing conundrum: ``Where is theinterstellar iron?'. Much higher depletion of Fe in the warm neutral mediumcompared to Si is reproduced by the models, in which a large fraction ofinterstellar iron (70%) is locked as inclusions in silicate grains, where it isprotected from sputtering by SN shocks. The slope of the observed[Fe$_{gas}$/H]$-n_H$ relation is reproduced if the remaining depleted ironresides in a population of metallic iron nanoparticles with sizes in the rangeof 1-10nm. Enhanced collision rates due to the Coulomb focusing are importantfor both silicate and iron dust models to match the observed slopes of therelations between depletion and density and the magnitudes of depletion at highdensity.

Iron and silicate dust growth in the Galactic interstellar medium: clues from element depletions