Metals in the interstellar medium ( ISM ) of essentially all types of galaxies are observed to be depleted compared to the expected values .
The depletion is most likely due to dust condensation in , for example , cold molecular clouds and various circumstellar and interstellar environments .
The relative observed metal abundances should thereby reflect the composition of the ISM dust components .
We aim at identifying the most dominant dust species or types , including silicate and iron oxide grains present in the ISM , using recent observations of dust depletion of galaxies at various evolutionary stages .
We use the observed elemental abundances in dust of several metals ( O , S , Si , Mg , and Fe ) in different environments , considering systems with different metallicities and dust content , namely damped Lyman- \alpha absorbers ( DLAs ) towards quasars and the Galaxy .
We derive a possible dust composition by computationally finding the statistically expected elemental abundances in dust assuming a set of key dust species with the iron content as a free parameter .
Carbonaceous dust is not considered in the present study .
Metallic iron ( likely in the form of inclusions in silicate grains ) and iron oxides are important components of the mass composition of carbon-free dust .
The latter make up a significant mass fraction ( \sim 1 / 4 in some cases ) of the oxygen-bearing dust and there are good reasons to believe that metallic iron constitutes a similar mass fraction of dust .
Wüstite ( FeO ) could be a simple explanation for the depletion of iron and oxygen because it is easily formed .
There appears to be no silicate species clearly dominating the silicate mass , but rather a mix of iron-poor as well as iron-rich olivine and pyroxene .
To what extent sulphur depletion is due to sulfides remains unclear .
In general , there seems to be little evolution of the dust composition ( not considering carbonaceous dust ) from low-metallicity systems to the Galaxy .