tokens: 2003 ). Its depletion covers a relatively narrow range of values ( Fig. 1 ) showing that it is not a strong function of environmental density. It has been hypothesized that the majority of carbon should be locked in graphite grains, as a likely explanation for the 2175 Å emission feature ( Draine 1989 , 2003 , and references therein). Graphite has been commonly adopted in ID models (e.g., Mathis et al. 1977 ). However, observational evidence pointed out that graphite cannot explain the variability of the 2175 Å feature ( Fitzpatrick Massa 2007 ). Furthermore, in analogy with the silicates, which are found to be amorphous, also graphite was deemed unlikely to survive in large quantities in the harsh environment of the ISM. Graphite should therefore face a natural process of amorphization ( Compiègne et al. 2011 ). The idea of carbon as a single and separate phase from the silicate population does not agree with a scenario of a constantly evolving and mixing medium ( Jones et al. 2017 ). Hydrogenated amorphous carbon (HAC) may indeed coat the silicate grains, forming a single population (e.g., Duley et al. 1989 ) with different characteristics with respect to the environment in which they reside and depending on the particle size (e.g., Jones et al. 2017 , and references therein). Polarization studies however have so far not confirmed this scenario. The carbon feature at 3.4 μ m shows a negligible degree of polarization with respect to the silicate feature at 9.7 μ m, pointing to two distinct grain populations ( Whittet 2011 ). Finally, under special condition of high pressure, for instance in a shocked environment, graphite and amorphous carbon can turn into nano-diamonds, which can constitute as much as 5% of the amount of C in the ISM ( Tielens et al. 1987 ), possibly with H and N inclusion ( Van Kerckhoven et al. 2002 ; Bilalbegović et al. 2018 ). Diamonds of possible ISM origin have been found in meteorites ( Lewis et al. 1987 ). An important carbon carrier are polycyclic aromatic hydrocarbons (PAH), large molecules (Ångstrom-sized) formed by carbon and hydrogen in a honeycomb structure. These molecules constitute up to about 10% of the carbon abundance (e.g., Tielens 2013 ). PAHs are very sensitive to ionizing radiation from far-ultraviolet to X-rays and they are easily destroyed near star formation sites at AU distance scale (e.g., Siebenmorgen Krügel 2010 ), up to kiloparsec scale, for active galaxies ( Voit 1992 ). Apart from C, other constituents can be studied in detail by future generation telescopes. Sulfur in the dust phase seems to be absent from the diffuse ISM ( Sembach Savage 1996 ). However, a relative fast transition to a depletion approaching −1 dex is reported in dense media, such as molecular clouds ( Joseph et al. 1986 ). In molecular clouds, sulfur can be included in aggregates such as H 2 S, SO 2 , OCS, SO, H 2 CS, NS SiS, CS, HNCS CH 3 SH ( Duley et al. 1980 , and references therein) as well as other
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