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The formation and growth of refractory matter on pre-existing interstellardust grain surfaces was studied experimentally by annealing neon-ice matricesin which potential precursors of silicate grains (Mg and Fe atoms, SiO andSiO$_2$ molecules) and of solid carbon (C$_n$ molecules, $n$ = 2-10) wereinitially isolated. Other molecules, mainly O$_3$, CO, CO$_2$, C$_3$O, andH$_2$O, were embedded at the same time in the matrices. The annealing procedurecaused the cold dopants to diffuse and interact in the neon ice. Monitoring theprocedure in situ with infrared spectroscopy revealed the disappearance of thesilicon oxide and carbon molecules at temperatures lower than 13 K, and therise of the Si-O stretching band of silicates. Ex situ electron microscopyconfirmed the formation of silicate grains and showed that their structure wasamorphous. It also showed that amorphous carbon matter was formedsimultaneously next to the silicate grains, the two materials being chemicallyseparated. The results of the experiments support the hypothesis that grains ofcomplex silicates and of carbonaceous materials are re-formed in the cold ISM,as suggested by astronomical observations and evolution models of cosmic dustmasses. Moreover, they show that the potential precursors of one material donot combine with those of the other at cryogenic temperatures, providing uswith a clue as to the separation of silicates and carbon in interstellargrains.

Separate silicate and carbonaceous solids formed from mixed atomic and molecular species diffusing in neon ice