  The formation and growth of refractory matter on pre-existing interstellar
dust grain surfaces was studied experimentally by annealing neon-ice matrices
in which potential precursors of silicate grains (Mg and Fe atoms, SiO and
SiO$_2$ molecules) and of solid carbon (C$_n$ molecules, $n$ = 2-10) were
initially isolated. Other molecules, mainly O$_3$, CO, CO$_2$, C$_3$O, and
H$_2$O, were embedded at the same time in the matrices. The annealing procedure
caused the cold dopants to diffuse and interact in the neon ice. Monitoring the
procedure in situ with infrared spectroscopy revealed the disappearance of the
silicon oxide and carbon molecules at temperatures lower than 13 K, and the
rise of the Si-O stretching band of silicates. Ex situ electron microscopy
confirmed the formation of silicate grains and showed that their structure was
amorphous. It also showed that amorphous carbon matter was formed
simultaneously next to the silicate grains, the two materials being chemically
separated. The results of the experiments support the hypothesis that grains of
complex silicates and of carbonaceous materials are re-formed in the cold ISM,
as suggested by astronomical observations and evolution models of cosmic dust
masses. Moreover, they show that the potential precursors of one material do
not combine with those of the other at cryogenic temperatures, providing us
with a clue as to the separation of silicates and carbon in interstellar
grains.
