  Laboratory spectral measurements of relevant analogue materials were
performed in the framework of the Rosetta mission in order to explain the
surface spectral properties of comet 67P. Fine powders of coal, iron sulphides,
silicates and their mixtures were prepared and their spectra measured in the
Vis-IR range. These spectra are compared to a reference spectrum of 67P nucleus
obtained with the VIRTIS/Rosetta instrument up to 2.7 {\mu}m, excluding the
organics band centred at 3.2 {\mu}m. The species used are known to be chemical
analogues for cometary materials which could be present at the surface of 67P.
Grain sizes of the powders range from tens of nanometres to hundreds of
micrometres. Some of the mixtures studied here actually reach the very low
reflectance level observed by VIRTIS on 67P. The best match is provided by a
mixture of sub-micron coal, pyrrhotite, and silicates. Grain sizes are in
agreement with the sizes of the dust particles detected by the GIADA, MIDAS and
COSIMA instruments on board Rosetta. The coal used in the experiment is
responsible for the spectral slope in the visible and infrared ranges.
Pyrrhotite, which is strongly absorbing, is responsible for the low albedo
observed in the NIR. The darkest components dominate the spectra, especially
within intimate mixtures. Depending on sample preparation, pyrrhotite can coat
the coal and silicate aggregates. Such coating effects can affect the spectra
as much as particle size. In contrast, silicates seem to play a minor role.
