  Carbon-enriched rocky exoplanets have been proposed around dwarf stars as
well as around binary stars, white dwarfs and pulsars. However, the
mineralogical make up of such planets is poorly constrained. We performed
high-pressure high-temperature laboratory experiments ($P$ = 1$-$2 GPa, $T$ =
1523$-$1823 K) on carbon-enriched chemical mixtures to investigate the deep
interiors of Pluto- to Mars-size planets the upper mantles of larger planets.
  Our results show that these exoplanets, when fully-differentiated, comprise a
metallic core, a silicate mantle and a graphite layer on top of the silicate
mantle. The silicate mineralogy (olivine, orthopyroxene, clinopyroxene and
spinel) is largely unaffected by the amount of carbon. Metals are either two
immiscible iron-rich alloys (S-rich and S-poor) or a single iron-rich alloy in
the Fe-C-S system with immiscibility depending on the S/Fe ratio and core
pressure. Graphite is the dominant carbon-bearing phase at the conditions of
our experiments with no traces of silicon carbide or carbonates. If the bulk
carbon content is higher than needed to saturate the mantle and the core,
graphite would be in the form of an additional layer on top of the silicate
mantle assuming differentiation. For a thick enough graphite layer, diamonds
would form at the bottom of this layer due to high pressures.
  We model the interior structure of Kepler-37b and show that a mere 10 wt%
graphite layer would decrease its derived mass by 7%, suggesting future space
missions that determine both radius and mass of rocky exoplanets with
insignificant gaseous envelopes could provide quantitative limits on their
carbon content. Future observations of rocky exoplanets with graphite-rich
surfaces would show low albedos due to the low reflectance of graphite. The
absence of life-bearing elements other than carbon on the surface likely makes
them uninhabitable.
