  In the solar system, oldhamite (CaS) is generally considered to be formed by
the condensation of solar nebula gas. Enstatite chondrites, one of the most
important repositories of oldhamite, are believed to be the representative of
the material which formed Earth. Thus, the formation mechanism and the
evolution process of oldhamite are of great significance to the deeply
understanding about the solar nebula, meteorites, the origin of Earth, and the
C-O-S-Ca cycles of Earth. To date, no report about the oldhamite in the mantle
exists. However, here we show the formation of oldhamite through the reaction
between sulfide-bearing orthopyroxenite and molten calcite at 1.5 GPa/1510 K
and 0.5 GPa/1320 K. Surprisingly the oxygen fugacities in our experiments are
within the range of mantle conditions, which is 6 orders of magnitude higher
than that of the solar nebula mechanism. Oldhamite is easily oxidized to
calcium sulfate. Both low oxygen fugacity of magma and extreme low oxygen
content of atmosphere are necessary for existence of oldhamite on the surface
of a planet; otherwise, anhydrite or gypsum will exist in large quantities. The
widespread existence of oldhamite on the planet surface indicates the planet is
definitely not habitable because of the scarcity of oxygen. The formation and
oxidation of oldhamite are accompanied by the production of carbon dioxide and
the consumption of oxygen, which may have an impact on the Earth atmosphere
before the Great Oxidation Event and during the Permian-Triassic Boundary.
