Oldhamite (CaS) is a rare mineral, which is only observed naturally in
enstatite meteorites.No occurrence of CaS has been documented from other groups
of meteorites and terrestrial samples. However, in experiments at 1.5 GPa/1510
K and 0.5 GPa/1320 K, when the lgfo2 is lower than -10.57 (FMQ-0.52), CaS was
produced in this study by a two-melt mechanism involving the reaction between
molten pyrrhotite-pentlandite-bearing orthopyroxenite and molten CaCO3. CaS can
be easily oxidized to form CaSO4 or hydrolyzed to produce calcium hydroxide,
which may explain why it has never been found in geological samples from Earth.
We speculate that part of the anhydrite and gypsum in black smokers along
mid-ocean ridges are related to the oxidation or hydrolysis of CaS in the
underlying mantle. CaS can be produced when the Siberian mantle plume intruded
into the lithosphere.Comment: The third versio