  Sub-Neptune exoplanets are commonly hypothesized to consist of a
silicate-rich magma ocean topped by a hydrogen-rich atmosphere. Previous work
studying the outgassing of silicate material has demonstrated that such
atmosphere-interior interactions can affect the atmosphere's overall structure
and extent. But these models only considered SiO in an atmosphere of hydrogen
gas, without considering chemical reactions between them. Here we couple
calculations of the chemical equilibrium between H, Si, and O species with an
atmospheric structure model. We find that substantial amounts of silane,
SiH$_4$, and water, H$_2$O, are produced by the interaction between the
silicate-rich interior and hydrogen-rich atmosphere. These species extend high
into the atmosphere, though their abundance is greatest at the hottest, deepest
regions. For example, for a 4 $M_\oplus$ planet with an equilibrium temperature
of 1000 K, a base temperature of 5000 K, and a 0.1 $M_\oplus$ hydrogen
envelope, silicon species and water can comprise 30 percent of the atmosphere
by number at the bottom of the atmosphere. Due to this abundance enhancement,
we find that convection is inhibited at temperatures $\gtrsim 2500$ K. This
temperature is lower, implying that the resultant non-convective region is
thicker, than was found in previous models which did not account for
atmospheric chemistry. Our findings show that significant endogenous water is
produced by magma-hydrogen interactions alone, without the need to accrete
ice-rich material. We discuss the observability of the signatures of
atmosphere-interior interaction and directions for future work, including
condensate lofting and more complex chemical networks.
