  Little is known about the interaction between atmospheres and crusts of
exoplanets so far, but future space missions and ground-based instruments are
expected to detect molecular features in the spectra of hot rocky exoplanets.
We aim to understand the composition of the gas in an exoplanet atmosphere
which is in equilibrium with a planetary crust. Methods. The molecular
composition of the gas above a surface made of a mixture of solid and liquid
materials is determined by assuming phase equilibrium for given pressure,
temperature and element abundances. We study total element abundances that
represent different parts of the Earth's crust (Continental Crust, Bulk
Silicate Earth, Mid Oceanic Ridge Basalt), CI chondrites and abundances
measured in polluted white dwarfs. For temperatures between 600K and 3500K, the
near-crust atmospheres of all considered total element abundances are mainly
composed of H2O, CO2, SO2 and in some cases of O2 and H2. For temperatures
500K, only N2-rich or CH4-rich atmospheres remain. For 3500K, the atmospheric
gas is mainly composed of atoms (O, Na, Mg, Fe), metal oxides (SiO, NaO, MgO,
CaO, AlO, FeO) and some metal hydroxides (KOH, NaOH). The inclusion of
phyllosilicates as potential condensed species is crucial for lower
temperatures, as they can remove water from the gas phase below about 700K and
inhibit the presence of liquid water. Measurements of the atmospheric
composition could, in principle, characterise the rock composition of exoplanet
crusts. H2O, O2 and CH4 are natural products from the outgassing of different
kinds of rocks that had time to equilibrate. These are discussed as biomarkers,
but do emerge naturally as result of the thermodynamic interaction between
crust and atmosphere. Only the simultaneous detection of all three molecules
might be a sufficient biosignature, as it is inconsistent with chemical
equilibrium.
