  Extensive experimental studies show all major rocky elements (Si, Mg, Fe, Ni,
Ca, Al, Na, K) dissolve in steam to a greater or lesser extent. We use these
results to compute chemical equilibrium abundances of rocky element-bearing
gases in steam atmospheres equilibrated with silicate magma oceans. Rocky
elements partition into steam atmospheres as volatile hydroxide gases and via
reaction with HF or HCl as volatile halide gases in much larger amounts than
expected from their vapor pressures over volatile-free solid or molten rock at
the same temperature. We compute the extent of fractional vaporization by
defining gas to magma partition coefficients and show Earth's sub-solar Si to
Mg bulk elemental ratio may be due to loss of a primordial steam atmosphere. We
conclude hot rocky exoplanets that are undergoing or have undergone escape of
steam atmospheres may experience fractional vaporization and loss of Si, Mg,
Fe, Ni, Ca, Al, Na, and K. This loss may modify their bulk composition,
density, heat balance, and internal structure.
