  We use thermochemical equilibrium calculations to model iron, magnesium, and
silicon chemistry in the atmospheres of giant planets, brown dwarfs, extrasolar
giant planets (EGPs), and low-mass stars. The behavior of individual Fe-, Mg-,
and Si-bearing gases and condensates is determined as a function of
temperature, pressure, and metallicity. Our results are thus independent of any
particular model atmosphere. The condensation of Fe metal strongly affects iron
chemistry by efficiently removing Fe-bearing species from the gas phase.
Monatomic Fe is the most abundant Fe-bearing gas throughout the atmospheres of
EGPs and L dwarfs and in the deep atmospheres of giant planets and T dwarfs.
Mg- and Si-bearing gases are effectively removed from the atmosphere by
forsterite (Mg2SiO4) and enstatite (MgSiO3) cloud formation. Monatomic Mg is
the dominant magnesium gas throughout the atmospheres of EGPs and L dwarfs and
in the deep atmospheres of giant planets and T dwarfs. Silicon monoxide (SiO)
is the most abundant Si-bearing gas in the deep atmospheres of brown dwarfs and
EGPs, whereas SiH4 is dominant in the deep atmosphere of Jupiter and other gas
giant planets. Several other Fe-, Mg-, and Si-bearing gases become increasingly
important with decreasing effective temperature. In principle, a number of Fe,
Mg, and Si gases are potential tracers of weather or diagnostic of temperature
in substellar atmospheres.
