  We use chemical equilibrium calculations to model the speciation of alkalis
and halogens in volcanic gases emitted on Io. The calculations cover wide
temperature (500-2000 K) and pressure (10^-6 to 10^+1 bars) ranges, which
overlap the nominal conditions at Pele (T = 1760 K, P = 0.01 bars). About 230
compounds of 11 elements (O, S, Li, Na, K, Rb, Cs, F, Cl, Br, I) are
considered. We predict the major alkali and halogen species in a Pele-like
volcanic gas and the major alklai and halogen condensates. We also model
disequilibrium chemistry of the alkalis and halogens in the volcanic plume.
Based on this work and our prior modeling for Na, K, and Cl in a volcanic
plume, we predict the major loss processes for the alkali halide gases are
photolysis and/or condensation onto grains. On the basis of elemental
abundances and photochemical lifetimes, we recommend searching for gaseous KCl,
NaF, LiF, LiCl, RbF, RbCl, CsF, and CsCl around volcanic vents during
eruptions. Based on abundance considerations and observations of brown dwarfs,
we also recommend a search of Io's extended atmosphere and the Io plasma torus
for neutral and ionized Li, Cs, Rb, and F.
