ABSTRACT

The predominant chemical species of iodine in volcanic gases is suggested to be hydrogen iodide by thermodynamical calculations on the reactions, 2HI + 1/2 O₂ = H₂O + I₂, 2HI = H₂ + I₂ and SO₂ + 6HI = H₂S + 3I₂ + 2H₂O. Thermodynamical calculations were also made to explain the difference in I/Cl ratio between volcanic halide sublimates and the fumarolic gases from which the sublimates were formed. It is suggested that the partial pressure of hydrogen iodide in volcanic gases is not enough to form sodium iodide through fumarolic alteration of wall rocks, while that of hydrogen chloride is enough to form sodium chloride. Another possible factor controlling iodine content of volcanic halide sublimates is air oxidation of iodide to free iodine gas under fumarolic conditions near outlets of fumaroles.