Hawaiian volcanoes characteristically have but few of the many types of minerals found in incrustations of other volcanic areas. In Hawaii sulfates resulting from air oxidation of volatiles predominate, and fluorides produced during rock alteration by fumarolic gases are prominent. Halides are generally found where reducing conditions exist in fumaroles and lava lake drill holes. The most common mineral types are sulfur, opaline silica, gypsum, ralstonite, and thenardite. Minerals from the same deposit are found to vary markedly in the content of the less abundant components. Condensates from vapor issuing from fumaroles show little quantitative relationship in component content to incrustations deposited at the same fumaroles. It is believed that an energetically favorable isomorphic substitution of some elements in the crystal lattice of a depositing mineral may lead to the build-up of a high concentration of an element from a lean vapor. Equilibrium calculations applied to condensate studies give a good quantitative approximation to the concentrations of the elements found in natural systems, but when applied to incrustations they serve only to indicate general compositional relations. Laboratory studies have shown the important role of chlorides in metal transfer in the gas phase in high-temperature aqueous systems, but only in the absence of oxygen. These studies also demonstrated the important role of HF in rock alteration and in the transfer of silica.
Contribution No. 756, Hawaii Institute of Geophysics.