A singular silica sublimate mineral found in Satuma-Iwo-zima Volcano (I)

Abstract

A silica sublimate mineral which includes strong halogen acids solution and has a cell-like structure was found at Satuma-Iwo-Zima volcano. It is formed around some fumaroles of relatively low temperatures (above 100°C). The gases emitted from those fumaroles are rich in fluorine compounds.The freshly formed mineral apparently resembles alunogen. The liquid included in it is easily squeezed out by mechanical stimulus. The liquid content in the sublimate decreases gradually with time passed after its formation (about 97%→2%). Hyalite found in this volcano seems to be a final product of this series.Specimens which represent various stages of aging were collected and examined by chemical analysis, X-ray diffraction method and differential thermal analysis. The solid part of the mineral is mainly composed of amorphous silica and contains sulfur besides it in most cases. The liquid part contains high concentrations of hydrochloric (max. 4.5N) and fluorosilicic (max. 2.2N) acids but low concentration of sulfuric acid. Metallic ions do not exist in any appreciable amount. Some specimens contain large amounts of hydrogen iodide, hydrogen bromide and fluoroboric acid in their liquid part. They turn red on leaving in air owing to the oxidation of iodide to iodine.The formation mechanism of this silica sublimate is concluded to be as follows: At first, water vapor in the volcanic gas is condensed at the oriffice of the fumarole and then, silicon tetrafluoride in the gas is hydrolyzed by liquid water forming amorphous silcia and fluorosilcic acid: 3SiF4+2H2O=SiO2+2H2SiFe6. Hydrogen chloride, bromide and iodide are concentrated by the successive fractional distillation process.