Geothermal Blue Water Colored by Colloidal Silica

Abstract

Hydrothermal wat ers in hot pools commonly appear blue in color as a resul t of the incompl et e precipitation of silica sinter. In the case of the Beppu geothermal field, blue thermal wat ers of the neutralñchloride type initially contain monomeric silica whose concentrations are 2 to 3 times higher than the solubilities of amorphous silica at the water temperatures of the pools. In these pools, aqueous colloidal silica is formed by silicapolymerization. Color measurements using a colorimeter show that the hue of the blue waters in the pools agrees with that of a synthesized colloidal silica solution. Grain size analysis of colloidal silica in natural blue thermal wat er made before and aft er the coloration demonstrates that the coloration is caused by Rayleigh scattering of the sunlight by colloidal silica particles somewhat smaller in size than the wave lengths of visible radiation.