Abstract
Formation of clay minerals during experimental alteration of obsidian in acetic acid and oxalic acid solutions was investigated. The experiments were carried out using 0.01N or 0.02N acetic acid and oxalic acid solutions at 150°and 200°C for 1 to 60 days. The clay minerals formed in these organic acid solutions were examined by X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy. Variations of pHs and concentrations of dissolved elements in solutions during the reactions were measured. By reactions using acetic acid solution, pH-values of solutions retained weak acidic region of pH3.4-4.9, and formation of kaolinite proceeded during the reactions and crystallization of smectite occurred at the final stages. On the other hand, pH-values of oxalic acid solution increased rapidly to alkaline region, and then crystallization of smectite began to take place at the initial stages and proceeded throughout the reactions. The rapid increase in pH of oxalic acid was due to thermal decomposition of the organic acid, and therefore solution compositions changed drastically to the stability filed of smectite.
