Natural green tuff and rhyolitic tuff (Miocene age) were subjected to alter in sodium hydroxide and potassium hydroxide solutions under 200°C and 200kg/cm
2. Various chemical behaviors were shown under the influence of running times and chemical conditions. Al
2O
3 content in the alteration product shows an apparent increase. However, Al
2O
3 is absent from the present hydrothermal solutions. It is assumed that during hydrothermal alteration, fresh rock has decreased in weight so that Al
2O
3 has apparently increased. Column A in Table 5 shows the culculated weight in grams of each oxide remaining from the alteration of 100g of fresh rock, on the assumption of constant Al
2O
3. It may be concluded from the present experiments that the amount of analcime in the alteration product depends upon mineral mineral association of starting material and contents of SiO
2 in alteration products.
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