The hydration rate of rhyolitic glass has been determined at temperatures ranging from 5° C to 100° C. The relationship between the depth of hydration, x, and time, t is x2 = kt; k varies from 0.4 μ2/103 years at 5° C to 104 μ2/103 years at 100° C; k is independent of the water pressure from a few hundredths of a centimeter to 1 atm. water pressure. The activation energy of hydration is about 20 kcal/mole.

The determined hydration rates are consistent with the observation that perlite commonly forms by the hydration of shattered rhyolitic glass, either during the late cooling of a deposit or after the deposit has cooled to a surficial temperature.

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