Hydration Rate of Obsidian

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Abstract
The hydration rates of 12 obsidian samples of different chemical compositions were measured at temperatures from 95° to 245°C. An expression relating hydration rate to temperature was derived for each sample. The SiO 2 content and refractive index are related to the hydration rate, as are the CaO, MgO, and original water contents. With this information it is possible to calculate the hydration rate of a sample from its silica content, refractive index, or chemical index and a knowledge of the effective temperature at which the hydration occurred. The effective hydration temperature can be either measured or approximated from weather records. Rates have been calculated by both methods, and the results show that weather records can give a good approximation to the true EHT, particularly in tropical and subtropical climates. If one determines the EHT by any of the methods suggested, and also measures or knows the rate of hydration of the particular obsidian used, it should be possible to carry out absolute dating to ± 10 percent of the true age over periods as short as several years and as long as millions of years.