Refractive index and hydration of rhyolitic glass from Holocene tephras, North Island, New Zealand

Abstract

Abstract An accurate hot-stage microscope method of determining refractive index of rhyolitic glass shards, by using the thermal variation of refractive index of methyl phenyl ether in which the shards are immersed, enables the distinction of glass samples of Mairoa Ash from those of Tirau Ash and late Pleistocene tephras, The accuracy and precision of the method clearly show the contamination of Mairoa Ash with Oruanui Ash. Comparison of refractive index of glass of Oruanui Ash from various sites shows little difference. However, water contents (H20>−) of the glasses, calculated from the difference between the measured refractive index (being that of hydrated glass) and that determined empirically from chemical composition (being that of the glass before hydration-effectively obsidian), vary with the square root of the time of hydration and show an Arrhenian dependence with the temperature of hydration. The activation energy for diffusion of water into the glasses is calculated from these relationships as 166 kJ mol−1.