The nature of water in hydrous silica

Abstract

The nature of water in hydrous silica has been investigated by measuring the hydrogen isotopic composition of successive increments of water evolved under vacuum during heating of the silica to 1000°C. Water increments evolved from Monterey diatomite are progressively enriched in deuterium up to 87°C, behaving isotopically like free, structurally nonessential water subjected to distillation. Between 87° and 218°C successive water increments have constant δD values, suggesting a bound water component. Above 218°C water with relatively lower δD values is outgassed, and this water is interpreted as hydroxyl. Isotope exchange experiments at 25°C and 100°C in which the silica is exposed to D enriched water and analyzed as above suggest at least 5 types of water in Monterey diatomite. (1) At least 2 wt.% water occurs as adsorbed or mechanically trapped H_2O which cannot be completely pumped away at 25°C and exchanges readily with D-enriched water. (2) Approximately 3.6 wt.% occurs as H_2O in sites protected from interaction with external water at 23°C, but exchanges isotopically at 100°C. (3) Approximately 0.2 wt.% occurs as surface hydroxyls which exchange at 23°C. (4) Approximately 1.3 wt.% occurs as nonsurface hydroxyls which exchange at l00°C. (5) Approximately 0.9 wt.% water exists as nonsurface hydroxyls which exchange slowly, if at all, with waters external to the silica. The nonessential and easily exchangeable water fraction of diatomite cannot be readily separated from other water in the silica. However, water evolved above 700°C may be derived from hydroxyl groups which have preserved a geologically useful isotopic record. Applications of this analytical procedure to other hydrous minerals may help to refine the stoichiometry of various water components in these minerals.