Hydrothermal interaction between basaltic glass, deionized water and seawater

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This paper presents the results of a series of experiments on the interactions of a basaltic glass with deionized water and natural seawater at 200°C and autogenous pressure in a closed system. Investigations showed the different mineralogical and geochemical behavior of the basaltic glass in the two solutions. The initial stage of interaction with deionized water was characterized by hydration and rapid hydrolysis of the basaltic glass. Variations in ion concentration showed a parabolic trend from the first hours of interaction, indicating selective dissolution of the glass. Phillipsite, Mg-smectite and analcime were the alteration products according to pH-values reached in the solution. Our data also suggest that only the first reaction stages are affected by grain-size and that this controls the times of appearance of the secondary phases. Experiments on seawater fall into the category of rock-dominated but acid pH, and quite high Mg content persisted throughout. This behavior may be correlated to the alteration layer that covers the granules, reducing exchanges between glass and seawater and changing our system into the “seawater-dominated” type. The continuous acid pH throughout the experiments favoured 6-coordination of Al ions in aqueous solution and prevented the crystallization of zeolitic phases. Mg-smectite and anhydrite were the only alteration products. In this set of experiments, grain-size played a minor role in the alteration process, since it did not control the time of appearance of secondary phases.