Modification of residual glass and precipitation of a neoformation layer : experimental evidence for a double origin of the «palagonite » derived from the alteration of basaltic glass by sea-water

Abstract

To understand how palagonite forms during the basaltic glass — sea-water interactions, experimental investigations have been performed using the labelling of the initial interface with Ni, introduced at one ppm level in solution. This element is quickly concentrated in early hydrotalcite crystals grown on the initial interface. In Al-serpentine formed after epigenization of the hydrotalcite, Ni remains concentrated. On ultramicrotome thin sections, the onset of the Ni chemical profile labels the initial interface. Alteration products appear on both sides of this surface : on the inner part lies a Si-Fe bearing layer and a Si-Al-Mg one on the outer part. They respectively result from a in situ evolution of residual hydrated glass and a precipitation followed by an epigenization. Accordingly, palagonitization is a two ways process carried out with a volume increase.