A low temperature experimental alteration of a rhyolitic obsidian
Fiore, Saverio; Huertas, F. Javier; Tazaki, Kazue; Huertas, Francisco; Linares, José
European Journal of Mineralogy Volume 11 Number 3 (1999), p. 455 - 470
published: May 25, 1999
manuscript accepted: Jan 7, 1999
manuscript received: Jul 21, 1998
ArtNo. ESP147051103001, Price: 29.00 €
Abstract Experimental alteration of obsidian by distilled water in an open system (T = 82 ± 5 °C; pH = 5.5 - 6 ) was carried out to investigate on the mechanism of leaching and on the mobilization of major elements as well as some trace elements, including rare earths. Nine samples were altered at different times (up to 1717 hours) and products of leaching were studied by SEM, HRTEM, and XPS, and chemically analysed by XRF and ICP-MS. The most evident chemical change occurring in the rhyolitic obsidian after the experimental leaching is the loss of alkalis and the gain of H2O. Statistical analysis revealed that inward diffusion of molecular water into the bulk glass takes place during the early stage of alteration, followed by cation+-H3O+ or cation+-H+ interdiffusion and changes within the glass structure as well as disruption of the silica network at the glass-solution interface. The structural reorganization of the glass is accompanied by the formation of flake-leaf-needle morphologies as evidenced and observed by HRTEM. These morphologies might represent three steps of local crystallization within the glass, which culminates in the formation of an acicular mineral, probably halloysite in the interior of the glass. The specific weight-loss leach rate of the rhyolitic obsidian under study is very slow, i.e. 1.5 ± 0.51 × 10-8 gglass m-2s-1. Leach rates, estimable only for some of the detected elements show the following order as regards mobility: Na>Si≈K>Ca>Ti≈Al for major elements, and Pb>Lu>Er>Zn>La>Rb>Ce for trace elements. Values calculated for trace elements are higher than for major elements. There is no simple explanation at hand. Finally, XPS analysis clearly showed that the formation of a leached layer did not take place. However, the very small variation of Na/Al, Mg/Al, and Ca/Al ratios with leaching time might be indicative of the existence of an ultrathin layer, even lower than 30 Å. Discrepancies between the experimental findings reported in literature are explainable by structural differences of the obsidians.