Experimental alteration of obsidian with HCl solution was performed to elucidate dissolution mechanism and formation process of clay minerals in acid solution. Reactions were carried out using 0.1, 0.5, and 4.0 g of obsidian to 100 ml of 0.01 N HCl solution at 150° and 200°C for 1 to 60 days. The reaction products were examined by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive X-ray analysis. The surface composition of obsidian before and after alteration was investigated by X-ray photoelectron spectroscopy (XPS). TEM showed that boehmite precipitated at early stage and spherical kaolinite appeared subsequently by 200°C reactions. However, spherical halloysite occurred predominantly with small amounts of allophane, boehmite, and kaolinite by 150°C reaction in which formation process of the halloysite from allophane passing through an intermediate phase of small size rounded aggregate that consists of fine particles of allophane was observed. A boehmite exhibiting hexagonal platy habit with higher degree of crystallinity was formed by 200°C reaction as a stable phase in solution containing lower Si concentration at which the solution composition coincides with the stability field of boehmite on activity diagram for the system Na2O-Al2O3-SiO2-H2O. The fibrous boehmite having lower crystallinity appeared with increasing Si concentration, considered as a metastable phase in the stability field of kaolinite. XPS indicated that dissolution of obsidian in acid solution proceeded initially by cation exchange between Na ions and hydronium ions in solution and subsequently by preferential release of Al ions relative to Si from the Na depleted surface.

Alteration experiments have been performed using RTT7 and synthetic basaltic glasses in MgCl2−CaCl2 salt solution at 190°C. The duration of experiments ranged from 0.25 to 463 days. The alteration products were studied by Scanning Electron Microscope (SEM), Scanning Transmission Electron Microscope (STEM), X-ray diffraction (XRD) and Electron Spectrometry for Chemical Analysis (ESCA). For both glasses, the early alteration product is a hydrotalcite-like compound [Mg6Al2CO3(OH)16·4H2O] in which HPO42−, SO42− and Cl− substitutes for CO32−. The measured basal spacing is 7.68 Å for the hydrotalcite formed from R7T7 glass and 7.62 Å for the hydrotalcite formed from basaltic glass which reflect the high Al/Al + Mg ratios x (0.34 ≤ x ≤ 0.46). The chemical microanalyses show that the hydrotalcite is subsequently covered by a silica-rich gel which evolves into saponite after a few months. These results support the use of basaltic glasses alteration patterns in Mg-rich solution, to understand the long-term behavior of R7T7 nuclear waste glass.

Experimental alteration of volcanic glass has been carried out in distilled water at 200°C and 150°C. The formation and transformation processes of alteration products have been examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), infrared absorption analysis, and X-ray photoelectron spectroscopy. SEM and TEM clearly show that amorphous aluminum-silicate coatings with allophane particles precipitate on the surface of volcanic glass during the earliest alteration stage. Noncrystalline flaky and/or fibrous materials are formed from the allophane aggregates and from the amorphous coatings as new reaction products. The flaky and/or fibrous materials curl inward and transform into 100–500 nm circular smectite. The Al/Si atomic ratio of 1.09 for allophane decreases progressively to 0.65 for smectite through 0.86 for noncrystalline transitional material. The smectite has d(06) spacing of 1.497 Å and consists mainly of Si, Al, and small amounts of Fe, Ca, and Na.

Experimental alterations of K-feldspar in distilled-deionized water at 150°, 175°, 200°, and 225°C were performed. The alteration products and dissolution mechanism of K-feldspar were examined by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS). SEM, TEM, and EDX clearly showed formation of fibrous boehmite less than 1.0 μm in length at the early alteration stages. The boehmite fibers decreased in abundance and rounded platy 1 M mica was produced as alteration proceeded. The mica exhibited initially angular shaped small flakes of 0.69 μm in average size, which developed to rounded platy particles of 1.97 μm. The main chemical reactions occurring in this experimental system can be expressed by: $$\begin{array}{l} \mathop {KAlS{i_3}{O_3}}\limits_{\left[ {K - feldspar} \right]} + 6{H_2}O + {H^ + } \to \mathop {AlO\left( {OH} \right)}\limits_{\left[ {boehmite} \right]} + 3{H_4}Si{O_4} + {K^ + }, \\ \mathop {3KAlS{i_3}{O_8}}\limits_{\left[ {K - feldspar} \right]} + 12{H_2}O \to \mathop {KA{l_3}S{i_3}{O_{10}}{{\left( {OH} \right)}_2}}\limits_{\left[ {mica} \right]} + 2{K^ + } + 6{H_4}Si{O_4} + 22{H^ + }. \\ \end{array}$$ XPS showed no significant changes in intensities of photoelectron lines excited from K, Si, and Al in the K-feldspar surface before and after alteration, however the K/Si molar ratios in the solutions were considerably smaller than that of the original K-feldspar. The results of XPS strongly indicate that no dealkalized layer was produced on the surface, and that dissolution of K-feldspar in aqueous solution proceeded congruently by a surface-reaction mechanism. The discrepancy of mass balance in the solutions may be mainly caused by adsorption of K on the surface of boehmite.

The storage of CO2 in geological reservoirs requires an understanding of the impact of CO2 on clay-rich sealing cap-rocks to identify and explore critical parameters that modify petrophysical properties such as permeability and fracturing. The purpose of this study was to investigate the effect of heating, under different hydrated-CO2 partial pressures, on the chemical compositions and relative amounts of mineral phases in the Saint Martin de Bossenay (SMB, Paris Basin, France) cap-rock in order to identify possible mineral-phase transitions and to estimate reaction kinetics induced by the presence of excess dissolved CO2.

X-ray diffraction, transmission electron microscopy, and electron microprobe analyses were employed to study mineral alteration, with particular attention given to visualization and quantification of the mineral evolution of clay minerals. In all the altered mixtures investigated, the illitization of clays was combined with the formation of anhydrite. These changes were accompanied by a dolomitization and a slight increase in the quartz content. The CO2-rich samples crystallized Fe2+-and K+-enriched illites, whereas the CO2-free experiments precipitated Al3+-deprived and Mg2+-enriched illites. Advanced characterizations of cap-rock material allowed reaction paths, induced by the increase in dissolved CO2 in the porous media, to be determined precisely. The results place strong constraints on numerical models aimed at evaluating the safety of an SMB site.