EFFECTS OF TEMPERATURE, pH, AND IRON/CLAY AND LIQUID/CLAY RATIOS ON EXPERIMENTAL CONVERSION OF DIOCTAHEDRAL SMECTITE TO BERTHIERINE, CHLORITE, VERMICULITE, OR SAPONITE

International audienceIn deep geological repositories for high-level nuclear wastes, interactions between steel canisters and clay-rich materials may lead to mineralogical transformations with a loss of the confining properties of the clays. Experiments simulating the conversion of smectite to Fe-rich clay phases in contact with Fe metal have been carried out to evaluate such a possibility by taking into account the effects of a series of critical parameters, including temperature, pH, and Fe/clay (Fe/C) and liquid/clay (L/C) ratios. The mineralogical and chemical transformations observed in these experiments have been compared with data from the literature, and subsequently used to propose a conceptual model for the main mineralogical transformations which can be expected in clay formations surrounding high-level nuclear waste repositories. In the presence of Fe metal and under low oxygen fugacity ( 5: dioctahedral smectite (di-sm) ? 7 A˚ Fe-rich phase (berthierine, odinitecronstedtite) for large Fe/C ratios (>0.5), or di-sm ? Fe-rich di-sm + Fe-rich trioctahedral smectite (tri-sm) for small Fe/C ratios (0.1); (2) up to 150ºC, under alkaline pH (1012), and L/C > 5: di-sm ? Fe di-sm (Npalygorskite) for a small Fe/C ratio (0.1); (3) at 300ºC, Fe/C = 0.1, and L/C > 5: di-sm ? Fe-rich saponite ? trioctahedral chlorite + feldspar + zeolite (near-neutral pH); di-sm ? Fe-rich vermiculite + mordenite (pH 1012). Low temperatures (