High-pH alteration of argillaceous rocks: an experimental study

Diffusion and evaporatively-driven advection experiments were performed at 30 degrees C to investigate the effect of high-pH solutions on the mineralo gy of whole rock Opalinus Clay. Reacted rock samples were analysed by SEM a nd XRD after 4, 6, 12 and 18 months in the diffusion experiments and after 7 and 12 months in the advection experiments. The sequence of reaction prod ucts under these high-pH conditions includes first the precipitation of zeo lites and Ca-AL-Si-hydrate phases (C-A-S-H) followed by a clay mineral-calc ite paragenesis which is accompanied by sulphates in the advection experime nts. The formation of zeolites appears to be bound to closed-system reactio ns where conditions of low fluid fluxes accomplish a greater degree of loca l pH buffering of the solution thus providing suitable conditions for zeoli te precipitation. Massive formation of calcite during later stages of rock- water interaction is attributed to dissolution of dolomite/ankerite which i ncreases pCO(2) in solution and suppresses further precipitation of zeolite s and C-A-S-H phases. The results of these investigations show that the min eralogy of Opalinus Clay is significantly changed by attack of high-pH solu tions even at temperatures as low as 30 degrees C.