The interaction of single crystals of a zeolite, scolecite (CaAl2Si3O10·3H2O), with NaCl and NaOH solutions of different concentrations and pH values was investigated in autoclaves at temperatures up to 200 °C. Only the experiments with 1 or 2 M NaOH at 200 °C led to significant reactions, namely one or two reaction rims, depending on the reaction time. The products were studied using electron microscopy, electron-microprobe analysis and X-ray diffraction. An inner reaction rim of mesolite (Na2Ca2Al6Si9O30·8H2O) was only observed in longer-term experiments. At all reaction times, however, a reaction rim consisting of Na,Al-substituted tobermorite 11 Å (Ca4.5Na1.3Si5.2Al1.0O16(OH)2) formed. The reaction interfaces are sharp, and the product phases form porous pseudomorphs of the original scolecite. Some advancements of the tobermorite rim are connected to fractures formed in the scolecite. Additional Cs exchange experiments (2 × 10−4 M CsCl, 24 °C, 18 h) on the partly reacted samples showed that Cs-Na exchange in the tobermorite did not start simultaneously in the whole reaction rim, but only at the outer part.
The results indicate that replacement of scolecite takes place by coupled dissolution-precipitation, independent of the similarity between the structure of scolecite and the reaction product. There is less fluid mobility around separate crystals in a solution than could be expected, which emphasises the importance of interfacial fluids. The infiltration of the fluid into the crystal partially depends on reaction-induced fracturing.