Hydrothermal reactions in the system sepiolite/H2O have been examined between 149° and 316°C. Approximately 10–20% of the starting sepiolite was converted to a smectite (stevensite) at 204°C within 24 hr. Similar results were obtained when CaCl2, NaOH, Ca(OH)2, or Mg(OH)2 was added to the system. In the presence of NaCl, about 60% of the sepiolite was converted to stevensite, whereas, only 5% stevensite formed in the presence of MgCl2. Greater amounts of stevensite formed at 260°C in these systems. Above 316°C, 60–80% of the sepiolite was converted to stevensite in 24 hr, regardless of the presence or absence of salts. Within the experimental conditions used, temperature is the most important factor in the sepiolite-to-stevensite conversion.

At or below 216°C, sepiolite appears to transform into stevensite by dislocations involving c12 glides that are triggered by the stresses of the hydrothermal conditions. Above this temperature, stevensite seems to form by direct precipitation after dissolution of sepiolite.

Saponite crystallizes from amorphous gel having an ideal saponite composition within 7 days at all experimental temperatures between 300° and 550°C at 1 kbar pressure. Reactions subsequent to this initial crystallization vary in type and degree, depending on the temperature of reaction and the type of interlayer cation. Above 450°C, the intitially crystallized K-saponite dissolves, and talc and phlogopite nucleate and grow as discrete phases. At 450°C the initial K-saponite reacts to form talc and phlogopite layers, but the reaction proceeds via intracrystalline layer transformations rather than via dissolution and precipitation, producing a mixture of fully ordered, interstratified talc/saponite and fully ordered saponite/phlogopite. The K-saponite shows subtle signs of reaction at 400°C after 200 days: this temperature is at least 150°C lower than experimental reaction temperatures previously reported for saponites. No reactions beyond the initial crystallization of saponite were observed below 400°C. K-saponite reacts more rapidly than either Na-saponite or Ca-saponite above 400°C, and the Na-saponite and Ca-saponite produce no mica layers during their transformation to mixed-layer clays. Interstratified talc/saponite formed in the Na-saponite system, and the Ca-saponite system produced both talc/saponite and chlorite/saponite.

Many environmental applications in the inorganic remediation field are based on the swelling and ion-exchange capacities of smectites, even though these can be affected by hydrothermal treatment in water and acidic media. Here a systematic study of the properties of layered silicates that could affect their hydrothermal stability at different pH is described: type of layers, octahedral occupancy, layer charge, and origin of the layer charge. The silicates studied were selected on the basis of their different characteristics associated with these properties. Kanemite (1:0 phyllosilicate), kaolinite (1:1 phyllosilicate), and pyrophyllite and talc (2:1 phyllosilicates with no-layer charge) were examined in order to determine the effect of layer structure, whereas the hydrothermal reactivity of silicates with different layer charge was analyzed by comparing the talc-hectorite-Laponite® and talc-saponite-trioctahedral vermiculite series. Samples were treated hydrothermally at 300ºC for 48 h in pure water and in a 0.01 M HNO3 solution and the final products were analyzed by X-ray diffraction, scanning electronic microscopy, and solid-state nuclear magnetic resonance spectroscopy. All layered silicates, except for kanemite, were found to remain intact after hydrothermal treatment in water and acidic media, with only minimal short-range structural changes observed. The extent of the structural changes depended on the octahedral sheet occupancy (greater extent) and the number of isomorphic substitutions (lesser extent), both of which weaken the structure.