Abstract: 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 c/2 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.