Hydrothermal synthesis of dioctahedral smectites: The Al–Fe3+ chemical series

The hydrothermal synthesis of smectites in the Al–Fe ³⁺ chemical series was investigated at various pHs and temperatures using stoichiometric coprecipitated hydrogels. For the first time, Al–Fe ³⁺ smectites within a wide compositional range were successfully synthesized. The windows of experimental conditions to crystallize such smectites are narrow. Temperatures up to 150 °C and high pH (around 12) are suitable for the formation of the Fe ³⁺-rich smectitic end-member (nontronite), while higher temperatures (200 °C) and lower pH (down to 7) are suitable for the more Al-rich smectites (Fe ³⁺-beidellites). For intermediary compositions, the pH and temperature of synthesis leading to pure smectite crystallization should be concomitantly adjusted. Outside these windows, admixtures or other phases are obtained. This is probably the reason why smectites of the beidellite–nontronite series were not extensively synthesized previously.