The Hydrothermal Synthesis of Sulfate Cancrinite (SO4‐CAN): Relations between Si‐Al Sources and Crystal Quality

Sulfate cancrinite (SO₄‐CAN) Na₈AlSiO₄]₆(SO₄)(H₂O)_n (2.6 < n < 3.2) was synthesized under hydrothermal conditions at 200 °C and 48 hours. Three different Si‐Al sources were inserted: (a) kaolinite (K), (b) a gel of sodium‐waterglass and sodium aluminate (G), and (c) an oxide mixture of cristobalite and corundum (CK). The products were characterized by X‐ray powder diffraction (XRD), ²⁹Si and ²⁷Al MAS NMR spectroscopy, scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDXS) and BET surface area analysis as well as simultaneous thermoanalysis (TG‐DTA). Mostly nanocrystals of platelet habit and dimensions smaller than 100 nm (beside a very few larger crystals) were observed from K. In contrast the gel (G) yielded to cancrinites with typical rod‐like morphology and dimensions of 100 × 500 nm. Large prismatic cancrinites up to 5 μm width and 10 μm length were observed from the oxide mixture CK. Furthermore the transition kinetics of the cancrinites into nosean was investigated at a temperature of 1000 °C under open conditions. This process was already complete after 1 hour for the nanocrystals from K but needed 10 hours for the nanocrystals from G and the microcrystals from CK. The relations between the individual Si‐Al source and the crystal properties like size, habit, and thermal transition kinetics were discussed.