Isomorphous substitution vs. defect density in hydrothermally synthesized (200 °C) Fe3+, Ga3+ and Cr3+-substituted kaolinites

Fe3+, Ga3+ and Cr3+ substituted kaolinites had been synthesized at hydrothermal conditions (200 °C), starting from very disordered ground kaolinite and Fe3+- Ga3+- and Cr3+-bearing solutions. The synthetic kaolinites showed a roughly bimodal distribution with large particles (1000 × 10,000 Å) and smaller aggregates of rounded (< 500 Å), lath shaped (100 × 2000 Å), or very thin curved particles (< 100 × < 1000 Å). Fe3+ and Ga3+ substituted kaolinites contained only minor amounts of R3+ for Al substitution (on the order of 0.05 atoms per formula unit), whereas the Cr3+ content reached up to 0.58 atoms per formula unit. Similarly, the Fe3+ and Ga3+ substituted kaolinites showed relatively high Hinckley index (> 0.8), whereas Cr3+-substituted kaolinite showed values < 0.4, revealing a clear relationship between degree of isomorphous substitution and defect density in kaolinite. The amount of isomorphous substitution appeared to be controlled by the presence of associated metallic (hydr)oxides, which prevented the entry of the cations into the kaolinite structure.