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

Fe ³⁺, Ga ³⁺ and Cr ³⁺ substituted kaolinites had been synthesized at hydrothermal conditions (200 °C), starting from very disordered ground kaolinite and Fe ³⁺- Ga ³⁺- and Cr ³⁺ -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). Fe ³⁺ and Ga ³⁺ substituted kaolinites contained only minor amounts of R ³⁺ for Al substitution (on the order of 0.05 atoms per formula unit), whereas the Cr ³⁺ content reached up to 0.58 atoms per formula unit. Similarly, the Fe ³⁺ and Ga ³⁺ substituted kaolinites showed relatively high Hinckley index (> 0.8), whereas Cr ³⁺-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.