Insight into the spiral growth of disordered kaolinite nanocrystals

Structural disorder in kaolinite critically shapes its reactivity and performance across environmental and industrial contexts. Insights into spiral growth offer a mechanistic framework to decode the structural disorder of kaolinite and advance our understanding of its formation and properties. We identified a distinct expansion of the (001) interlayer spacing, from ∼7.16 Å in well-ordered kaolinite to ∼7.21 Å in disordered samples, along with selective enhancement of the (020) reflection in disordered nanocrystals. This structural disorder is closely linked to Al(4)-for-Si(4) substitution, with a high Al(4)/Al total ratio (∼2.83 %). The ionic radius ratio of $R_{{\mathrm{Al}}^{\mathrm{III}}\left(4\right)}$_:$R_{O^{2-}}$=0.438 substantially exceeds that of ideal tetrahedral packing, compared to $R_{{\mathrm{Si}}^{\mathrm{IV}}\left(4\right)}$_:$R_{O^{2-}}$=0.331, generating internal stress that exceeds the structural tolerance of triclinic kaolinite. When this stress exceeds a critical threshold, it likely promotes the formation of screw dislocations, which initiate spiral growth and generating stacking faults and in-plane lattice rotations (∼5°), as evidenced by Moiré fringe patterns and SAED. Spiral growth thus acts as a stress-adaptive mechanism, enabling the crystal to accommodate structural instability while maintaining long-range order and anisotropic deformation. These findings recast disorder as a stress-regulated growth strategy and offer a mechanistic blueprint for tuning structure in low-dimensional layered materials.