Kaolinite ($\mathrm{A}{\mathrm{l}}_2\mathrm{S}{\mathrm{i}}_2{\mathrm{O}}_5(\mathrm{OH}{)}_4$)

A clay mineral with composition $\mathrm{A}{\mathrm{l}}_2\mathrm{S}{\mathrm{i}}_2{\mathrm{O}}_5(\mathrm{OH}{)}_4$. Relative to gibbsite, its greater silica content allows higher dissolved silica thermodynamic activity to favor its formation, as quantified by the appropriate equilibrium constant.