Incongruent dissolution occurs when a mineral breaks down into a different solid phase plus dissolved species, rather than dissolving completely into solution. In humid climates, hydrolysis of orthoclase (K-feldspar) proceeds incongruently, producing a residual clay mineral (kaolinite) plus dissolved silica (silicic acid) and potassium ions, which are leached away.
In a humid climate weathering profile, orthoclase undergoes hydrolysis: 2KAlSi3O8 (orthoclase) + 2H⁺ + H2O → Al2Si2O5(OH)4 (kaolinite) + 4SiO2(aq, as H4SiO4 silicic acid) +...
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