Granite weathering and clay mineral formation impart distinct and interpretable stable Si isotope (d30Si) signatures to their solid and aqueous products. Within a saprolite, clay minerals have d30Si values ;2.0‰ more negative than their parent mineral and the d30Si signature of the bulk solid is determined by the ratio of primary to secondary minerals. Mineral-specific weathering reactions predominate at different depths, driving changes in differing d30Sipore water values. At the bedrock-saprolite interface, dissolution of plagioclase and hornblende creates d30Sipore water signatures more positive than granite by up to 1.2‰; these reactions are the main contributor of Si to stream water and determine its d30Si value. Throughout the saprolite, biotite weathering releases Si to pore waters but kaolinite overgrowth formation modulates its contribution to pore-water Si. The influence of biotite on d30Sipore water is greatest near the bedrock where biotite-derived Si mixes with bulk pore water prior to kaolinite formation. Higher in the saprolite, biotite grains have become more isolated by kaolinite overgrowth, which consumes biotite-derived Si that would otherwise influence d30Sipore water. Because of this isolation, which shifts the dominant source of pore-water Si from biotite to quartz, d30Sipore water values are more negative than granite by up to 1.3‰ near the top of the saprolite.