Rapid SRO aluminosilicate formation in a narrow pH interval fractionates silicon isotopes

Kayla Russo, Hunter T Jamison, Louis A Derry, Nicole M. Fernandez 

Short-range ordered (SRO) hydrous aluminosilicates (HAS) are among the earliest secondary phases formed during incongruent silicate weathering, yet the timescales and silicon isotope (δ30Si) fractionation associated with their initial formation remains poorly constrained. We use stepwise room temperature NaOH titration to resolve pH buffering, Si–Al removal, and dissolved δ30Si evolution in two complementary organosilicate-derived Si experiments and an experiment with an inorganic silica source. All three titrations (initial Si/Al ≈ 1) showed coupled Si–Al removal across a narrow pH interval and converged on Al-rich net uptake in solid phases (Si/Al ≈ 0.47 – 0.52). FTIR, XRD, and SEM of recovered precipitates support poorly ordered allophanic SRO HAS products. In the organosilicate-derived experiments, dissolved δ30Si increased by ~ 2 ‰ as Al was removed. First-order Rayleigh models yield apparent kinetic isotope fractionation factors of – 2.56 to -2.08 ‰, although late-stage deviations show that no single factor captures the full trajectory. These results identify allophane-like SRO HAS formation as a rapid process for generating measurable δ30Si signatures during incipient weathering.