Precipitation of short-range order hydroxy aluminosilicate (HAS) and hydrous ferric silicate (HFS) at ambient temperature: Insights into mineral formation pathways, crystal chemistry and solubility-stability relationships

Abstract

Chemical weathering of silicates on continents and the subsequent formation of clay minerals are important processes within the Earth’s critical zone, controlling pH, water-holding capacity and ion exchange properties of soils. Short-range ordered (SRO) hydroxy aluminosilicate (HAS) and hydrous ferric silicate (HFS) phases, such as allophane (~Al₂O₃(SiO₂)₁.₃–₂⋅2.5-3H₂O) and hisingerite (~Fe³⁺Si₂O₅(OH)₄•2H₂O), are such common soil clays, but their crystal-chemical properties, solubilities and formation paths remain disputed. In this study, pure HAS and HFS phases were precipitated at molar [Al]ₐq/[Si]ₐq and [Fe]ₐq/[Si]ₐq ratios of 1.0, 1.3, 1.5 and 2.0 and ambient temperature using equilibrium-approaching experiments. The formation of HAS-HFS minerals was studied at [(Al + Fe)]ₐq/[Si]ₐq = 1 using replacement levels of [Fe]ₐq for [Al]ₐq of 10%, 25%, 50%, 75% and 90%. HAS, HFS and HAS-HFS minerals were formed at pH ~3–6 through condensation of silica tetrahedrons onto Al/ Fe-O-OH octahedral templates. The [Al]s/[Si]s, [Fe]s/[Si]s and [(Al + Fe)]s/[Si]s ratios of the precipitated SRO phases ranged from 0.7 for HAS and 0.7–1.0 for HAS-HFS to 1.0–1.3 for HFS minerals, and correlate linearly with the values of the solubility constants (pK) obtained herein and from literature as follows: pKHAS = 2.9⋅[Al]s/[Si]s + 7.9 (r² = 0.96; n = 6) pKHAS-HFS = - 23.2⋅[(Al + Fe)] /[Si]s + 24.8 (r² = 0.94; n = 5) pKHFS = 23.5⋅[Fe]s/[Si]s–26.3 (r² = 0.86; n = 4) The faster formation kinetics and lower solubility of HFS phases (pK = -2.2 to 4.7) and HAS-HFS phases (pK = -1.0 to 6.0) compared to HAS phases (pK = 10.2 ± 0.3) suggests that hisingerite-like and Fe-substituted allophane-like minerals are probably more abundant in the Earth’s critical zone than previously thought, thus hly reactive substrates for the formation of thermodynamically more stable kaolinite and smectite minerals.