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

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 (∼Al ₂O ₃(SiO ₂) _1.3–2·2.5-3H ₂O) and hisingerite (∼Fe ₂ ³⁺Si ₂O ₅(OH) ₄∙2H ₂O), 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: pK _HAS=2.9·Al _s/Si _s+7.9r ²=0.96n=6 pK _HAS−HFS=−23.2·Al+Fe _s/Si _s+24.8r ²=0.94n=5 pK _HFS=23.5·Fe _s/Si _s–26.3r ²=0.86n=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 providing highly reactive substrates for the formation of thermodynamically more stable kaolinite and smectite group minerals.