TY - JOUR
T1 - Precipitation of short-range order hydroxy aluminosilicate (HAS) and hydrous ferric silicate (HFS) at ambient temperature
T2 - insights into mineral formation pathways, crystal chemistry and solubility-stability relationships
AU - Baldermann, Andre
AU - Stamm, Franziska M.
AU - Farkaš, Juraj
AU - Löhr, Stefan
AU - Ratz, Bettina
AU - Letofsky-Papst, Ilse
AU - Dietzel, Martin
N1 - Copyright the Author(s) 2023. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.
PY - 2024/2/20
Y1 - 2024/2/20
N2 - 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 (∼Fe23+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)]s/[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 providing highly reactive substrates for the formation of thermodynamically more stable kaolinite and smectite group minerals.
AB - 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 (∼Fe23+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)]s/[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 providing highly reactive substrates for the formation of thermodynamically more stable kaolinite and smectite group minerals.
KW - Allophane
KW - Aluminosilicate
KW - Clay nanoparticles
KW - Hisingerite
KW - Soils
KW - Weathering
UR - http://www.scopus.com/inward/record.url?scp=85181775155&partnerID=8YFLogxK
UR - https://purl.org/au-research/grants/arc/DP210100462
U2 - 10.1016/j.chemgeo.2023.121911
DO - 10.1016/j.chemgeo.2023.121911
M3 - Article
AN - SCOPUS:85181775155
SN - 0009-2541
VL - 646
SP - 1
EP - 16
JO - Chemical Geology
JF - Chemical Geology
M1 - 121911
ER -