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Synthetic Allophanes Formed in Calcareous Environments: Nature, Conditions of Formation, and Transformations
Author(s) -
Farmer V. C.,
McHardy W. J.,
Palmieri F.,
Violante A.,
Violante P.
Publication year - 1991
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1991.03615995005500040044x
Subject(s) - allophane , saponite , silicate , aluminosilicate , chemistry , imogolite , halloysite , calcareous , infrared spectroscopy , inorganic chemistry , mineralogy , clay minerals , geology , organic chemistry , geochemistry , paleontology , catalysis
Synthetic allophanes were prepared by adding excess CaCO 3 to solutions containing Al(NO 3 ) 3 and Si(OH) 4 (concentrations ≤1.5 m M ) with molar Si/Al starting ratios ranging from 1:8 to 4:1. The systems were incubated at 20 and 80 °C until the concentration of Si(OH) 4 in solution approached stability after 6 to 10 wk, when the allophanes were isolated and characterized by infrared (IR) spectroscopy and electron microscopy. At 20 °C, proto‐imogolite allophane, rather than Al(OH) 3 species, formed when Si in solution exceeded about 50 µ M , but hydrous feldspathoids, containing tetrahedral Al, were progressively formed when Si in solution exceeded about 150 µ M . At 80 °C, partial or nearly complete transformation of these products to proto‐halloysite occurred for Si in solution above 50 µ M . The presence of 1 m M Mg and K in the starting solutions did not affect the allophanes formed at 20 °C, but modified those formed at 80 °C. For starting Si/Al ratios of 1:2 and 1:1, the formation of proto‐halloysite was partially inhibited, but for starting ratios of 2:1 and 4:1, evidence for the initiation of 2:1 trioctahedral layer silicate saponite was obtained. It is postulated that an alkaline environment favors the transformation of allophanes to layer silicate clays, and that this may account for the scarcity of reports of allophanes in calcareous horizons. A marked decrease in Si in solution when acid soils are limed is explained.