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Conversion of Ferruginous Allophanes to Ferruginous Beidellites at 95 °C Under Alkaline Conditions With Alternating Oxidation and Reduction

Published online by Cambridge University Press:  28 February 2024

V. C. Farmer
V. C. Farmer*
Affiliation:
Division of Soils, Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, United Kingdom
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Abstract

Ferruginous beidellites with Al:Fe atomic ratios up to 2.36 were obtained when solutions containing Al, Fe2+ and H4SiO4 were adjusted to pH 8.5 with Ca(OH)2 and incubated at 95 °C in the presence of CaCO3 as a pH buffer. Incubation took place under cyclic reducing and oxidizing conditions achieved by adding 2 mM hydrazine at 14–15-d intervals over a period of 10–13 weeks. During the 14–15-d cycle, atmospheric oxygen slowly diffused through the high-density polyethylene bottles used, causing a slow oxidation of Fe(II) to Fe(III). The infrared (IR) spectra of the products approached that of natural beidellite, but indicated little change in octahedral Al:Fe ratio in the products for starting Al:Fe ratios from 2.5 up to 3.5, which was the highest Al:Fe ratio at which a well-crystallized product was obtained. Chemical analysis showed the presence of more Al+Fe in the products than could be incorporated into a dioctahedral formula. After the excess was assigned to a hydroxy-aluminium interlayer, the formula of the most Al-rich beidellite was calculated to be 0.575Ca(Si6.85A1.15)(Al2.47Fe1.53)O20(OH)4. This composition lay within the range recorded for the ferruginous beidellites that form in Vertisols.

Keywords

AllophaneBeidelliteInfrared SpectraNontroniteVertisolsX-ray Diffraction (XRD)
Type
Research Article
Information
Clays and Clay Minerals , Volume 45 , Issue 4 , August 1997 , pp. 591 - 597
Copyright
Copyright © 1997, The Clay Minerals Society

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