Conversion of ferruginous allophanes to ferruginous beidellites at 95 degrees C under alkaline conditions with alternating oxidation and reduction

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 degrees 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 AI: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(Si6.85Al1.15)(Al2.47Fe1.53)O-20(OH)(4). This composition lay within the range recorded for the fermginous beidellites that form in Vertisols.