An interstratified illite-smectite from Denchworth series soil in weathered Oxford Clay

The very fine (<0·04 µm) clay fraction of a Denchworth series subsoil in weathered Oxford Clay has been investigated in detail. It contains a layer silicate mineral, almost free from impurities, that is a randomly interstratified mixture of approximately 45% expanding and 55% non-expanding interlayers with the following unit formula: Na 0.33 K 0.29 (Al 1.41 Fe 0.34 Mg 0.26 )(Si 3.60 Al 0.40 )O 11 . Computer-simulated diffraction patterns showed that experimental patterns are best matched when an average diffracting packet consists of between 4 and 5 2:1 layer silicate units. Correction of the chemical composition for external exchangeable ions associated with such small packets gives non-expanding interlayers a charge density of 0·69K/Si 4 O 11 and expanding interlayers one of 0·57M + /Si 4 O 11 . The relationship between these values and those of mica and regularly expanding minerals is briefly discussed.