The stabilities of hydroxy-Al and -Fe interlayers in montmorillonite and vermiculite were evaluated by subjecting the systems to HCl treatment and to sequential treatments which included boiling 2% Na2CO3, buffered sodium citrate-dithionite, a second citrate-dithionite treatment, and boiling NaOH.
The stability of hydroxy interlayers was dependent on kind of mineral matrix, whether the interlayer was Fe or Al, and the conditions of acidity under which they were initially formed. Hydroxy-Al interlayers formed under acid conditions were more stable than those formed under alkaline conditions in both montmorillonite and vermiculite systems. Iron interlayers formed under neutral and alkaline conditions in montmorillonite and vermiculite were more stable than those formed in acid media. The hydroxy-Al and -Fe interlayers in montmorillonite exhibited a higher stability than their counterparts in vermiculite.
A residual 14A line after K-saturation and heating to 550C was observed in some samples which received the dithionitecitrate and NaOH treatments. This occurred to a greater extent in vermiculite and particularly with Fe-hydroxy systems. The data suggested a resolution of a mixed system into collapsible and noncollapsible components.
In montmorillonite, the hydroxyl-Al interlayer, formed in alkaline conditions, showed the highest stability to the HCl treatment, whereas that formed in acid medium was the least stable. In vermiculite, the stability of hydroxy-Al interlayer to the HCl treatment decreased from acid to alkaline media.
Iron interlayers in montmorillonite formed in all media exhibited high stabilities to the HCl treatments. In vermiculite, the HCl treatments removed more interlayers from acid media than from alkaline media.
Some of the dissolution treatments tested in this study are often used as pretreatments for removal of coatings and cementing agents prior to separation or analysis of clay fractions. These reagents will remove hydroxy interlayers and result in changes of properties of clays from soils and sediments. This must be recognized for proper interpretation of data for clay mineral identification and characterization.