The cis- and trans-vacant variety of a montmorillonite: an attempt to create a model smectite

Na+-saturated samples (<2 μm) of a cis-vacant (cv) montmorillonite from Linden (Bavaria) were converted into the trans-vacant (tv) variety by dehydroxylation between 510°C and 630°C and subsequent rehydroxylation under steam between 1.5 and 8.0 MPa. The tv variety is characterised by a dehydroxylation temperature which is about 190°C lower than that for the starting material. The large Na+ (ionic ratio>0.97 A) are located on fully exchangeable and hydratable interlayer positions after transformation and the montmorillonite retained its interlamellar swellability. Both varieties of the montmorillonite are different in their rheological properties because of the aggressive conditions during conversion and not because of the transformation itself. The initial Na+-rich cv montmorillonite delaminated nearly completely in salt-free water, producing a high viscous suspension. In contrast, aqueous suspensions of the obtained tv montmorillonite coagulated because of diluted Al3+ and as a result of sintered particles. These changes of the rheological properties are not reversible by ultrasonic or soda treatment.