The layer charge density (LCD) of montmorillonite represents the permanent negative charge, its most important property. The LCD can be determined by two different methods, the structural formula method (SFM) and the alkylammonium method (AAM). Other methods of determining the LCD are calibrated against one or the other of these. The results of the two methods differ systematically: SFM values are larger than AAM values and the difference increases with increasing layer charge density.
In the present study, the critical parameters of both methods were considered quantitatively in order to identify the most likely reason for the systematic difference. One particularly important argument against the validity of the SFM is that typical SFM values correspond to unrealistically large CEC values that have never been reported. In addition, SFM does not consider the variable charge which causes cations to be adsorbed to the outer surface (at pH >4). In contrast to minor constituents, which can of course also affect SFM values, the variable charge can explain only part of the systematic difference. The exchange of pure smectite samples with both Cu-trien and alkylammonium revealed the presence of non-exchangeable, non-structural cations (Na, K, Ca). These cations, together with 10% (or more) variable charge, may explain the differences in LCD values. The non-exchangeable, non-structural cations could stem from undetected traces of feldspar or volcanic glass. The present samples indicated that the systematic difference in LCD values between the two methods is related to the amount of non-exchangeable, non-structural cations only, indicating that the two LCD methods probe different features of smectites. Using the SFM on pure smectite provides a value for the total number of charges (permanent with and without fixed (= non-exchangeable, non-structural) cations plus variable charge). The AAM, on the other hand, provides the charge density of the exchangeable cations (without variable charge).