Validity of the structural formula method for layer charge determination of smectites: A re-evaluation of published data

The validity of the structural formula (SF) method for calculation of layer charge of smectites is examined through re-interpretation of published data, which suggest that the SF method overestimates layer charge. The overestimation of layer charge by SF is based on assumptions about the permanent CEC (CECperm) of smectites i) on the association of the molar mass of half unit cell (MHUC) with the CECperm of the smectitic clay fraction and ii) on imbalanced SF calculated for a series of used smectites. The CECperm of smectites should not be determined at pH 4 because of competitive adsorption of H+ cations at exchangeable sites. This was verified by monitoring the pH of acidified smectite suspensions. Instead the pH at the isoelectric point (iep) should be used for determination of permanent charge of smectites. Moreover it is suggested that the equation of Lagaly [Lagaly, G., 1994. Layer charge determination by alkylammonium ions. In: Mermut, A.R. (Ed.), Layer Charge Characteristics of 2:1 Silicate Clay Minerals. CMS Workshop lectures, vol. 6. The Clay Minerals Society, Boulder Colorado, pp. 2–46] which relates the smectite content with layer charge may be used only if CECperm is calculated on a totally anhydrous basis, otherwise it may lead to significant underestimation either of smectite content or of layer charge.