Comparison of methods for the determination of the layer charge density (LCD) of montmorillonites

Abstract

Two wide spread methods exist for the determination of the layer charge density (LCD) of montmorillonites. 1) Structural formula method (SFM): calculation of the structural formula based on the chemical composition (Ross, C.S., Hendricks, S.B. (1945): Minerals of the montmorillonite group. — U. S Geol Surv Profess Paper 205-B, p. 23–79.; Stevens, R.E. (1946): A system for calculating analyses of micas and related minerals to end members. — Contributions to geochemistry, 1942–45, Geological survey bulletin, 950, p. 101 – 119.; Grim, R.E., Güven, N. (1978): Bentonites – geology, mineralogy, properties and uses. – Developments in sedimentology 46, Elsevier, New York, pp. 254.; Newman, A.C.D., Brown, G. (1987): The chemical constitution of clays. In: Chemistry of clays and clay minerals, Mineralogical Society Monograph No. 6, Newman, A.C.D. (ed.), John Wiley and sons, New York, p. 1 – 128.; Laird, D.A. (1994): Evaluation of the structural formula and alkylammonium methods of determnining layer charge. — In: Mermut, A.R., editor: Layer charge characteristics of 2:1 silicate clay minerals, CMS Workshop Lectures Vol. 6, Boulder, CO, Clay Miner. Soc., p. 79 – 104). 2) Alkylammoniummethod (AAM): calculation of the LCD from measuring the d-spacing after intercalation of a set of n-Alkylammoniumions (Lagaly, G. (1994): Layer charge determination by alkylammonium ions.—In: Mermut, A.R., editor: Layer charge characteristics of 2:1 silicate clay minerals, CMS Workshop Lectures Vol. 6, Boulder, CO, Clay Miner. Soc., p. 1 – 46). The SFM generally provides higher values (up to 40%, Laird, D.A. (1994): Evaluation of the structural formula and alkylammonium methods of determnining layer charge. — In: Mermut, A.R., editor: Layer charge characteristics of 2:1 silicate clay minerals, CMS Workshop Lectures Vol. 6, Boulder, CO, Clay Miner. Soc., p. 79 – 104.). Currently, most clay mineralogists from all over the world use either SFM or AAM which leads to incomparable results. In order to unambiguously prove which method provides correct values for the LCD and to be able to calculate correct montmorillonite contents two alternative methods (method A1 and A2) are tested in this paper. These procedures are independent from the well accepted SFM and AAM. Method A1, based on the determination of CEC and inner surface area by water adsorption, failed because water adsorption depends to a higher degree on the amount of exchangeable cations than on the surface area. Method A2 is based on measuring the variable charge, the total charge (CEC) of a pure smectite fraction, and the molar mass of a formula unit. This method (A2) is applicable if fine grained impurities do not interfere with the production of the pure smectite fraction. In conclusion, by applying a fully independent method for determining the LCD the results obtained by the AAM were confirmed. This data strongly indicates that the AAM as proposed by Lagaly (1994)[Lagaly, G. (1994): Layer charge determination by alkylammonium ions.—In: Mermut, A.R., editor: Layer charge characteristics of 2:1 silicate clay minerals, CMS Workshop Lectures Vol. 6, Boulder, CO, Clay Miner. Soc., p. 1 – 46.] provides reasonable values for the LCD. Additionally, it is shown that high LCD values as commonly determined by SFM for medium to high charged smectites cannot lead to the calculation of correct montmorillonite contents. However, there is no doubt, that high charged and hence non-swellable 2:1 layer silicates cannot be investigated by the AAM (as discussed by Mermut, A.R. (1994): Problems associated with layer charge characterisation of 2:1 phyllosilicates. — In: Mermut, A.R., editor: Layer charge characteristics of 2:1 silicate clay minerals, CMS Workshop Lectures Vol. 6, Boulder, CO, Clay Miner. Soc., 79 – 104).