Layer charge location in layer silicates and its implications

Layer charge value and location are commonly considered key factors for predicting interaction of layer silicates with external agents, such as, for example, pollutants, bio-molecules, and, more generally, ionic complexes. Several methods are discussed in literature for the experimental determination of layer charge, always, however, giving an average value from the different micro-crystals used for the analysis. The most commonly used methods are: the structural formula method [1, 2], the alkylammonium method [3] and the potassium saturation method [4]. The purpose of this paper is to demonstrate that the determination of the average value of the layer charge can sometimes lead to inconsistent results, as not correctly representing the layer charge location at mineral surface. This goal was achieved by analyzing crystal chemical results obtained from different crystals of muscovite characterized by a different content of celadonitic substitution. Muscovite sometimes presents heterovalent substitutions both in tetrahedral (e.g., Al3+ for Si4+) and in octahedral sites (e.g., Mg2+, Fe2+, and vacancies for Al in octahedral sites). Our results seem to demonstrate that these substitutions can be ascribed to trioctahedral cells, all occupied by cations different than Al, in a dominating dioctahedral pattern, with cis-octahedral sites entirely occupied by Al and the trans-site vacant. This interpretation suggests that mean layer charge may not represent effective layer charge at mineral surface because it refers not to randomly distributed substitution, but to domains showing different chemistry which can be locally charge-neutralized. Furthermore, also distortion parameters which affect tetrahedral topology and that can also affect the formation of surface complexes, such as alpha angle and delta z oxygen plane flattening, can be different inside a same structure and locally not corresponding strictly to the determined mean value via single crystal diffraction.