On the structural formula of smectites: a review and new data on the influence of exchangeable cations

The understanding of the structural formula smectite minerals is basic to predicting their physicochemical properties, which depend on location cation substitutions within 2:1 layer. This implies knowing correct distribution and positions cations, allows assigning source layer charge tetrahedral or octahedral sheet, determining total number cations and, consequently, type smectite. However, sometimes obtained not accurate. A key reason for complexity obtaining presence different exchangeable especially Mg. Most smectites, some extent, contain Mg 2+ that can be both interlayer positions. indeterminacy lead errors when constructing formula. To estimate position ions, over positions, it necessary substitute work with samples saturated a known (homoionic samples). Seven smectites dioctahedral trioctahedral types were homoionized Ca , substituting natural cations. Several differences found between formulae homoionic samples. Both charges increased, calculated numbers in closer four six respectively, respect values non-homoionic change was limited sheet interlayer, because content also changed. For samples, improved considerably after homoionization although higher accuracy more magnesic were. Additionally, changes resulted changing classification