The smectitic minerals in a bentonite deposit from Melo (Uruguay)

A nearly monomineralic 1.5 in thick bentonite bed sampled ill Melo (Uruguay) appears to be a pure high-charge montmorillonite: [Si3.94Al0.06](Al1.40Fe0.113+Ti0.02Mg0.49Mn0.01)O-10 (OH)(2)Na0.01K0.08Ca0.18. However, contrasting swelling behaviours have been demonstrated by fitting the experimental X-ray diffraction patterns which were recorded oil oriented preparations of the same sample in different saturation states. According to the expandability of the layers in the Ca-, K- and K-Ca-saturated (i.e. saturated first with K+ and subsequently With Ca2+) States. three 'layer types' were defined. Low-, intermediate-, and high-charge layers are fully, partly. and not expandable, respectively, after K-saturation. Collapse of high-charge layers is not reversible after subsequent Ca-saturation, probably because of tetrahedral substitutions. These three different layer types are segregated in two distinct randomly interstratified mixed-layer phases. Total surface area and cation exchange capacity are shown to depend on the interlayer cation composition.