CHARGE HETEROGENEITY AND NANOSTRUCTURE OF 2 1 LAYER SILICATES BY HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY/

Several soil and reference smectites and vermiculites and one referenc e illite were examined by high-resolution transmission electron micros copy (HRTEM) to decipher the nanostructure and layer charge heterogene ity in these minerals. HRTEM results were compared with those obtained from powder X-ray diffraction (XRD) analysis. Samples were either exc hanged with Na+ ions followed by equilibration with a very dilute solu tion of NaCl in a pressure membrane apparatus at 316 hPa (pF = 2.5) to see the effect of hydration and applied pressure on layer organizatio n, or exchanged with dodecylammonium ions to see the expansion behavio r. Oriented samples were embedded in a low viscosity resin and cut app roximately 500 angstrom thick perpendicular to d(001) using an ultrami crotome fitted with a diamond knife. In general, Na-saturated soil cla ys possessed crystallites that were thinner (c-direction) and shorter (ab-direction) as compared with reference clays. In all cases, samples treated with dodecylammonium chloride exhibited nanostructures that w ere more disintegrated as compared with Na-saturated samples. In a soi l vermiculite, dodecylammonium ion exchange showed frayed edges indica ting the initiation of mica transformation to vermiculite from edge to ward core. In a reference vermiculite (Transvaal) treated with dodecyl ammonium ions, in addition to completely expanded crystallites, a regu lar interstratification between expanded vermiculite and mica (phlogop ite) layers was clearly observed in some crystallites. Such nanostruct ural details were not detected by XRD. HRTEM of the Na-treated illite showed thick crystallites having 10 angstrom layer separations, wherea s the dodecylammonium-exchanged illite showed three types of layers wi th different degrees of expansion indicating charge heterogeneity in i llite: 1) unexpanded (10 angstrom, highest charge) crystallites; 2) ex panded high-charge vermiculite-like (24 angstrom) crystallites; and 3) occasionally expanded high-charge vermiculite-like (24 angstrom) laye rs interspersed in the matrix of 10 angstrom crystallites.