Particle orientation distributions and stacking arrangements in size-fractionated montmorillonite measured by neutron and X-ray diffraction

Abstract

The advantages of using neutron diffraction techniques to obtain platelet orientation distributions are described. These distributions have been obtained for Na⁺-, Ca²⁺- and Mg²⁺-exchanged, particle-size-fractionated, montmorillonite clay-film preparations containing two water layers between adjacent silicate layers. The measurements employed neutron diffraction ‘rocking curves’ corrected for attenuation effects. The data could be described by single broad Gaussians of f.w.h.m. ca. 50° except for the finest fraction of Na⁺-montmorillonite (< 0.05 µm e.s.d.), which required a narrow Gaussian (f.w.h.m. ca. 25°) superimposed on a broad Gaussian (f.w.h.m. ca. 50°) to fit the data adequately. Moreover, the orientation distributions of the Ca²⁺- and Mg²⁺-exchanged clays showed no dependence on particle size except when the samples were prepared from suspensions that had been subjected to ultrasonic vibrations. These results are discussed in terms of the association of particles into microaggregates and flocs in suspension.

X-ray and neutron diffraction patterns were obtained for both coarse and fine fractions of Na⁺-montmorillonite. For the coarse materials the widths of the measureable (00l) X-ray reflections increased with order. This behaviour is described by a model which takes account of finite crystallite size and of variable planar spacing within the crystallites. For the fine fraction on the other hand the widths were independent of order, hence only crystallite size effects were important. The information gained from these results underlines the importance of applying both X-ray and neutron techniques to the study of polydisperse lamellar crystallites.