THE NATURE OF DIFFRACTION EFFECTS FROM ILLITE AND ILLITE-SMECTITE CONSISTING OF INTERSTRATIFIED TRANS-VACANT AND CIS-VACANT 2 1 LAYERS - A SEMIQUANTITATIVE TECHNIQUE FOR DETERMINATION OF LAYER-TYPE CONTENT/

Previous work has described the structural and diffraction criteria fo r distinguishing between tv-1M, cv-1M, and m-1M illite varieties, wher e tv-1M illite corresponds to a structure with trans vacant (tv) 2:1 l ayers, cv-1M illite consists of cis-vacant (cv) 2:1 layers, and m-1M i llites consist of 2:1 layers with random distributions of octahedral c ations over trans and cis sites within each layer. Detailed analysis o f calculated and experimental X-ray diffraction (XRD) patterns from il lites and mixed-layered illite-smectites (I/S) with a range of cis- an d trans-vacant layer interstratification, and consideration of unit-ce ll parameters for pure tv-1M and cv-1M provides insight into the natur e of the diffraction effects from the interlayered structures with dif ferent types and contents of rotational stacking defects. Structural f ragments of the 2M(1) and 2M(2) polytypes exist within 1Md structures in which rotational disorder is dominated by n . 120 and n . 60 degree s, respectively. Such structural fragments constitute coherent-scatter ing domains and affect the peak positions of the diagnostic 11l reflec tions for rotationally disordered I/S and illites in different ways de pending on the proportion of cis- and trans-vacant 2:1 layers. Simple techniques that do not require computer programs were developed for th e semiquantitative determination of the proportion of tv and cv 2:1 la yers in I/S and illites where such layers are interstratified. These t echniques may help to reveal the diversity of I/S and illite samples r elating to the conditions of their formation and transformation.