Tarasovite, a mixed-layer illite-smectite which approaches an ordered 3:1 layer ratio

Abstract Basal X-ray peaks from normal and ethylene glycol-saturated tarasovite have been measured to 0.0.36 and 0.0.38, respectively; the mean basal spacings were 43·73 ± 0·44 and 46·05 ± 0·33 Å, with CV values of 1·31 and 0·97. The spacings indicate a near approach to a 3 : 1 illite-smectite but the CV values indicate a greater variability than is normally consistent with the allocation of a special name. Comparison of the observed spacings with those for illite showed that the more deviant values were always adjacent to positions of illite reflections. A modified Mering treatment of diffraction by mixed-layer sequences was used to estimate the proportion of illite layers; the average result was 79·9 ± 2·0%. The mineral is considered to have 3 : 1 illite-smectite units with randomly added illite layers, bringing the ratio from 75% for the ideally ordered sequence to near 80%.