Abstract--Mixed-layer illite/smectite (I/S) was formed by reacting the Chambers or Polkville montmorillonite hydrothermally at 270 ~ and 350~ from several hours to more than 15 weeks. Reactions were conducted in closed vessels containing K or mixed K-Na, K-Ca, or K-Mg solutions of varying concentrations. The reaction rate and the rate of ordering of I/S for the reaction smectite + K § ~ mixed-layer I/S + SiO2 was inhibited by the addition of Na § Ca 2+, and Mg2+; the inhibitory strength of Na +, Ca 2 § and Mg 2+, on an equivalent basis, increased approximately in the ratio 1:10:30. The first order reaction-rate constants for the reactions at 270 ~ and 350~ indicate an activation energy of about 30 kcal/mole.In the experimental system studied, the reaction smectite -~ mixed-layer I/S appeared to proceed by solid state transformation, as suggested by: (1) rapid dissolution of large amounts of silica, probably creating an AI-enriched residue; (2) similarity of particle size and morphology of the mixed-layer products to those of the original montmorillonite, implying no extensive dissolution of A13+; and (3) relatively high activation energy compared to published values for silicate dissolution.

No abstract

No abstract

No abstract

Abstract-Electron microscopy of particles in size fractions (0.5-1.0 ~z, 0.1-0-5 ~, 0.05-0.1/~ and < 0-05 tz e.s.d.) obtained by centrifugation of Na-saturated montmorillonites from Wyoming (two samples); Chambers, Arizona; and Fayette County, Texas, has shown details of particle morphology. The finest fraction of each montmorillonite consists predominantly of very thin, separate flakes whereas all coarser fractions, totalling 80 per cent or more by weight of the samples studied, are composed of microaggregates. For all the samples, preferred orientation is best developed in specimens formed from flakes of the finest fractions. Microaggregates are stable in dilute suspension although they swell to give large interlayer spacings, but can be disrupted into smaller, thinner flakes by ultrasonic vibration. Differences in dispersion behavior between separate flakes and microaggregates are not due to differences in interlayer charge or chemical composition, which are very small between fractions of each sample, but are thought to be due to the interlocking of flakes in microaggregates during crystal growth, which prevents their complete separation in dilute suspension.INTRODUCTION SEVERAL recent publications have described studies of size-fractionated montmorillonites. McAtee (1958) observed that untreated Na-Camontmorillonites from Wyoming bentonites can be separated by centrifugation into fine fractions and coarse fractions that are respectively predominantly Na-and Ca-montmorillonite. He suggested that the greater preference for Ca-ions of montmorillonite in coarser fractions arose from a difference in chemical composition compared with that of the montmorillonite in the finer fractions. Mungan and Jessen (1963) separated four untreated montmorillonites into several size-fractions; exchangeable Na-and Mg-ions were more abundant in the finer than the coarser fractions, and suspensions of different size-fractions had different rheological properties. Grim and Kulbicki (1961) separated a montmorillonite from Marnia, Algeria, into coarse and fine fractions (1-2~ and <1/~ respectively); the coarse fraction developed hightemperature phases characteristic of montmorillonites of the Cheto-type (minerals with many octahedral Mg-ions and large values of interlayer charge), and the fine fraction developed hightemperature phases characteristic of those of the Wyoming-type (minerals with fewer octahedral Mg-ions and smaller interlayer charge values).