A technique for preparing vermiculites for examination by high-resolution transmission electron microscopy (HRTEM) has been developed. A TEM-stable expanded phase can be obtained by intercalating n-alkylammonium ions between the silicate layers of a parent biotite. The vermiculite particles were embedded in Spurr resin and centrifuged to improve orientation. Ultra-thin specimens were prepared using an ultramicrotome, the quality and thickness of the sections being monitored by TEM. Lattice images of biotite, Ba-vermiculite and octylammonium-vermiculite, the latter showing a perpendicular arrangement of the alkyl chains relative to the silicate layers, were obtained with a resolution ~2 A. The reliability of these images was confirmed by computer simulation.High-resolution transmission electron microscopy is a powerful tool for investigating structural defects and deformations in layer-silicates (Brown & Rich, 1968;Lee et al., 1975;Amouric et al., 1981;Vali & K6ster, 1986). Whereas X-ray or neutron diffraction can characterize the overall structure without providing any information about the spatial distribution of structural inhomogeneities, HRTEM permits direct observation of microstructural features such as regular or random interstratification formed during the transformation of mica to vermiculite, or the arrangement of single silicate layers at the micavermiculite interface.A highly developed preparation technique is required to observe the details of layered structures necessitating the preparation of very thin specimens with basal planes oriented parallel to the electron beam. On the other hand, the stability of thin specimens is reduced further if layer-silicates are intercalated with either hydrated cations or organic molecules.In the course of a study on the mechanisms of mica-vermiculite transformation, an attempt was made to improve the preparation of highly charged alkylammonium-saturated layersilicates for inspection of the layer arrangement at the mica-vermiculite interface with HRTEM. The results of this investigation are presented in this report. MATERIALS AND METHODS Production of alkylammonium-vermiculitesA biotite from Miask (Ural) was wet ground and separated into the particle size fractions 0.5-2, 2-5, 5-20 and 20-50 #m by sedimentation.As complete exchange of interlayer K from biotite by alkylammonium cations requires several months of reaction time (Weiss et al., 1956), alkylammonium-vermiculites were prepared in a two-step exchange procedure.9 1989 The Mineralogical Society

Particle size fractions of biotite (0.5-2; 2-5; 5-20; 20-50 ~) were gradually transformed to vermiculite by exchanging the interlayer potassium with barium. In a subsequent exchange procedure, interlayer barium was removed by n-alkylammonium ions in order to preserve the expanded structure under the conditions in the electron microscope. Ultramicrotome sections cut perpendicular to the plane of silicate layers were prepared from samples representing different states of mica-vermiculite transformation.Inspection of lattice images in the HRTEM revealed two characteristic phenomena. During the initial phase of the transformation, mineral particles are dissected by a growing number of single expanded interlayers separating packets of contracted layers. In a later stage, a step-wise expansion of adjacent interlayers can be observed producing a wedge-shaped profile of the mica-vermiculite interface.A statistical treatment of these observations is necessary for relating the mechanism of lattice expansion quantitatively to cooperative phenomena frequently found in studying the kinetics of intercalation.