Abstract-Completeness of exchange of K from muscovite by Ba 2+ ions decreased with particle size below 20/~. Accompanying K exchange at 120~ using a repeated batch technique, was a marked loss of Si and the formation of boehmite in the finer fractions. Several possible explanations for the unexpected high K retention of fine mica fractions are discussed.

A B S T R A C T : The interlayer structures in one-and two-layer hydrates of Na-vermiculite are described by presenting the positional parameters of their constituent atoms, including protons.The revision of these structures has been accomplished by: (1) determining accurate water contents of the two hydrates by thermoanalysis (TG, DSC); (2) using earlier X-ray diffraction data published by Slade et al. (1985) and de la Calle et al. (1984) as a reference; and (3) applying least-squares refinements when considering the constraints for atomic distances and bond angles between interlayer constituents.In the 1.485 nm hydrate of Na-vermiculite (n H 2 O /n Na & 4), sharing edges of Na(OH 2 ) 6 octahedra cause their chainlike arrangement. The chains are aligned along [100] and are stabilized by H-bonds between interlayer water molecules and oxygens of the silicate layer. The partial loss of these bonds during dehydration forces the stacking order to change from V3 to Vc in the resulting 1.185 nm (n H 2 O /n Na & 2) hydrate.This new understanding may help to explain differences in the rotational correlation time of water molecules between one-and two-layer hydrates of vermiculite as observed by quasielastic neutron scattering (Swenson et al., 2000).

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

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Dehydration and rehydration of a phlogopitic Mg-vermiculite were studied by thermo-analysis (TG, DSC) and in situ X-ray powder diffractometry. A detailed analysis of the relation between temperature and state of hydration of the vermiculite was rendered possible by use of slow heating rates in combination with a fast-recording position-sensitive detector. The results confirm the existence of a number of definite states of hydration characterized by basal spacings of 1.441,1.429,1.376, 1.165, 1.151,1.002 and 0.926 nm, all of which have already been described in the literature. In addition, so far undetected regular 1 : 1 interstratifications have been found. They formed during those transformations which lead to a change in the number of sheets of interlayer water molecules. These structures exhibit integral series of 00l reflections and d(001) spacings of 2.54 and 2.15 nm for the 2 : 1 and 1 : 0 transitions in the ratio of molecular layers of water, respectively.