Preparation of vermiculites for HRTEM

Abstract

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

Characterization of Untreated and Alkylammonium Ion Exchanged Illite/Smectite by High Resolution Transmission Electron Microscopy

“…On the other hand, a practical estimation of the range of layer charge density is also possible by inferring the type of alkylammonium arrangement from expanded interlayer thicknesses on HRTEM images. Several recent HRTEM studies have indeed proven the utility of such a technique by confirming the XRD basal spacings observed for monomineralic smectite and vermiculite on high resolution images (Vali and K6ster 1986, Rfihlicke and Niederbudde 1985, Ghabru et al 1989, Marcks et al 1989. In the present study, hexadecylammonium (Nc = 16) and heptadecylammonium (Nc = 17) ions have been used to form expanded complexes of US.…”

“…The technique described above differs from those of Vali and KSster (1986) and Marcks et al (1989) in that during the entire process dispersed clay particles were not allowed to dry into aggregates. In addition, other techniques involve centrifugation of the resin/sample mold to obtain oriented aggregates of materials at the base of the resin block whereas the present technique maintains a good degree of dispersion of particles due to polimerization of resin synchronous with particle settling.…”

“…The purpose of the study was to obtain high resolution structural information on the layer sequences and ordering of the samples as well as layer charge information by means of alkylammonium expanded interlayers. Alkylammonium ion treatment has been shown to provide a stable expansion ofinterlayers under HRTEM conditions (Riihlicke and Niederbudde 1985, Bell 1986, Klimentidis and Mackinnon 1986, Vali and K6ster 1986, Marcks et al 1989, Ghabru et al 1989, even though it causes some disruption of the clay fabric (Lee and Peacor 1986). Furthermore, the type of structural arrangement adsorbed alkylammonium ions adopt can be inferred from the thickness of expanded interlayers on HRTEM images which, in turn, can be used to estimate the layer charge density of the expandable layers (Marcks et al 1989, Ghabru et al 1989, Olis et al 1990.…”

Identification of vermiculite by transmission electron microscopy and X-ray diffraction

“…However, the distinction between vermiculite and mica by TEM or XRD is not without problems. Banfield & Eggleton (1988) described a component in weathered biotite as vermiculite that shows 14/~ spacing by HRTEM whereas other HRTEM studies report a 10/k spacing for weathered biotite (vermiculite) similar to unweathered biotite (Graf von Reichenbach et al, 1988;Marcks et al, 1989;Ghabru et al, 1989). X-ray and TEM studies also reveal 9.4-10 ,~ basal spacings for anhydrous vermiculite (Vali & K6ster, 1986;de la Calle & Suquet, 1988).…”

“…High-resolution transmission electron microscopy (HRTEM) has been applied to study the transformation of biotite to vermiculite (Graf yon Reichenbach et al, 1988;Marcks et al, 1989;Ghabru et al, 1989;Banfield & Eggleton, 1988). However, the distinction between vermiculite and mica by TEM or XRD is not without problems.…”