Materials representing common interstratified clay minerals are shown to be composed of aggregates of fundamental particles. Transmission electron microscopy and x-ray diffraction demonstrate that the x-ray diffraction characteristics of a wide range of interstratification can be modeled experimentally by utilizing materials containing only three types of particles. The data have been incorporated into a new model that regards interstratified clay minerals as populations of fundamental particles whose x-ray diffraction patterns result from interparticle diffraction.

Transmission electron microscopic (TEM) examination of the <0.1-/zm fraction of montmorillonite and regularly interstratified illite-smectite (I-S) shows that these clays, when dried from suspension, consist primarily of particles 10 A and 20 A thick respectively. However, X-ray diffraction (XRD) examination of sedimented aggregates of montmorillonite indicate that the effective number of unit cells that are diffracting coherently is ~9. This discrepancy can be reconciled by postulating an interparficle diffraction effect from the sedimented aggregates of oriented particles. The interfaces of these particles are capable of adsorbing water, ethylene glycol etc. so that on this basis smectite is composed of elementary silicate particles 10 A thick, and regularly interstratified I-S is primarily composed of elementary 'illite' particles 20 A thick, values which are in agreement with the TEM observations. This concept is confirmed experimentally by XRD examination of sedimented aggregates from mixed suspensions of both materials; the resulting patterns are identical to those of randomly interstratified illite and smectite layers, which indicates that the layer sequence examined by XRD has been entirely rearranged. It is concluded that the use of XRD peak breadth to determine mean crystal thickness cannot be reliably applied to these systems. Standard XRD data from sedimented aggregates may not be able to distinguish between true interstratification and interparticle diffraction effects of intimate physical mixtures.

SUMMARY. Infra-red examination of a weathered biotite and of biotites that have been converted to vermiculites and subsequenOy oxidized, indicates that oxidation of octahedral ferrous ions to ferric ions is associated with a reversible conversion of hydroxyl ions to oxide ions. Subsequently, in highiron biotites, there is an irreversible loss of ferric ions from the octahedral layer, resulting in an increased number of dioctahedral sites. Electron microscopy and X-ray diffraction indicate that ejected ferric ions form either amorphous interlayer oxides or, when bromine is used as an oxidant, a crystalline external phase of ~-FeOOH. The high refractive index of some oxidized vermiculites is shown to be due largely to submicroscopic iron oxides.

Diagenetic illitic clays from seven North American bentonites of Ordovician, Devonian, and Cretaceous ages and from three subsurface North Sea sandstones of Permian and Jurassic ages have been examined by X-ray diffraction (XRD) and transmission and scanning electron microscopy (TEM and SEM). XRD indicates that the clays from the bentonites are randomly and regularly interstratified illite/smectites (I/S) with 30–90% illite layers, whereas the clays from the Jurassic and Permian sandstones are regularly interstratified I/S, with 80–90% illite layers, and illite respectively. TEM of shadowed materials shows that randomly interstratified I/S consists primarily of mixtures of elementary smectite and ‘illite’ particles (10 and 20Å thick respectively) and that regularly interstratified I/S and illite consist mainly of ‘illite’ particles 20–50 Å thick and > 50 Å thick respectively. Regularly interstratified I/S from bentonites and sandstones are similar with regard to XRD character and particle thickness distribution. These observations can be rationalized if the interstratified XRD character arises from an interparticle diffraction effect, where the smectite interlayers perceived by XRD, result from adsorption of exchangeable cations and water or organic molecules at the interfaces of particles generally < 50Å thick. A neoformation mechanism is proposed by which smectite is converted to illite with increasing depth of burial in sedimentary rocks, based on dissolution of smectite particles and the precipitation/growth of ‘illite’ particles occurring within a population of thin phyllosilicate crystals.

A B S T R A C T: A new conceptual model for interstratified clays is proposed, based on complementary use of the transmission electron microscope (TEM) and X-ray diffraction (XRD). Clays yielding interstratified XRD patterns are shown to be composed of aggregates of fundamental particles (,here defined as an individual or free particle which yields a single crystal pattern by electron diffraction) whose interfaces are capable of adsorbing water and organic molecules. In these circumstances, the interstratified XRD character results from an interparticle diffraction phenomenon. Smectites are shown to be primarily composed of particles 10 A thick corresponding to elementary 2:1 silicate layers. Nearest-neighbour regularly interstratified illitesmectite (K-rectorite) is composed of particles of elementary 'illite' 20 A. thick, corresponding to two silicate layers coordinated by a single plane of potassium ions. Regularly interstratified chlorite-smectite (corrensite) consists of particles 24 A thick corresponding to two silicate layers coordinated by a single brucitic sheet. The layer sequence examined by XRD is determined by the types and proportions of particles present within the clay material. Thus a sedimented aggregate of 20 • elementary 'illite' particles appears to be a regularly alternating sequence of illite and smectite layers by XRD. Randomly interstratified itlite-smectite and chlorite-smectite can be synthesised by making mixed suspensions of the <% 1 gm fractions of smectite with those of K-rectorite and corrensite respectively, providing experimental confirmation of the proposed model. Unusual associations, such as three-component illite-chlorite-smectite interstratified systems can be synthesised by making mixed suspensions of K-rectorite and corrensite. Long range regularly interstratified i[lite-smectite of the IIS and IIIS types are composed primarily of fundamental 'illite' particles 20-50 A thick. Conventional illite is composed primarily of particles > 50 A thick. This conceptual model has implications for the chemistry, behaviour and genesis of interstratified clays which are common constituents of soils and sediments.Interstratified clays are regarded as phyllosilicates composed of two or more kinds of layers stacked in fixed sequences normal to the (00/) planes. Many different stacking sequences have been demonstrated by comparing theoretically calculated one-dimensional d(OOl) diffraction patterns with those experimentally recorded from oriented specimens of natural clays (Reynolds & Hower, 1970;Reynolds, 1980). These calculated patterns assume that interstratified clays consist of crystatlites 5 to 15 layers thick, which define the one-dimensional coherent scattering domain for well-oriented samples, a form usually achieved by sedimentation on to glass slides. The question of whether these clays actually exist as a fixed sequence of layers within such crystallites has, however, recently been raised by studies involving the complementary use of TEM and XRD (McHardy et al., 1982;Nadeau et ...