High-resolution transmission electron microscopy (HRTEM) and electron diffraction experiments have been performed on R1 and R> 1 illite/smectite (I/S) samples that from X-ray powder diffraction (XRD) experiments appear to contain well-ordered layer sequences. The HRTEM images confirmed earlier computer image simulations, which suggested that periodicities due to I/S ordering can be imaged in TEM instruments of moderate resolution. The experiments also confirmed that in instruments of this sort, the strongest contrast arising from the compositional difference between I and S layers occurs under rather unusual imaging conditions of strong overfocus. Some selected-area electron diffraction (SAD) patterns showed additional diffraction spots consistent with R1 and R3 ordering. SAD patterns and cross-fringes arising in HRTEM images from non-00l reciprocal lattice rows indicated that the stacking vectors of most adjacent 2:1 layers were not randomly oriented with respect to each other. Thus, the I/S was not fully turbostratic, but instead consisted of very thin, coherently stacked crystallites that extended across the fundamental particles postulated by Nadeau and coworkers.

S/(I + S) ratios were determined for about seventy HRTEM images obtained and interpreted by three different TEM operators. These ratios were consistent with those obtained from standard XRD procedures, suggesting that results obtained by XRD can be used to infer the initial structural state of mixed-layer I/S prior to treatment of samples for XRD experiments. The HRTEM experiments thus demonstrated that the two specimens examined consisted of ordered I/S existing as small crystals, most of which contained more layers than the fundamental particles of Nadeau and coworkers. The non-turbostratic stacking suggests an energetic interaction between the individual fundamental particles, leading to at least two alternative thermodynamic descriptions of these materials. Although the I/S crystals in the present experiments probably were disaggregated into fundamental particles during sample preparation for XRD, the I/S crystals appear to have separated only along the smectite interlayers. If the term “fundamental particle” is to be used for primary, untreated I/S, its original definition should be modified to include not only free particles, but also those that occur as layers within small crystals. It further should be recognized that these particles can interact thermodynamically and crystallographically with their neighbors.

Samples of mixed-layer illite/smectite were investigated from a single bentonite bed zoned with respect to expandability from 90 to 30%. Chips of natural rocks were embedded in a resin, using a procedure designed to preserve the original fabric, cut with an ultramicrotome, and observed by high-resolution transmission electron microscopy (HRTEM). These observations confirmed the X-ray powder diffraction (XRD) model of mixed-layer clays, i.e., that illite/smectite grains in natural rocks are built of mixed-layer crystals, from 1 to as many as 15 silicate layers thick (4–6 interlayers per crystal on average). These crystals are present either as individual particles (loose crystals) or, typically, they form nearly parallel face-to-face groupings called here quasi-crystals. Free fundamental smectite and illite particles as defined by Nadeau and coworkers were essentially absent.

Illite and smectite interlayer spacings were 10 and 13.5 Å, respectively. Crystal thickness and number of interlayers were measured for 35–100 mixed-layer crystals per sample. Illite/smectite expandabilities were calculated from these data in two ways: either neglecting the crystal edges or accounting for them. The former determinations agree well with XRD estimates of expandability and the latter, with expandabilities calculated from the distributions of fundamental particle thickness measured by a shadowing technique in the TEM. This result explains the systematic discrepancy between XRD and TEM measurements of illite/smectite expandability.

Area-weighted thickness distributions of fundamental illite particles for samples of illite and illite-smectite from seven locations (including bentonites and hydrothermally altered pyroclastics) were measured by Pt-shadowing technique, by transmission electron microscopy. Most thickness distributions are described by lognormal distributions, which suggest a unique crystallization process. The shapes of lognormal distributions of fundamental illite particles can be calculated from the distribution mean because the shape parameters α and β2 are interrelated: β2= 0.107α − 0.03. This growth process was simulated by the mathematical Law of Proportionate Effect that generates lognormal distributions. Simulations indicated that illite particles grow from 2-nm thick illite nuclei by surface-controlled growth, i.e., the rate of growth is restricted by how rapid crystallization proceeds given a near infinite supply of reactants, and not by the rate of supply of reactants to the crystal surface. Initially formed, 2-nm thick crystals may nucleate and grow within smectite interlayers from material produced by dissolution of single smectite 2:1 layers, thereby transforming the clay from randomly interstratified (Reichweite, R = 0) to ordered (R = 1) illite-smectite after the smectite single layers dissolve. In this initial period of illite nucleation and growth, during which expandable layers range from 100 to 20%, illite crystals grow parallel to [001]* direction, and the dimensions of the (001) plane are confined to the size of the original smectite 2:1 layers. After nucleation ceases, illite crystals may continue to grow by surface-controlled growth, and the expandable-layer content ranges from 20 to 0%. This latter period of illitization is characterized by three-dimensional growth. Other crystal-growth mechanisms, such as Ostwald ripening, supply-controlled growth, and the coalescence of smectite layers, do not produce the observed evolution of α and β2and the observed shapes of crystal thickness distributions.

Fundamental particle thickness measurements of Dolná Ves hydrothermal illite/smectite (I/S) samples confirmed earlier findings regarding the content of fixed cations in illite interlayers (ca. 0.9/O10(OH)2). The distributions of fundamental particles and mixed-layer crystals of a given sample are internally consistent. In samples dominated by bilayer fundamental particles, mixed-layer crystals most often contain even numbers of layers. The expandabilities measured by XRD are much higher than so-called minimum expandabilities obtained from HRTEM measurements. This discrepancy is explained by assuming that the coherent scattering domains of Dolná Ves clays do not correspond to natural mixedlayer crystals but are thicker, probably due to parallel association of crystals on the oriented XRD slide. This tendency to produce intercrystal contacts is probably related to the unusually large ab dimensions of crystals of Dolná Ves clays.

It has been suggested that interstratified illite-smectite (I-S) minerals are composed of aggregates of fundamental particles. Many attempts have been made to measure the thickness of such fundamental particles, but each of the methods used suffers from its own limitations and uncertainties. Small-angle X-ray scattering (SAXS) can be used to measure the thickness of particles that scatter X-rays coherently. We used SAXS to study suspensions of Na-rectorite and other illites with varying proportions of smectite. The scattering intensity (I) was recorded as a function of the scattering vector, q = (4 π/λ) sin(θ/2), where λ is the X-ray wavelength and θ is the scattering angle. The experimental data were treated with a direct Fourier transform to obtain the pair distance distribution function (PDDF) that was then used to determine the thickness of illite particles. The Guinier and Porod extrapolations were used to obtain the scattering intensity beyond the experimental q, and the effects of such extrapolations on the PDDF were examined. The thickness of independent rectorite particles (used as a reference mineral) is 18.3 Å. The SAXS results are compared with those obtained by X-ray diffraction peak broadening methods. It was found that the power-law exponent (α) obtained by fitting the data in the region of q = 0.1–0.6 nm−1 to the power law (I = I0q−α) is a linear function of illite particle thickness. Therefore, illite particle thickness could be predicted by the linear relationship as long as the thickness is within the limit where α <4.0.

It has been previously reported that late diagenetic fibrous illite, separated out from oilfield sandstones for dating by the K-Ar method, has systematically shown an increasing K-content with burial depth in the UK northern and central North Sea. This is surprising as fibrous illite is believed to form rapidly, in response to a geological event such as oil charging, and to retain its composition during subsequent burial. If the composition of the illite is related to present-day conditions, rather than the conditions of initial formation, then argon loss may have occurred, making K-Ar ages of questionable validity. Variations in crystal chemistry of the illite and their fundamental particle size and shape (length/width) distribution suggest alteration of the illite from the time of formation. The extent to which K-Ar ages of illite might need to be re-interpreted as a result of this post-formation alteration is not quantifiable at present; however there is evidence to suggest that the ages may be interpreted in terms of burial models involving both crystal nucleation and growth, and that a high proportion of Ar within the crystals may be retained during post-growth recrystallization.