A closed-form equation was derived that describes the powder-ring distribution factor as a function of 2θ, soller slit collimation, and σ*, which is defined as the standard deviation of an axially symmetrical Gaussian orientation function. Methods were developed for measuring σ* in the reflection mode by means of a θ/2θ diffractometer. Six experimental arrangements for a sedimentary chlorite showed widely different intensity ratios of the 001/005 reflections and gave a standard deviation of ±5.8% when corrected by the theory. The absolute integrated intensities of the 003 reflection from eleven illite samples provided an eight-fold maximum range which, when corrected, yielded a standard deviation of ±7.7%.

The intensity distributions within each of two X-ray powder diffraction patterns obtained from instruments with different soller-slit configurations could not be directly compared at low diffraction angles unless corrections, based on σ*, were introduced to allow for the differences in axial divergence.

Chemical data from three different series of diagenetic illite/smectites (I/S), analyzed statistically by two regresion techniques, indicate that the content of fixed-K per illite layer is not constant, but ranges from ~0.55 per O10(OH)2 for illite layers in randomly interstratified I/S (R=0; >50% smectite layers) to ~ 1.0 per O10(OH)2 for illite layers formed in ordered I/S (R>0; <50% smectite layers). By extrapolation of the experimental data, the following chemical characteristics were obtained for end-member illite derived from the alteration of smectite in bentonite: average fixed-K per illite layer = 0.75 per O10(OH)2; total charge = about -0.8; cation-exchange capacity = 15 meq/100 g; surface area (EGME) = 150 m2/g.

The 001 spacing of Na-smectite was found to vary from 9.6 Å at 0% relative humidity (RH) to 12.4 Å at 60-65% RH. The 9.6-Å spacing corresponds to dehydrated Na-smectite, and the 12.4-Å corresponds to Na-smectite with one water layer. A regular series of intermediate values resulted from ordered interstratification of the 9.6- and 12.4-Å units. Ordered interstratification was confirmed by the presence of a 001 spacing of 9.6 + 12.4 Å = 22 Å. This peak appeared under experimental conditions at about 35% RH. It appeared for calculated simulations of ordered stacking of 50/50 mixtures (±10%) of 9.6- and 12.4-Å units. The 004 peak of this 22-Å spacing interacted with the 002 of the 9.6-Å spacing of ordered mixtures of more than 50% 9.6-Å units and with the 002 of the 12.4-Å spacing of ordered mixtures of more than 50% 12.4-Å units. The result of this interaction was a complex peak, the position of which was a function of the ratio of 9.6- and 12.4-Å units. This complex peak was noted for experimental and for calculated conditions. Calculated tracings assuming ordered stacking matched the experimental tracings closely, whereas those assuming random stacking did not.

Ordering was apparently due to the interaction of the positive charge of the interlayer cation repelling the positive charge of the hydrogens of the hydroxyl ions, one above and one below, closest to the interlayer space. The collapse of a single interlayer space (dehydration) brought the interlayer cation closer to the hydrogens of the hydroxyls causing the hydroxyls to rotate such that the hydrogens shifted toward the adjacent interlayer spaces. Collapse of these two interlayer spaces was therefore more difficult. This same mechanism helps explain ordering in illite/smectite. The difference is that hydration/dehydration is quick and reversible, whereas the change from smectite to illite is slow and irreversible.

A high-pressure, high-temperature cell was used to monitor the basal X-ray powder diffraction spacing of Na-saturated Cheto montmorillonite in contact with NaCl solutions at temperatures as high as 200°C and hydraulic pressures as high as 6700 psi (456 bar). The 003 and 005 reflections were used to determine d(001) of the smectite. The montmorillonite, in 1 molal NaCl, exhibited a d(001) of 15.4 Å at room temperature and pressure and a d(001) of 15.6–15.7 Å under 500–2200 psi hydraulic pressure. The basal spacing of the clay in 5 molal NaCl was 15.2 Å and 15.33–15.45 Å at 1 bar and 750–6700 psi (53–456 bar), respectively. Because no changes in the basal spacing with increasing temperature to 200°C were detected in any of the experiments, this Na-smectite probably exists as a two-water-layer complex under diagenetic conditions.

The percentage of expandable layers in illite/smectite (I/S) mixed-layer clay decreases with increasing temperature and depth in a section through marine Cretaceous shales in the Champlin 1 Hartley Federal well in the Powder River basin, Wyoming. This systematic change in I/S expandability is evidence that low-expandable I/S in Cretaceous shales of the northern Rocky Mountain area reflects, at least in part, thermal alteration during burial diagenesis. In eastern Montana and western North Dakota where I/S in the Lower Cretaceous Mowry and Skull Creek source rocks is diagenetically unaltered, only trace amounts of hydrocarbons have been found in the Lower Cretaceous and other Cretaceous sandstones. Elsewhere in the northern Rocky Mountain-Great Plains region, hydrocarbons in the Lower Cretaceous Muddy Sandstone and its equivalents occur within or immediately adjacent to areas in which I/S clay in the Mowry and Skull Creek shales has been diagenetically altered during burial. Altered I/S and thermally mature organic matter, as defined by Rock-Eval pyrolysis values, coexist in these source rocks. Both may be used as maturation indicators in the search for Cretaceous-source hydrocarbons in the northern Rocky Mountain area.

Oxygen isotope ratios of <0.1-μm smectite in bottom sediments of the Mississippi River and the Gulf of Mexico near the mouth of the river have been determined to investigate diagenesis of land-derived clay minerals during sedimentation in the sea. No difference was detected in δ18O (SMOW) between the river and the Gulf samples indicating that no smectite alteration or addition of neoformed smectite to the river samples took place during sedimentation. Thus, authigenic minerals in the river sediments cannot make up more than a few tenths of a percent of the bulk sediments.

Similar results were obtained from 3 × 106-yr b.p. sediments buried to 80 to 600 m at Deep Sea Drilling Project site 323, Bellingshausen Abyssal Plain. No significant change with depth was noted in the δ18O of the <0.3-μm size fraction, mostly smectite, of these land-derived sediments. On the basis of the δ18O of the deepest sample, the maximum amount of authigenic minerals in the land-derived sediments during burial in the sea cannot be more than one or two percent of the bulk sediments. Hence, the alteration at seafloor temperatures of 25-45% of the <0.1-μm size clays in 3 × 106 yr b.p. sediments reported in a previous study is not substantiated. The data demonstrate that land-derived smectite is stable in the sea, and that oxygen isotopes can be used to investigate the modes and the temperatures of formation of authigenic smectites in marine sediments that are younger than 25 × 106 yr and that formed below 25°C.

Burial diagenetic reactions of di- and trioctahedral clay minerals were compared in Brazilian offshore, basinal sediment sequences of Cretaceous age. Originally dioctahedral smectite-rich shales of three basins—Potiguar,Ceara, and Ilha de Santana—exhibited the classical smectite-to-illite burial pattern. Trioctahedral clay-rich shales and trioctahedral clay-mineral cements in sandstones, however, showed a burial sequence of saponite to mixed-layer chlorite/saponite with progressive increase in the percentage of chlorite layers with increasing burial depth.

The change from disordered to ordered interstratifications of chlorite/saponite occurred in the temperature range 60°-70°C at a vitrinite reflectance value of about 0.65. These values are lower than those found for the ordering of illite/smectite clays. Increasing substitution of Al for Si in tetrahedral sites, followed by fixation of interlayer hydroxide sheets was found to be the major chemical change promoting transformation of saponite to chlorite via corrensite.

In Upper Cretaceous-Tertiary marine shales (Campos Formation) from the Campos basin, Brazil, mixed-layer illite/smectite (I/S) has remained randomly interstratified to depths of 3500 m and to temperatures as high as 100°C, in contrast to the typical pattern of shale diagenesis in, for example, the Gulf Coast area. X-ray powder diffraction analysis of the bulk shale and several size fractions from samples from one well in the Brazilian basin and from one in the Gulf Coast region were carried out to assess the factors that might have controlled the lack of illitization in the Campos Formation shales.

In samples from the Gulf Coast well, the clay minerals are I/S (montmorillonite-type), discrete illite, chlorite, and minor kaolinite. In contrast, the clay minerals in samples from the Campos basin well are kaolinite, clay-size biotite, and I/S (nontronite-type). Kaolinite is abundant in this well, and the variation of its abundance with depth seems to reflect variations in sea-level stands. The original composition of the I/S (nontronite-type) was probably the main factor controlling the lack of illitization in the shales of the Campos Formation.

X-ray powder diffraction (XRD) studies of the clay fraction of Upper Cretaceous mudstones from a shallow (2.5 km) drill hole in the Niger delta indicate a high geothermal gradient (about 100°C/km) during diagenesis. The mineralogy of the clays is similar to that observed elsewhere and consists of interstratified illite/smectite (I/S), kaolinite, and chlorite. Detrital mica and K-feldspar are also present throughout the section. The composition-depth relationship of the I/S is different from that observed in deeply buried Gulf Coast shales. The Niger delta rocks show a linear change in composition of the I/S as a function of depth in the drill hole from 60 to 10% smectite layers. The I/S ordering is R=0 in shallower samples and progresses to R=1 in deeper samples. The I/S in the deepest samples has R=3 ordering. No R=1 I/S showing a first-order 27-Å XRD reflection was found. The Niger delta sequence differs from the Gulf Coast sequence b. (1) a lack of R=1 I/S showing a 27-Å XRD reflection (which are common in Gulf Coast samples and contain 20% smectite layers), an. (2) the existence of a simple, continuous linear relation between the composition of the R=1 and R=0 I/S and depth.

A comparison of composition-temperature curves for I/S formed under different diagenetic regimes shows different types of I/S ordering in which the presence of R=1 I/S showing a 27-Å reflection and R=3 I/S types changes the composition-depth (and thus, composition-temperature) relations. These changes suggest a difference in the energy necessary to form the various ordering types. Also, geothermal gradient during burial appears to be responsible for different composition-temperature gradients found for the same type of I/S ordering.

The < 1-μm fraction of 17 bentonite samples from Kinnekulle, southwest Sweden, were studied by chemical analysis, X-ray powder diffraction, and cation-exchange capacity. The bentonites are interbedded with undeformed, flat-laying Ordovician and Silurian sediments and were formed by the transformation of volcanic ash (dated at about 450 Ma) into smectite, which later converted to mixed-layer illite/smectite (I/S). The reaction, possibly driven by heat from an overlaying diabase intrusion (about 300 Ma), stopped at different stages of conversion, as evidenced by the I/S which ranges in composition from 60 to 10% smectite layers. A 2-m-thick bed shows zonation, with decreasing smectite proportions towards the upper contact. The zonation is not symmetrical towards the lower contact. In thin beds the illite proportion is higher and the regularity of ordering is inversely proportional to the thickness of the bed. K:Sr and K:Rb ratios follow the illite pattern; the ratios are highest at the contact and in thin beds. The inhibiting effect of Ca and Mg on the smectite-to-illite conversion probably was the cause of less-reacted smectite in the center of the thick bed.

Chemical analysis by X-ray fluorescence (XRF) and calculated structural formulae of clay-size fractions of smectites from Cretaceous bentonites and illitic clays from Cretaceous, Devonian, and Ordovician bentonites and Jurassic and Permian sandstones indicate the nature and extent of various types of ionic substitution. The determination of tetrahedral (Al, Si) and octahedral (Al, Mg, Fe) composition shows the variable chemistry of these materials. Structural formulae of the illitic clays show that they have tetrahedral charges between 0.4 and 0.8 per half unit cell, and can be divided into phengitic types having octahedral charges of 0.2-0.4 and muscovitic types having octahedral charges <0.2. Evaluation of the formulae in the light of X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) data shows that the occupancy of non-exchangeable interlayer sites (predominantly K) varies from 47% to 90% of that of ideal muscovite. In some minerals as much as 20% of these sites is occupied by ammonium ions (determined independently). The amount of surface silicate charge balanced by non-exchangeable cations versus that balanced by exchangeable cations has been examined in conjunction with TEM data and suggests that in most samples the charges are about equal. The octahedral composition of smectites in Cretaceous bentonites precludes their having served as transformation precursors for most of the Cretaceous illitic bentonites. The results suggest that these illitic clays originated by neoformation.

The depth distribution of illite/smectite (I/S) compositions was investigated for a well drilled to a depth of 3462 m on Barbados Island, the only subaerial exposure of the Barbados accretionary complex. The classical pattern of increasing percentage of illite interlayers in the mixed-layer clay with increasing burial depth was not observed. Rather, the data describe an irregular, zig-zag trend with depth. This trend is probably the result of reverse faulting in the section. I/S data were also used to infer several kilometers of uplift and subsequent erosion of the section. This study shows that irregular patterns of clay diagenesis with depth can be anticipated for sequences that have undergone complicated tectonism and deformation. Combined with other geologic information, these patterns can help to determine the tectonic history of the sedimentary sequence.

On the basis of progressive acid dissolution the Thompson-Hower model states that mixed-layer glauconite/smectite and illite/smectite contain potassium in two structural sites: a mica-type K position (site 1) and a position of uncertain structural status more prone to dissolution (site 2). Site 2 was thought not to retain radiogenic argon (40Ar*). Using extensive progressive acid dissolution and K/Ar studies on a sized illite/smectite (I/S), determining the amount of K in site 2 is shown to be somewhat more complicated than previously thought because the dissolution pattern depends on acid normality. More important, site 2 fully retains 40Ar*, and no age correction is thus necessary as is required by the Thompson-Hower model, further affirming the geochronologic value of illite in mixed-layer clay. These data are also relevant to understanding the crystal and particle structure of I/S. Site 2 is probably a partly filled K interlayer that develops as an intermediate kinematic step on the way to being fully filled during the transformation of smectite to illite.

To test the ability of illitic clay minerals to retain argon, K/Ar ages were measured on grain-size separates from the Cretaceous Mancos Shale and associated bentonites that have been transformed into K-bentonite near the contact with a large Tertiary igneous stock. The ages of size separates of illite/ smectite from the K-bentonite nearest the contact were internally concordant and matched the hornblende K/Ar age of the stock. In contrast, K/Ar data from clay size fractions from shales adjacent to each K-bentonite were internally discordant with measured ages that were much greater than the age of the intrusion. Thus, significant radiogenic argon was retained by fine-grained detrital illite, even in shale samples very near the igneous contact. These results are convincing evidence that illitic clay minerals are excellent K/Ar clocks under conditions prevailing in sedimentary and diagenetic environments.

An unusual vanadium chlorite precipitated during the formation of a vanadium-uranium ore deposit in the Henry Basin, southeastern Utah. The ore deposit formed by reduction and precipitation of U and V in the presence of organic matter at the interface between a stagnant brine and overlying, circulating meteoric water. Some samples of the vanadium chlorite (heated before analysis) contain ≥ 10% V2O5. In fresh samples, most of the vanadium is in the trivalent oxidation state. X-ray powder diffraction data suggest that Fe and V are concentrated preferentially in the interlayer hydroxide sheets of the chlorite. A d(060) value of 1.52 Å indicates that the chlorite probably has a dioctahedral structure which is distended by the presence of octahedral Fe and V. The vanadium ore zone is flanked by peripheral zones containing perfectly ordered chlorite/smectite. This chlorite/smectite contains much less V than the pure chlorite. This chlorite may have formed by the progressive precipitation of vanadium-rich interlayer hydroxide sheets in the mixed-layer chlorite/smectite in the most reducing portion of the ore zone. The pure chlorite is a IIb polytype, which, for nonvanadiferous analogs, is ordinarily found in high-temperature environments; however, no evidence exists to show that these rocks have ever been exposed to elevated temperatures. In fact, the presence of unreacted smectite in a potassium-rich setting and the low vitrinite reflectance of coalified plant debris indicate a low-temperature history for these sediments.