The smectite to illite reaction was studied by transmission and analytical electron microscopy (TEM/AEM) in argillaceous sediments from depths of 1750, 2450, and 5500 m in a Gulf Coast well. Smectite was texturally characterized as having wavy 10- to 13-Å layers with a high density of edge-dislocations, and illite, as having relatively defect-free straight 10-Å layers. The structures of smectite and illite were not continuous parallel to (001) at smectite-illite interfaces. AEM data showed that the smectite and illite were chemically distinct although smectite had a more variable composition. Illite formation appeared to have initiated with the growth of small packets of illite layers within subparallel layers of smectite matrix. With increasing depth, ubiquitous thin packets of illite layers increased in size until they coalesced.

A model for the transition requires that the structure of smectite was largely disrupted at the illite-smectite interface and reconstituted as illite, with concomitant changes in the chemistry of octahedral and tetrahedral sites. At least partial Na-K exchange of smectite preceded illite formation. Transport of reactants (K, Al) and products (Na, Si, Fe, Mg, H20) through the surrounding smectite matrix may have taken place along dislocations.

The smectite-to-illite conversion process for the studied samples does not necessarily appear to have required mixed-layer illite/smectite as an intermediate phase, and TEM and AEM data from unexpanded samples were found to be incompatible with the existence of mixed-layer illite/smectite in specimens whose XRD patterns indicated its presence.

A complete conversion series for mica/smectites was found in a hydrothermal alteration envelope around Kuroko-type ore deposits at the Shinzan area, Akita Prefecture, Northeast Japan. The minerals are an alteration product of volcanic glass of Miocene age and are commonly associated with zeolites and silica minerals. Degrees of ordering of interstratification of the minerals change discontinuously from Reichweite g = 0 (100–55% expandable layers) to g = 1 (45–20% expandable layers), and from g = 1 to g = 2 (<20% expandable layers). This pattern of conversion differs from the behavior of mica/smectites during burial diagenesis which undergo a continuous change in ordering type, and from the behavior of rectorite which displays a constant expandability and ordering (45–55%) over a wide range of conditions. Differences between these minerals were also found in the relationships between expandability and total layer charge, and between expandability and number of non-exchangeable interlayer cations. In mica/smectites from the Shinzan area, chemical changes in the interlayers and tetrahedral and octahedral sites are consistent with a reaction in which K-enrichment and K-fixation in the interlayers are controlled by an increase in negative layer charge. This conversion occurred in response to a steep geothermal gradient and migrating hydrothermal solutions.

Petrographic observations indicate that dolomite, siderite, pyrite, Ti oxides, quartz, and K-feldspar formed as by-products of mica alteration during diagenesis of Jurassic reservoir sandstones of the Haltenbanken area (offshore central Norway). These minerals precipitated on a mica-grain scale or a thin-section scale. Modal analyses and mass-balance calculations of muscovite alteration to kaolinite and of biotite to kaolinite, illite, and chlorite suggest limited elemental mobility during sandstone diagenesis. The alteration of mica to kaolinite occurred in sandstones buried <2200 m deep (present temperature < 70°C), whereas illitization and chloritization of biotite occurred during burial to depths > 3000 m (present temperature >95°C).

The morphology of illite/smectite (I/S) from deeply buried bentonites and hydrothermally altered Tertiary volcanic rocks from Japan changes in parallel with the proportion of expandable layers in the I/S. As viewed by scanning electron microscopy, the morphologies range from the typical “cornflake,” “maple leaf,” or “honeycomb” habit of smectite to the typical platy or scalloped (with curled points) habit of illite. Although the changes are more subtle near either end member, at a composition of 60-70% illite layers, the morphology changes from sponge-like or cellular to platy or ribbon-like. The change of morphology at this composition correlates with a change in layer stacking from turbostratic to rotational ordering of the 1Md type. Turbostratic stacking can be thought of as randomly distributed translations of successive layers by any magnitude and in any direction. The rotationally ordered structure, which allows nearly precise juxtaposition of quasihexagonal oxygen surfaces from adjacent layers, probably permits more crystalline regularity in the a-b plane, which promotes a more plate-like or sheet-like habit.

Ba-bearing clinoptilolite (with >2 wt. % BaO) occurs in Neogene marine siliceous sediments from ODP Leg 127 Site 795 in the Japan Sea. Ba-bearing clinoptilolite is formed by reaction of volcanic glass and Ba-rich pore water during burial diagenesis.

The composition of clinoptilolite formed from volcanic glass was investigated from cuttings collected from the offshore MITI-Somaoki borehole, 52 km east of Soma City, Japan. Unaltered volcanic glass and clinoptilolite-replaced glass were found coexisting in the same silicic tuff at 1000 m depth, but unaltered volcanic glass and clinoptilolite did not coexist in individual glass shards, and each material analyzed in this study was pure. The SiO2 and Al2O3 contents of the pure clinoptilolite and the pure, unaltered glass were 69.32 and 13.05 wt. %, and 70.03 and 11.54 wt. %, respectively. The SiO2 content was almost the same and the Al2O3 content was greater in the clinoptilolite-replaced glass compared with the unaltered volcanic glass. As a result, the SiO2/Al2O3 ratio in the clinoptilolite-replaced glass (SiO2/ A12O3 = 5.31) was slightly lower than that of the unaltered volcanic glass (SiO2/Al2O3 = 6.07). The extra-framework cation composition of the clinoptilolite-replaced glass was three times richer in K (K2O = 7.84 wt. %) than the unaltered volcanic glass (K2O = 2.59 wt. %). The cation selectivity of clinoptilolite, which shows a preference for K to Ca and Na, played an important role in the concentration of K in clinoptilolite during clinoptilolite formation from volcanic glass, although the initial pore waters and the volcanic glass were not especially relative K.

Clinoptilolite-replaced glass in the calcite-cemented tuff at 1100 m depth was not enriched in Ca, suggesting that the exchangable-cation composition of clinoptilolite did not change by exposure to Ca-rich solutions at this depth. Apparently, the exchangable-cation composition of clinoptilolite, once formed, is relatively stable and difficult to change, even under conditions in which calcite is precipitating on clinoptilolite crystals.

The <0.5-μm fraction of 120 samples from a lithologically complex Permian sedimentary sequence, underlying dolerite intrusive sheets, has been characterized by means of X-ray diffraction to establish I/S compositions as a function of temperature, lithology and time duration. Illitization has been active over the entire 210 m depth range and the clay data reflect both the local pattern of contact metamorphism and the more regional trend of heat flow during burial. A continuum exists in the illite proportions of the illite/smectite interstratifications with increasing distance from the intrusive sheet ranging from R = 3 with less than 5% smectite via R = 2 and R = 1 to R = 0 with up to 70% smectite. In the mixed-lithology section, individual component layers in the I/S within similar distance levels, but between contrasting lithologies, appear to vary only within a very restricted compositional range. In the massive mudstone strata, however, more silty parts contain I/S of a higher degree of ordering and lower expandability. Calcite contents are reflected in a higher rate of chlorite formation, but not in the I/S composition. A satisfactory inverse correlation was found between percent smectite in I/S and vitrinite reflectance in the lithologically complex section. R = 1 interstratifications are associated with a maximum vitrinite reflectance of 1.07–1.29 and R > 1 phases with 1.93–2.7, indicating that time duration is not a controlling factor in the illitization process in this facies. R = 0 interstratifications are present in a massive mudstone/siltstone sequence situated furthest from the igneous intrusives, and display vitrinite reflectance values of 1.42–1.52. No satisfactory explanations for this finding can be offered.

Investigation of mixed-layer illite/smectites with far-infrared (FIR) spectroscopy indicates the presence of torsional mode absorption bands associated with interlayer fixed-K sites. By contrast, hydrated montmorillonitic interlayer cation sites are transparent in the far IR. The presence or absence of bands for interlayer cation sites appears to be related to both the magnitude and site of negative layer charge within the 2:1 layer structure. The bimodal nature of illite/smectite spectra leads to the suggestion that two different fixed-K environments occur within illite/smectite structures. These two environments are controlled by the composition of the octahedral sheet. The torsional modes at 112 and 89 cm-1 represent fixed-K sites influenced, respectively, by an Al-rich, high-charge dioctahedral layer and a heterogeneous Al-Fe-Mg-bearing, low-charge layer. A general trend of increasing absorption of the 112 cm-1 band, relative to the 89 cm-1 band, is observed in a typical diagenetic illite/smectite sequence of Miocene shales from the Gulf of Mexico sedimentary basin. The absorbance strength of both torsional bands is also seen to increase with increasing degree of illitization and the amount of fixed potassium in the illite/smectite. These observations are consistent with the concept of shales undergoing illitization during burial diagenesis by both the collapse of high-charge smectite layers to form illite layers (i.e., transformation) and the formation of new high-charge (-0.9) illite layers at the expense of smectite layers (i.e., dissolution/ neoformation).

This study assesses the distribution of Al and Fe in mixed-layer illite/smectites (I/S) in shales undergoing burial diagenetic changes, using evidence from 27Al NMR, XRD, and chemical analyses. Samples studied include a sequence of mixed-layer I/S (ranging from 40% to 68% illite layers) in shales from a well located in the Caillou Island Oil Field, Terrebonne Parish, Louisiana, as well as synthetic mica-montmorillonite (Syn-1), Silver Hill illite (IMt-1), K-benonite (ISMt-1), an Fe-bearing muscovite, phengitic muscovite, and a randomly interstratified mixed-layer I/S with 50% illite layers. Using a simplified model, where Fe3+ isomorphously substitutes randomly for [6]Al in the dioctahedral 2:1 structure, the 27Al NMR signal intensities are examined with regard to the paramagnetic deshielding effect of the Fe3+. The rapid decrease in paramagnetic deshielding with distance allows for a spherical “wipeout” model with a radius of 6 Å, over which there is complete effective paramagnetic line broadening (i.e., Al within the sphere is not “seen”). Using the average dimensions of a dioctahedral mica, the expected relative intensities of the octahedral and tetrahedral Al signal are determined as a function of Fe2O3 content.

Observed 27Al signals, normalized per unit weight of Al2O3 and relative to the lowest Fe-bearing phase, show a clear trend of decreasing intensity with increasing Fe2O3 content. Normative fitting of oxide data to structural formulae reveals a similar trend of decreasing 27Al intensity with increasing fraction of dioctahedral site occupied by Fe3+. Agreement between the observed 27Al intensities of low Fe-bearing 2:1 phyllosilicates and 27Al intensities predicted using the wipeout model indicate regular ordering of Fe and Al within the low Fe-bearing phases. However, observed 27Al intensities for the I/S specimens fall into a region where the amount of Al seen is in excess for the given XFe, thus indicating segregation of Al and Fe domains.

The second order quadrupole effect for the [6]Al site in the I/S fraction of shales decreases very slightly with increasing depth and percent of illite in the I/S, but not enough to effect site quantitation. Quantitative apportionment of elements into the I/S phase of the <0.2 µm fraction using NMR constraints shows directly a trend of increasing number of [4]Al sites and no change in the number of [6]Al sites with increasing degree of illitization for samples from the Gulf Coast diagenetic environment. Stoichiometry indicates an approximate 1:1 substitution of tetrahedral Al for Si over the 40–68% range of illite in I/S examined.

The K-Ar systematics of illite/smectite (I/S) mixed layers in deeply buried shales from the Gulf Coast, the North Sea and the Mahakam Delta basins have been compared to provide additional perspectives on the diagenetic evolution of these minerals. Comparison of the results suggests that illitization proceeds similarly in the 3 basins, at least for the increase in the illite-layer and K contents, despite differences in the provenance of the detrital components, the ages of deposition, the depths of burial and the tectonic history of the basins. Analysis of the trends with depth in the illite-layer and K contents of I/S-enriched size fractions of shales in the North Sea and the Mahakam Delta basins shows that these trends represent segments of the more complete trends from I/S minerals of the Gulf Coast area.

The trends with depth in the radiogenic 40Ar contents and in the K-Ar ages of the I/S-rich fractions in the North Sea and Mahakam Delta basins suggest that, relative to the reference trends of the Gulf Coast area, the K-Ar system of the clay material is more dependent on the behavior of the radiogenic 40Ar than on the occurrence or non-occurrence of detrital grains in the size fractions. Recasting of the available data suggests that retention of radiogenic 40Ar by the illite-type minerals occurs in the intense illitization zone and release occurs in the deeper part of the basins. We therefore speculate that the illitization process of the I/S mixed layers of progressively buried shale-type sediments could be controlled by a transformation process integrating dissolution of detrital components in poral rock environments relatively impermeable to radiogenic 40Ar. These excesses, which might be partly or completely erased in deeper parts of the sedimentary basins, question the application of the K-Ar dating method on clay minerals extracted from shales.

Reinterpretation of published data for shale cuttings from the Gulf of Mexico sedimentary basin identifies three reaction zones for illite formation with increasing depth for well CWRU6. In a shallow zone (1.85 to 3 km), non-expanding illite-like layers formed primarily by the coalescence of smectite 2:1 layers around interlayer K+. In a middle zone (3 to 4 km), illite crystals neoformed from solution as coarser K-bearing phases and smectite were dissolved by organic acids. In the deepest zone (>4 km), illite recrystallized as less stable illite crystals dissolved, and more stable illite crystals grew during mineral ripening. The progressive loss of radiogenic argon in the deepest zone yielded a constant apparent age for the clays with depth, an effect previously attributed to “punctuated diagenesis.” The above hypothesis for illite formation emphasizes the need to establish the zone (i.e., the reaction mechanism) from which shales were derived before making detailed geologic interpretations based on illite mineralogy.

Burial-induced and hydrothermal-related illitization in bentonites and in sandstones can be modeled on the basis of isotopic studies of fundamental particles separated from mixed-layer illitesmectite. The model envisages different reaction rates and durations relative to the varied impacts of temperature, considering that the water:rock ratio also has an influence. The different pathways for illitization are suggested on the basis of the K-Ar, Rb-Sr and δ18O compositions of previously studied materials.

New information is provided on why fundamental particles separated from mixed-layer illite-smectite in shales yield K-Ar age data that are systematically greater than the ages of the fundamental particles from associated bentonites and/or sandstones, and greater than the reported stratigraphic ages. The study of pure authigenic, recent to present-day smectite from Pacific sediments shows that (1) those collected from active hydrothermal vents have 40Ar/36Ar ratios identical to that of the atmosphere, and (2) those of mud sediments have 40Ar/36Ar ratios above the atmospheric value, indicating addition of 40Ar not generated in situ by radioactive decay. A preliminary but detailed analysis of the noble-gas (Ar, Xe, Kr) contents of authigenic smectite-rich size fractions from Pacific deep-sea red clays suggests trapping of these gases by smectite. Therefore, the results point to the fact that fundamental particles can incorporate excess 40Ar into their structure when nucleating in restricted to closed systems, such as shales. This excess 40Ar, which represents radiogenic 40Ar released from nearby altered silicates, might be temporarily adsorbed at the surface of the rock pore spaces and is therefore available for incorporation in nucleating and growing particles.

The clay fraction of the sedimentary succession beneath the Mahakam Delta (eastern Kalimantan, Indonesia) consists mainly of mixed-layer illite-smectite, with minor amounts of kaolinite and/or dickite, discrete detrital illite and chlorite. On the near-shore anticline, evolution of this mixed-layer material is characterized by a decrease in expandability with depth. The mechanism of conversion of smectite to illite layers depends on the lithology of the host rocks: it evolves along a solid-state transformation in the shales and a dissolution-precipitation in the sandstones.

Illite fundamental particles from two sandstones buried at ∼4000 m in the Tambora field next to the Handil field on the same near-shore anticline yield a mean K-Ar age of 15.7±1.6 Ma (2σ), which is younger than the stratigraphic age but still slightly biased by minute amounts of discrete detrital illite. Recalculated after modeling, which takes into account the occurrence of the discrete detrital illite, this K-Ar age becomes 14.4±0.7 Ma. The K-Ar values of the fundamental particles from associated buried shales are significantly older, which can be explained by a mixture of (1) a precursor material similar to that presently deposited in the delta of the Mahakam River, and (2) authigenic fundamental particles incorporating Ar with a 40Ar/36Ar ratio above atmospheric value during nucleation and growth on the same detrital precursor.

Application of the modeling to the burial evolution of <0.4 µm particles from both the shales and the sandstones of the basin points to a contribution of detrital micaceous material in both lithologies, (1) in the sandstones as a detrital component dissolving progressively and mixed mechanically with the authigenic fundamental particles, and (2) in the shales as a detrital precursor progressively releasing radiogenic 40Ar by burial alteration, and at the same time acting as a support for the fundamental particles for the growth of authigenic fundamental particles that were found to incorporate Ar characterized by an excess of 40Ar.

Shales have undergone a complex burial diagenesis that involved a severe modification of the pore structure. Reconstituted shales can provide new insights into the nature of the pore structure in natural materials. The effects of diagenesis on the microfabric, pore size distribution, and porosity of Opalinus shale were measured by comparing the behavior of natural and reconstituted specimens. The parent material (Opalinus shale) was reconstituted through multiple grinding operations, sedimentation from a dispersed slurry, and one-dimensional isothermal consolidation. This process produced uniform specimens that were not cemented and had replicable microfabric and engineering properties. The microfabric and mineralogy of the materials were examined using high-resolution scanning/backscattered electron microscopy (SEM/BSEM) and energy-dispersive X-ray spectroscopy (EDS) for specimens with broken and milled surfaces. Mercury intrusion porosimetry (MIP) and N2 adsorption were used to assess the pore size distributions and specific surface areas of the materials. The microstructure of natural shale was characterized to be highly heterogeneous with significant concentrations of calcareous microfossils, calcite, and quartz particles embedded within the clay matrix. The microfossils were observed to be locally infilled and rimmed by a calcite cement that showed evidence of dissolution. The reconstituted specimens showed a double-structure microfabric that evolved with the level of consolidation stress and converged into a single-structure material (comparable to the natural shale) at a consolidation stress of more than twice the estimated maximum in situ effective stress. The natural shale had a lower specific surface area in comparison to the reconstituted material, which was consolidated at large effective stresses. These differences can be attributed to cementation at a submicron pore scale and highlight chemical diagenesis effects that were not replicated in the reconstituted specimens.

In the Nuussuaq Basin, West Greenland, a thick succession of Tertiary dolerites has penetrated Upper Cretaceous mudstone. The mixed-layer minerals of mudstone core samples have been analyzed by X-ray diffraction, solid-state 29Si and 27A1 magic-angle spinning nuclear magnetic resonance, Mössbauer and infrared spectroscopies, thermal analysis, chemical analysis, stable isotopes (18O/16O), and K/Ar dating. The mixed-layer minerals include for each sample two mixed-layer phases consisting of pyrophyllite, margarite, paragonite, tobelite, illite, smectite and vermiculite layers. The main, 80 m thick intrusion resulted in the formation of pyrophyllite, margarite, paragonite and tobelite layers. However, the tobelite layers are absent in samples <21 m from this intrusion. Furthermore, chlorite was formed and kaolinite destroyed in samples adjacent to minor intrusions and at distances <60 m from the large intrusion. For the first time, the detailed, complex mixed-layer structures formed during contact metamorphism of kaolinitic, oil-forming mudstones have been investigated accurately. The formation of tobelite layers reveals that oil formation has taken place during contact metamorphism. Furthermore, K/Ar dating of mixed-layer minerals from shale indicates that the intrusives are of early Eocene age. The 80 m thick intrusive is responsible for the main mixed-layer transformations, whereas two thin (3 m and 0.5 m thick) intrusions contribute little. Thus, the detailed mixed-layer investigation has contributed significantly to the understanding of the regional geology and the contact metamorphic processes.

The Tarbert Formation on a north±south oriented structure along the eastern flank of the Viking Graben has been studied. The reservoir in the two wells studied is buried ~100 m deeper in the northern than in the southern well. The present reservoir temperature is ~130°C. The reservoir quality is good, but due to extensive illitization of kaolin, the northern well shows lower permeability values than the southern well, for similar porosity values. The initial composition of the analysed samples in the two wells is very similar. There are no significant differences in total clay content and both wells contain K-feldspars and kaolin. However, the diagenetic alteration is more advanced in the deeper well. Total porosity is about the same in both wells but, due to illitization, the ratio of micro- vs. macro-porosity is higher in the deeper well. Petroleum emplacement in the structure occurred in two phases: oil emplacement predates, whereas gas emplacement postdates, most of the quartz cementation. In the deeper well, illitization occurred after gas emplacement.

In the western High Atlas basin, the evolution of the clay assemblage of the Upper Jurassic–Cretaceous sedimentary series is controlled by palaeogeographic changes of the basin and by burial diagenesis.The effects of burial are expressed by an increase in the proportions of illite and chlorite with depth, a decrease of expandability of smectite minerals and by a decrease in the Kübler Index.Kaolinite is preserved at greater depths than smectite, but also tends to diminish in abundance. TEM observations show that idiomorphic pseudo-hexagonal crystals of illite increase in size and abundance downwards, especially in the sandy levels.In the Agadir section, the main change in the clay mineral evolution related to late diagenesis occurs at the Mid-Albian, corresponding to a burial depth of about 1800 m; in the Essaouira section it occurs at the Lower Tithonian at a burial depth of about 2100 m.Considering the different geothermal gradient between Agadir (36ºC km–1) and Essaouira (27ºC km–1), the disappearance of smectite occurs at the same temperature in these two sites (60 to 65ºC).

K-Ar dating of mixed-layer illite-smectite from clay fractions extracted from pyroclastic horizons was used to address the controversy about the age and mechanism of the thermal alteration of Carboniferous rocks from the Upper Silesia Coal Basin (USCB). The clay fractions were also investigated by X-ray diffraction in order to select for dating samples possibly rich in illitesmectite, and to evaluate the K-Ar dates for possible contamination by K-bearing pre-diagenetic minerals, of pyroclastic or epiclastic origin.

The K-Ar dates document intense Variscian tectonic burial illitization produced by thrusting (~290 Ma) in the SW of the basin, and the lack of intense burial illitization in the NE, which is consistent with sedimentological models of the basin. The burial illitization in its final phase (<30%S in illite-smectite) involved incorporation of measurable amounts of ammonium cation in the illite structure (substitution for K).

Advanced illitization in the NE of the basin is much younger than its tectonic inversion (uplift and erosion started in Permian), and the corresponding K-Ar dates have to be interpreted as the result of a Mesozoic thermal event, which produced widespread pervasive illitization at shallow depth. This event was dated at 175 Ma, but it may have started earlier and could have lasted longer. This conclusion is consistent with widespread evidence of a major Mesozoic thermal event all over Central Europe, produced by rifting and lithospheric thinning during the opening of the Tethys and Atlantic oceans.

This study demonstrates that smectite illitization histories may be very complex, and that the nature of the illitization mechanism results in mixed K-Ar dates encompassing pro-longed or multiple illitization histories. Dating of several grain-size fractions may help to unravel such histories.

When a calibration using data from Neogene sedimentary basins is applied, vitrinite reflectance and %S in I-S indicate similar palaeotemperatures of tectonic burial diagenesis in the USCB, but they produce very different estimates of the temperature of the thermal event.

The unstable glassy component of the Oligocene tuffaceous sediments of the Metaxades area, Thrace, Greece, has undergone extensive zeolitic diagenetic alteration. The authigenic minerals occur as crypto- to micro-crystalline aggregates making up most of the matrix in the altered tuffs and as precipitates in cavities produced by dissolved glass fragments. Glass-shard pseudomorphs are ubiquitous and most of them are partly filled by one or more of the minerals smectite, heulandite, mordenite and silica.

The heulandite group zeolites are mostly high-silica calcium-rich heulandites showing intermediate thermal behaviour between most heulandites and clinoptilolites. Their Si/Al ratios are similar to clinoptilolite (4.74–5.19) but their divalent/monovalent cation ratios (1.5–3.24) are partly superposed onto the ratios of heulandite group 1 and 2 and differ considerably from the values of the relevant ratio in clinoptilolite. Rare K-rich clinoptilolite crystals have been identified in one sample only.

Based on field observations, compositions and paragenetic relationships of coexisting authigenic minerals and the absence of critical phases such as laumontite, analcime or authigenic albite, it is suggested that the Metaxades pyroclastic rocks underwent burial diagenesis intermediate between Iijima's (1978) zones II and III commonly developed in silica saturated environments.