Illite Crystals Research Articles

Geochemical signature of paleofluids in microstructures from Main Fault in the Opalinus Clay of the Mont Terri rock laboratory, Switzerland

The present study reports on elemental and Sr isotopic analyses of calcite and associated celestite infillings of various microtectonic features collected mostly in the Main Fault of the Opalinus Clay from Mont Terri rock laboratory. Based on a detailed microstructural description of veins, slickensides, scaly clay aggregates and gouges, the geochemical signatures of the infillings were compared to those of the leachates from undeformed Opalinus Clay, and to the calcite from veins crosscutting Hauptrogenstein, Passwang and Staffelegg Formations above and below the Opalinus Clay. Vein calcite and celestite from Main Fault yield identical 87Sr/86Sr ratios that are also close to those recorded in the Opalinus Clay matrix inside the Main Fault, but different from those of the diffuse Opalinus Clay calcite outside the fault. These varied 87Sr/86Sr ratios of the diffuse calcite evidence a lack of interaction among the associated connate waters and the flowing fluids characterized by a homogeneous Sr signature. The 87Sr/86Sr homogeneity at 0.70774 ± 0.00001 (2σ) for the infillings of most microstructures in the Main Fault, as well as of veins from nearby limestone layer and sediments around the Opalinus Clay, claims for an “infinite” homogeneous marine supply, whereas the gouge infillings apparently interacted with a fluid chemically more complex. According to the known regional paleogeographic evolution, two seawater supplies were inferred and documented in the Delemont Basin: either during the Priabonian (38–34 Ma ago) from western Bresse graben, and/or during the Rupelian (34–28 Ma ago) from northern Rhine Graben. The Rupelian seawater that yields a mean 87Sr/86Sr signature significantly higher than those of the microstructural infillings seems not to be the appropriate source. Alternatively, Priabonian seawater yields a mean 87Sr/86Sr ratio precisely matching that of the leachates from diffuse calcite of the Opalinus Clay inside the Main Fault, as well as that of its microstructures and the same features of the sediments above and below. To envision a Priabonian seawater supply, there is a need for its storage without a significant evolution in its Sr isotopic composition until the final deformation of the area. The paleo-hydrogeological context calls for a possible infiltration of the seawater into a limestone karst located above the Opalinus Clay that could have acted as the storage reservoir. The karstic nature of this reservoir also explains why the 87Sr/86Sr of the fluids was not modified significantly until expulsion. An alternative storage could have been provided by the regional faulting system that developed during the contemporary regional rifting of the Rhine Graben. The fluid expulsion started along these extensional faults during the further Upper Eocene–Lower Oligocene rifting phase. Later, the thin-skinned deformation of the Jura Belt affected the Mont Terri region in the form of the Main Fault, probably between approximately 9 and 4 Ma on the basis of preliminary K–Ar ages of nanometer-sized authigenic illite crystals recovered from gouge samples.

SEM and TEM evidence of mixed-layer illite-smectite formed by dissolutioncrystallization processes in continental Paleogene sequences in northwestern Argentina

AbstractIn the northernmost Calchaquí Valley (Salta, Argentina), the Paleogene continental sediments show a transition from smectite, at the top, to R3 I-S (>90% illite) through R1 I-S (65–80% illite), in contrast to the remaining sectors, containing smectite up to the bottom. Samples at the base of the succession were characterized by high-quality step-scan X-ray diffraction (XRD), scanning electron microscopy (SEM) and analytical high-resolution transmission electron microscopy (HRTEM). Analysis by SEM demonstrated dissolution of primary phases (feldspars, micas and quartz) and crystallization of illite, I-S and kaolinite. As this alteration is not pervasive, an intermediate fluid/rock ratio could be inferred. The lattice-fringe images of the samples from upper parts of the sequence show abundant I1-rich areas, whereas in the lower parts of the sequence, illite packets and I3 I-S coexist and compositions evolve towards muscovite (tetrahedral-charge increase, principally compensated by Mgby- Al substitution in octahedral sites and by a slight decrease in Ca in interlayer sites). As burial temperatures were probably similar in all the samples, depth was not responsible for the illite formation at the bottom. The TEM textures suggest that illitization proceeded mainly by dissolution-crystallization. The active faults close to the northern Calchaquí Valley probably promoted the circulation of hot, deep fluids, favouring illitization.

Contributions of separate reactions to the acid–base buffering of soils in brook floodplains (Central Forest State Reserve)

The acid–base buffering of gleyic gray-humus soils developed in brook floodplains and undisturbed southern-taiga landscapes has been characterized by the continuous potentiometric titration of soil water suspensions. During the interaction with an acid, the major amount of protons (>80%) is consumed for the displacement of exchangeable bases and the dissolution of Ca oxalates. In the O and AY horizons, Mn compounds make the major contribution (2–15%) to the acid buffering. The buffer reactions with the participation of Al compounds make up from 0.5 to 1–2% of the total buffering capacity, and the protonation of the surface OH groups of kaolinite consumes 2–3% of the total buffering capacity. The deprotonation of OH groups on the surface of Fe hydroxides (9–43%), the deprotonation of OH groups on the surface of illite crystals (3–19%), and the dissolution of unidentified aluminosilicates (9–14%) are the most significant buffer reactions whose contributions have been quantified during the interaction with a base. The contribution of the deprotonation of OH groups on the surface of kaolinite particles is lower (1–5%) because of the small specific surface area of this mineral, and that of the dissolution of Fe compounds is insignificant. In the AY horizon, the acid and base buffering of soil in the rhizosphere is higher than beyond the rhizosphere because of the higher contents of organic matter and nonsilicate Fe and Al compounds.

XRD-based 40Ar/39Ar age correction for fine-grained illite, with application to folded carbonates in the Monterrey Salient (northern Mexico)

Due to their minute size, 40Ar/39Ar analysis of illite faces significant analytical challenges, including mineral characterization and, especially, effects of grain size and crystallography on 39Ar recoil. Quantifying the effects of 39Ar recoil requires the use of sample vacuum encapsulation during irradiation, which permits the measurement of the fraction of recoiled 39Ar as well as the 39Ar and 40Ar∗ retained within illite crystals that are released during step heating. Total-Gas Ages (TGA) are calculated by using both recoiled and retained argon, which is functionally equivalent to K–Ar ages, while Retention Ages (RA) only involve retained Ar in the crystal. Natural applications have shown that TGA fits stratigraphic constraints of geological processes when the average illite crystallite thickness (ICT) is smaller than 10nm, and that RA matches these constraints for ICTs larger than 50nm. We propose a new age correction method that takes into account the average ICT and corresponding recoiled 39Ar for a sample, with X-ray Corrected Ages (XCA) lying between Total-Gas and Retention Ages depending on ICT. This correction is particularly useful in samples containing authigenic illite formed in the anchizone, with typical ICT values between 10 and 50nm.In three samples containing authigenic illite from Cretaceous carbonates in the Monterrey Salient in northern Mexico, there is a range in TGAs among the different size-fractions of 46–49, 36–43 and 40–52Ma, while RAs range from 54–64, 47–52 and 53–54Ma, respectively. XCA calculations produce tighter age ranges for these samples of 52.5–56, 45.5–48.5 and 49–52.5Ma, respectively. In an apparent age vs ICT or %2M 1illite plot, authigenic illite grains show a slope that is in general slightly positive for TGA, slightly negative for RA, but close to zero for XCA, with thinner crystallites showing more dispersion than thicker ones. In order to test if dispersion is due to a different formation history or the result of retention capability, degassing spectra were modeled for site XCA averages and overall XCA average. Modeling shows that local site age average best match the measured spectra, instead of a global average age, indicating that illite growth reflects local deformation, and is not the result of regional metamorphism. Modeling also shows that Ar-degassing spectra are very sensitive to grain size, such that age interpretation based on Ar-plateaus is meaningless for most fine-grained clays.

Clay mineralogy of the Boda Claystone Formation (Mecsek Mts., SW Hungary)

AbstractBoda Claystone Formation (BCF) is the host rock of the planned site for high level nuclear waste repository inHungary. Samples representing the dominant rock types of BCF were studied: albitic claystone, claystone with high illite content, and analcime bearing claystone. Clay minerals in these three rock types were characterized by Xray powder diffraction (XRD), transmission electron microscopy (TEM) and thermal analysis (DTA-TG), and the results were discussed from the point of view of the radionuclide sorption properties being studied in the future. Mineral compositions of bulk BCF samples vary in wide ranges. In the albitic sample, besides the dominant illite, few percent of chlorite represents the layer silicates in the clay fraction. Illite is the dominating phase in the illitic sample, with a few percent of chlorite. HRTEM study revealed that the thickness of illite particles rarely reaches 10 layers, usually are of 5-6 TOT layer thick. Illite crystals are generally thicker in the albitic sample than in the illitic one. The significant difference between the clay mineral characterisitics of the analcimous and the other two samples is that the former contains regularly interstratified chlorite/smectite beside the dominant illite.Based on the structural and chemical data two illite type minerals are present in the BCF samples: 1M polytype containing octahedral Fe and Mg besides Al, 2M polytype illite generally is free of Fe andMg. Close association of very thin illite plates and nanosized hematite crystals is typical textural feature for BCF.The goal of this study is to provide solid mineralogical basis for further studies focusing on radionuclide sorption properties.

Reconstructing fluid-flow events in Lower-Triassic sandstones of the eastern Paris Basin by elemental tracing and isotopic dating of nanometric illite crystals

Lower- to Middle-Triassic sandstones from eastern Paris Basin were buried to a maximum depth of 2500m at a paleo-temperature of about 100°C. They contain extensive amounts of authigenic platy and filamentous illite particles similar to those reported in reservoirs generally buried at 3000 to –5000m and subjected to temperatures of 120 to –150°C. To evaluate this unexpected occurrence, such sandstones were collected from drill cores between 1825 and 2000m depth, and nanometric-sized sub-fractions were separated. The illite crystals were identified by XRD, observed by SEM and TEM, analyzed for their major, trace, rare-earth elements and oxygen isotope compositions, and dated by K–Ar and Rb–Sr.Illite particles display varied growth features in the rock pore-space and on authigenic quartz and adularia that they postdate. TEM-EDS crystal-chemical in situ data show that the illite lath/fiber and platelet morphologies correspond at least to two populations with varied interlayer charges: between 0.7 and 0.9 for the former and between 0.8 and 1.0 for the latter, the Fe/Fe+Mg ratio being higher in the platelets. Except for the deeper conglomerate, the PAAS-normalized REE patterns of the illite crystals are bell-shaped, enriched in middle REEs. Ca-carbonates and Ca-phosphates were detected together with illite in the separates. These soluble components yield 87Sr/86Sr ratios that are not strictly in chemical equilibrium with the illite crystals, suggesting successive fluids flows with different chemical compositions. The K–Ar data of finer <0.05μm illite separates confirm two crystallization events at 179.4±4.5 and 149.4±2.5Ma during the Early and Late Jurassic. The slightly coarser fractions contain also earlier crystallized or detrital K-bearing minerals characterized by lower δ18O values. The δ18O of the finest authigenic illite separates tends to decrease slightly with depth, from 18.2 (±0.2) to 16.3 (±0.2)‰, suggesting different but contemporaneous crystallization conditions deeper in the section.The illite platelets and filaments crystallized in changing physical–chemical crystallization conditions induced by fluids flows through the host-rock pore system. These flow events were probably driven by repetitive rifting episodes of the North Atlantic Ocean, although located several hundreds kilometers away from eastern Paris Basin, and/or by fracturing events in the nearby basement of the Vosges Massif. Complex relationships between geodynamical events, thermal anomalies, and advective fluids confirm that remote tectonic activities can impact quiescent basins, even if located far from tectono-thermal activities, by discrete and long-distance fluid flows.

The Influence of Texture and Firing on Thermal and Elastic Properties of Illite-Based Ceramics

The texture in illite-based ceramics was introduced by spreading a large number of thin layers of a wet plastic mass one on top of the other. The basal planes of the plate-like illite crystals are predominantly oriented parallely with the direction of spreading. The samples were fired at different temperatures ranging from room temperature up to 1100 °C with a heating rate of 5 °C/min, then freely cooled and measured at room temperature. We determined Young modulus (E), thermal diffusivity (a) and relative dimension changes (Δl/l0) in two directions: parallely to the basal planes (a subscript ||) of the illite crystals and perpendicularly to these planes (a subscript ⊥). It was found that the ratio E|| / E⊥ ≈ 2.9 and a|| / a⊥ ≈ 2.0 up to 900 °C. Above 900 °C, the values of E and a increase due to sintering, and the differences between the values measured in the two directions decrease. In the case of the thermal diffusivity these differences do not disappear even after firing at the temperature of 1200 °C.

THERMOMECHANICAL ANALYSIS OF ILLITE FROM FÜZÉRRADVÁNY

Young’s modulus of green illite from Fuzerradvany (Hungary) was measured in-situ in the temperature interval 20 °C – 1100 °C and auxiliary analyses DTA, TG and TDA, XRD and EGA, were also performed. It was found that a removal of the physically bound water (20 °C – 250 °C) sets illite crystals closer which leads to a significant increase of Young’s modulus from its initial value of 7 GPa and reaches the maximum 12 GPa at 300 °C. Young’s modulus slightly decreases in the temperature interval of dehydroxylation of illite (300 °C – 800 °C), and which then reaches up to 45 GPa at 1100 °C, increasing exponentially as a consequence of the sintering above 800 °C. DOI: http://dx.doi.org/10.5755/j01.ms.21.3.7152

Caesium incorporation and retention in illite interlayers

Radioactive caesium (chiefly 137Cs) is a major environmental pollutant. The mobility of Cs in temperate soils is primarily controlled by sorption onto clay minerals, particularly the frayed edges of illite interlayers. This paper investigates the adsorption of Cs to illite at the molecular scale, over both the short and long term. Transmission electron microscopy (TEM) images showed that after initial absorption into the frayed edges, Cs migrated into the illite interlayer becoming incorporated within the mineral structure. Caesium initially exchanged with hydrated Ca at the frayed edges, causing them to collapse. This process was irreversible as Cs held in the collapsed interlayers was not exchangeable with Ca. Over the long term Cs did not remain at the edge of the illite crystals, but diffused into the interlayers by exchange with K. Results from extended X-ray absorption fine structure spectroscopy (EXAFS) and density functional theory modelling confirmed that Cs was incorporated into the illite interlayer and revealed its bonding environment.

Time constraints for the tectono-thermal evolution of the Cantabrian Zone in NW Spain by illite K–Ar dating

Contrasting sedimentary facies and tectonic structures diversely distributed and affected by diagenetic to low-grade metamorphic activity form a complex pattern in the northwestern Spanish Cantabrian area of the Variscan orogen. The distribution of these metamorphic features is based on field observations, clay mineralogy, mineral crystallinity indices, vitrinite reflectance data, and observation of rock microtextures. K–Ar dating of fine-grained illite-rich size fractions provides a complete set of ages for the episodic deformation and thermal overprints in the southeastern Cantabrian area. The usual K–Ar isochron plotting was combined with the bench-type age distribution to unravel the successive illite crystallization episodes. Several tectono-thermal episodes occurred in the southeastern Pisuerga–Carrion Province: initially during the Early Permian at 293 ± 4 Ma in the Ruesga Line, Valsurvio Dome and Sierra del Brezo area. A further mid-Permian event was detected at 268 ± 6 Ma in the Palentine Nappes, and during the mid-Triassic at 243 ± 5 Ma, again in the Valsurvio and Sierra del Brezo areas, and along the Ruesga Line. A later reactivation of the Ruesga Line was detected during mid-Jurassic time at 175 ± 6 Ma.

Episodic and simultaneous illitization in oil-bearing Brent Group and Fulmar Formation sandstones from the northern and southern North Sea based on illite K-Ar dating

Size fractions (0.4 and 0.4–1.0 m) of Brent Group sandstones from the northern North Sea contain mostly illite-smectite mixed layers with kaolinite, whereas the same size fractions of Fulmar Formation sandstones from the south-central North Sea consist of illite-smectite mixed layers with minor chlorite. Transmission electron microscope observations show elongated illite laths or agglomerates consisting of small laths when larger individual laths are lacking. The K-Ar data of the fractions less than 0.4 m of Brent Group samples plot on two arrays in a 40Ar/36Ar vs. 40K/36Ar diagram that have isochron characteristics with ages of 76.5 4.2 and 40.0 1.5 Ma, and initial 40Ar/36Ar ratios of 253 16 and 301 18, respectively. For the Fulmar Formation samples, the data points of the fractions less than 0.2 and less than 0.4 m also fit two isochrons with ages of 76.6 1.4 and 47.9 0.5 Ma and initial 40Ar/36Ar ratios of 359 52 and 304 2, respectively. Some of the coarser 0.4–1.0-m fractions also plot on the two isochrons, but most plot above indicating the presence of detrital components more than 0.4 m. The almost identical ages obtained from illite-type crystals of sandstones with different deposition ages that are located about 600 km (373 mi) apart record two simultaneous illitization episodes. These events were not induced by local burial conditions, but are related to episodic pressure and/or temperature increases in the studied reservoirs, probably induced by hydrocarbon injection. This interpretation is indirectly supported by notably different K-Ar illite ages from cores of a nearby reservoir at hydrostatic pressure. Illite is not as well crystallized as expected for potential crystallization temperatures above 160C measured by fluid-inclusion determinations. In both the northern and south-central North Sea, the two illite generations remain unaffected after crystallization despite continued burial, suggesting notably higher crystallization temperatures than those estimated from geothermal gradients. No recent illite crystallization or alteration is recorded in the K-Ar ages, despite a dramatic regional acceleration of the subsidence in the southern North Sea.

New insights into the formation of diagenetic illite from TEM studies

Diagenetic illite from the Proterozoic Chuanlinggou Formation, China, has been investigated using the techniques of transmission electron microscopy (TEM), selected-area electron diffraction (SAED), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectrometer (EDS). Polytypes of 1Md and 1M illite coexist in the diagenetic illite. The 1Md polymorph is predominant in the <0.2 μm size fraction, which has high Al and low Mg contents and irregular crystal shapes. Lattice-fringe imaging and one-dimensional structural imaging show abundant structure defects. The 1Md illite has both layer and sheet terminations caused by edge dislocations and layer displacements along c* caused by screw dislocations. These observations indicate that the disordered 1Md structure of the diagenetic illite resulted from abundant dislocations. The dislocation serves as nucleation sites for the illite nano particles to spontaneously nucleate in the initial stage of illite formation. Such a nucleation process obeys the Ostwald ripening rule. The illite only shows a 1.0 nm thickness for each layer in high-resolution TEM images, which suggests that the illite crystals have no mixed smectite layer, and the 1Md illites are authigenic and not illitized from smectite or illite/smectite. Heterogeneous nucleation led to directly crystallization of the 1Md illite during the initial growth of illite in shale under a low-temperature and tectonic-stress-free environment. The results have implications for interpreting the distribution of 1Md illite in sedimentary rocks.

Time-constrained illitization in gas-bearing Rotliegende (Permian) sandstones from northern Germany by illite potassium-argon dating

Permian Rotliegende sandstone cores were collected from an area of about 14,800 km2 (5710 mi2) mostly to the east-southeast of the city of Bremen in northern Germany, at depths between 4596 and 5330 m (15,079–17,487 ft). The separated size fractions (0.2, 0.2–1.0, and 1.0–2.0 m) consist of illite (90–100%) with small to minute amounts of chlorite and detectable quartz and feldspars in the coarser fractions. Scanning electron microscopic and transmission electron microscopic observations showed two types of illite morphologies: flakes coating detrital framework minerals and laths and fibers invading the pore space. The data points of most size fractions fit two isochrons, with slopes providing ages of 191 8 and 178 1 Ma, with initial 40Ar/36Ar ratios reasonably close to the atmospheric value. Microthermometric fluid-inclusion determinations in quartz and calcite characterize two types of percolating fluids: a highly saline (19% NaCl equivalent) fluid at variable temperatures depending on the reservoirs (185 to 150C) and a slightly saline one (2.6% NaCl equivalent) again at varied temperatures (170 to 145C), also depending on the locations. These temperatures are higher than paleotemperatures calculated on the basis of a present-day burial gradient of 30.5C/km, therefore favoring hydrothermal illitization, with the oldest illite crystallizing at a generally higher temperature than that of the younger illite. However, if illite coincidently precipitated with quartz, which is supported by petrographic observation, then illite with ages of about 200 Ma in the eastern to central Rotliegende sandstones of the area formed at a lower temperature but in more saline fluids than that of about 155 Ma. Alternatively, illite crystallization temperature and salinity of the interacting fluids could have been activated by the same hydrothermal activity at different locations in the studied area and could have decreased and changed, respectively, when the fluids moved in the sandstone reservoirs. The potassium-argon data fit a regional model of recurrent thermal activity, with the oldest occurrence of authigenic illite in the eastern Imbrock-Ganderkesee area and a time-dependent trend of the hydrothermal activity toward the west. Previous results from northern Germany, central Netherlands, and the southern North Sea complete the model with an initial activity at the same 200-Ma time in the western offshore and central Netherlands and an activity trend toward the east.

Anatomy of a metabentonite: Nucleation and growth of illite crystals and their coalescence into mixed-layer illite/smectite

The illite layer content of mixed-layer illite/smectite (I/S) in a 2.5 m thick, zoned, metabentonite bed from Montana decreases regularly from the edges to the center of the bed. Traditional X-ray diffraction (XRD) pattern modeling using Markovian statistics indicated that this zonation results from a mixing in different proportions of smectite-rich R0 I/S and illite-rich R1 I/S, with each phase having a relatively constant illite layer content. However, a new method for modeling XRD patterns of I/S indicates that R0 and R1 I/S in these samples are not separate phases (in the mineralogical sense of the word), but that the samples are composed of illite crystals that have continuous distributions of crystal thicknesses, and of 1 nm thick smectite crystals. The shapes of these distributions indicate that the crystals were formed by simultaneous nucleation and growth. XRD patterns for R0 and R1 I/S arise by interparticle diffraction from a random stacking of the crystals, with swelling interlayers formed at interfaces between crystals from water or glycol that is sorbed on crystal surfaces. It is the thickness distributions of smectite and illite crystals (also termed fundamental particles, or Nadeau particles), rather than XRD patterns for mixed-layer I/S, that are the more reliable indicators of geologic history, because such distributions are composed of well-defined crystals that are not affected by differences in surface sorption and particle arrangements, and because their thickness distribution shapes conform to the predictions of crystal growth theory, which describes their genesis.

New findings on the tectono-metamorphic history of the western Rhenish Massif (Germany) by K–Ar dating of metasedimentary illite

Metapelites, clay-rich sandstones and volcanics from Cambrian, Ordovician and Lower Devonian strata of the western Rhenish Massif underwent a complex regional Variscan tectono-thermal evolution, as shown by mineralogical and K–Ar isotopic analyses of the illite to mica components from three NW-SE transects. The metamorphic degree extended from an anchimetamorphic to an epimetamorphic intensity during two major episodes of illite crystallization at 328 ± 6 and 282 ± 12 Ma. A further late orogenic or post-orogenic extensional activity could also be detected, but not precisely, around 270 Ma, probably recorded by the precipitation of illite in new or reactivated extensional faults with upward moving heat flows. Three studied cross-sections through the massif display a distribution of the illite crystallinity index that scatters within the diagenesis-to-anchizone range with increased crystallization grades along the thrust zones. The determined K–Ar ages probably frame the Variscan convergence in the western Rhenish Massif, which allows calculation of a “shortening velocity” of 140 km in 40 Ma, or of about 0.35 cm/a, that can be suggested on the basis of a total shortening of about 50% for the entire massif.

HRTEM investigation of intralayer and interlayer stacking defects and pyrophyllite interlayers in illite

AbstractMetastable authigenic 1M illite from shale of diagenetic grade has been studied using a high-resolution transmission electron microscope (HRTEM) equipped with energy-dispersive spectrometer, X-ray diffraction, and scanning electron microscope. The illite occurs as deformed flakes deficient in interlayer K+ cations with 0.6 per half cell, and with abnormally high Al in both octahedral and tetrahedral sites. Complex structural adjustments reflecting the unusual chemical composition are observed in images of illite at near-atomic resolution. Different distances and directions of intralayer shift between the upper tetrahedral sheet and the lower tetrahedral sheet within 2:1 layers are found in this sample. Intralayer undershift structure coupled with interlayer displacement is found in a 1M illite crystal, and intralayer overshift structure coupled with no interlayer displacement is found in a 1M domain of a larger crystal. Two tetrahedral sheets across the interlayer region sometimes deviate from ideal positions causing interlayer displacement. Two pyrophyllite layers are found overlying a stack of ordered 1M illite layers, and are overlain by illite layers with anomalous interlayer offsets. This offset is considered to result from an increase in the lateral dimensions of the tetrahedral sheet due to anomalous high Al content. Our observation of intralayer and interlayer deficiencies indicate that authigenic illite that crystallized in the early stage of diagenesis at low temperatures tends to give rise to heterogeneous, disordered, and metastable structures.

K–Ar dating and δ18O–δD tracing of illitization within and outside the Shea Creek uranium prospect, Athabasca Basin, Canada

Abstract Isotope analyses (K–Ar, δ 18 O and δ D) were performed on illite from both the sandstone cover and the underlying basement, close to and distant from Shea Creek, an unconformity-type U deposit (Athabasca Basin, Canada); the illite had previously been characterized crystallographically. In the barren areas away from deposit, illite is mainly of the cis-vacant 1 M polytype occurring as relatively coarse-grained lath-shaped particles, while it occurs as fine-grained particles of the trans-vacant 1 M type next to and in the U mineralized strata. The tectonic-induced hydrothermal system that favored illite crystallization was multi-episodic 1453 ± 2, 1330 ± 20 and probably about 1235 Ma ago. These illite-forming episodes appear to have occurred contemporaneously to those favoring the concentration of the associated U oxides, which were dated independently by the U–Pb method in the Shea Creek deposits and elsewhere in the Athabasca Basin. No relationship was found between the illite polytypes and their crystallization ages, meaning that precipitation of each, either as the cis-vacant or the trans-vacant type, did not relate to a specific event, but to variable physical and chemical crystallization conditions during the same event. The change in the contemporaneous illite polytypes appears to relate to an increase in the δ 18 O with distance to the U deposit. Such a change could result from a progressively lower formation temperature with increasing distance to the U deposit, probably combined to a changing δ 18 O of the interstitial fluids due to variable water–rock interactions in the rocks. Variable water–rock ratios could have resulted from variable tectonic adjustments of the basement. The authors are inclined to believe that the cis-vacant 1 M type crystallized from chemically different fluids, at slightly lower temperatures and away from U concentrations than the equivalent trans-vacant 1 M type detected next to the U ores, both precipitating contemporaneously within analytical uncertainty. In addition, comparison of the δ D of the hydroxyls from cis-vacant and trans-vacant types suggests that the illite mineral structure was not affected by radiation related to radioactive decay within the deposit or by further natural alteration, as advocated for other occurrences.

K-Ar dating of the Lower Palaeozoic K-bentonites from the Baltic Basin and the Baltic Shield: implications for the role of temperature and time in the illitization of smectite

AbstractMixed-layer illite-smectite samples from the Ordovician and Silurian K-bentonites of the Baltic Basin and the Baltic Shield (Norway, Sweden, Denmark, Poland and Estonia) were dated by K-Ar on several grain fractions and were studied by X-ray diffraction (XRD), both on oriented and random preparations, in order to reveal the conditions of smectite illitization in the area. Authigenic K-feldspar was also dated. The geographic pattern of the degree of illitization (% smectite in illite-smectite measured by XRD) is consistent with other indicators of palaeotemperatures (acritarchs, conodont alteration index, vitrinite reflectance, apatite fission track ages). It reveals the highest maximum palaeotemperatures (up to at least 200ºC) along the Norwegian and the German-Polish branches of the Caledonides and the lowest palaeotemperatures (120ºC) in the central part of the studied area. The distribution of K-Ar ages is not well correlated with this pattern, revealing a zone of older ages (Lower Devonian-Lower Carboniferous) between Denmark and Estonia, and areas of younger ages (Upper Devonian to Carboniferous/Permian boundary) to the north and south of this zone. The zone of older ages is interpreted as the result of illitization induced by a thermal event in front of the Caledonian orogenic belt (migration of hot metamorphic fluids?). The areas of younger ages are considered as representing deep burial illitization under a thick Silurian-Carboniferous sedimentary cover, perhaps augmented by a tectonic load. The K-Ar dates invalidate the hypothesis of a long-lasting low-temperature illitization as the mechanism of formation of the Estonian Palaeozoic illite-smectite. The ammonium content of illite-smectite from the Baltic K-bentonites reflects the proximity of organic-rich source rocks that underwent thermal alteration at the time of illite crystallization.

Hydrothermal alteration of the Soultz-sous-Forets granite (Hot Fractured Rock geothermal exchanger) into a tosudite and illite assemblage

Abstract: The Soultz-sous-Forets granitic basement represents the reservoir of an experimental Hot Fractured Rock (HFR) geothermal exchanger presently tested in the northern Rhine Graben (France), referring now to the concept of Enhanced Geothermal System (EGS). The injected fluids circulate in the natural fracture network of the granite and through its hydrothermally altered matrix. One of these fractured and altered zones, located about 800 m below the granite-sediment boundary, contains tosudite, which is a rather rare mixed-layer chlorite/smectite that crystallized here ahead of a fibrous illite/quartz/calcite paragenesis. Tosudite occurs mainly in the relics of plagioclase grains that were progressively altered by interacting with Li-bearing hydrothermal fluids percolating in the granite fractures. The age of the hydrothermal alteration activity is inferred from K-Ar dating of varied particle sizes of the associated illite: two distinct hydrothermal episodes of illite crystallization could be set at about 63 and 18-Ma or less, without further detectable precipitation, especially during the rifting of the Rhine Graben. Precipitation of fibrous illite in the pore space of the altered granite is expected to have reduced its permeability, as frequently observed in sandstone reservoirs. Clay crystallization may, therefore, represent a significant drawback for engineering the geothermal programme, as the chemical composition of the injected fluids shall be designed to reduce and even prevent illite precipitation and promote tosudite precipitation, when mixing with the natural fluids still present in the granite.

Reductive biogenic transformation of Fe(III)-containing phyllosilicates (review of publications)

Microorganisms are capable of reducing the structural Fe(III) of some phyllosilicates, i.e., montmorillonite, nontronite, and illite. Two reduction results are possible. For the low-ferruginous minerals, the solid-phase partial reduction of the structural Fe(III) does not cause any decay of particles. In this case, reduction leads to a partial dehydroxylation of the lattice and magnetic disordering of the mineral, as well as to changes in the layer packaging, the crystallization degree, the texture, the color, the surface charge, and the cation-exchange capacity. Except for dehydroxylation and the changing Fe oxidation status, the transformations of minerals’ properties are irreversible for the most part. In highly ferruginous phyllosilicates, the structural Fe(III) reduction leads to the particle dissolution. As a result of microbial reductive dissolution of crystallized nontronite, new amorphous minerals are formed, i.e., aluminosilicates with an elevated Al/Si ratio, Si globules, and Fe hydroxides. Another biogenically crystallized Si mineral (tridymite βSiO2) is formed in light-textured soils and sediments after smectite decomposition. The bacterial reduction of highly ferruginous illite leads to transformation of the shape of some particles: initially acicular illite crystals were transformed into plates. The amorphous structure is formed in illite particles contacting with bacterial cells. The solid-phase reduction of structural Fe(III) does not lead to particle dissolution in the case of low-ferruginous illite.