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Bruno Lanson - One of the best experts on this subject based on the ideXlab platform.
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Diagenetic smectite-to-illite transition in clay-rich sediments: A reappraisal of X-ray diffraction results using the multi-specimen method
American Journal of Science, 2009Co-Authors: Bruno Lanson, Boris A. Sakharov, Francis Claret, Victor A. DritsAbstract:Smectite Illitization is a common mineralogical reaction occurring during the burial diagenesis of clay-rich sediments and shales, and has thus attracted sustained interest over the last fifty years. Prior studies have concluded that smectite Illitization proceeds through a steady set of homogeneous reactions involving intermediate mixed layers of varying compositions. In these intermediate structures, illite and smectite or, more generally, expandable layers (I and Exp layers, respectively) coexist among the same crystallites giving rise to non-periodic structures (I-Exp) characterized by specific diffraction effects. Consistent with this model, reaction progress was characterized by the simultaneous increase in the illite content in I-Exp and in their stacking order leading to the following mineralogical sequence: smectite → randomly interstratified I-Exp with high smectite contents (> 50% Exp layers) → ordered I-Exp with high illite contents (> 50% I layers) → illite. Although reaction mechanisms have been extensively debated, this structural characterization has not been challenged, possibly due to a methodological bias. In the present study, X-ray diffraction patterns typical of the diagenetic Illitization of smectite are interpreted using modern approaches involving profile fitting (multi-specimen method). Novel insights into the structure of intermediate reaction products are thus obtained. In particular, original clay parageneses are described that include the systematic presence of illite, kaolinite, chlorite and a mixed layer containing kaolinite and expandable layers (K-Exp). In contrast to previous descriptions, the early stages of smectite Illitization are characterized by the coexistence of discrete smectite and of a randomly interstratified I-Exp with a high content of illite layers (>50% I layers). Both the smectite and the I-Exp are authigenic and form under shallow burial, that is at low temperature conditions. With increasing burial depth, the relative proportion of I-Exp increases, essentially at the expense of discrete smectite, and the composition of I-Exp becomes slightly more illitic. In the second stage of smectite Illitization, two illite-containing mixed layers are observed. They result from two parallel reaction mechanisms affecting the randomly interstratified I-Exp present in the shallow section of the series. The first reaction implies the dissolution of this randomly interstratified I-Exp and leads to the crystallization of an ordered I-Exp without significant Illitization, possibly because of the low K-availability. The second reaction affecting the randomly interstratified I-Exp implies the growth of trioctahedral (Mg, Al) hydroxide sheets in Exp interlayers, thus developing di-trioctahedral chlorite layers (Ch layers) in the initial I-Exp to form an I-Exp-Ch. A layer-by-layer mechanism is hypothesized for this reaction. In this scheme, Mg cations released by the dissolution-recrystallization reaction of I-Exp likely represent the source of Mg for the formation of brucite-like sheets in expandable interlayers, and thus of the I-Exp-Ch. The reported structural characterization of smectite Illitization intermediate products contradicts the conventional wisdom of a homogeneous reaction through a series of pure mixed layers of variable composition. In contrast, the coexistence of different phases implies a heterogeneous reaction via a sequence of intermediate phases and requires reassessing the reaction mechanisms proposed in the literature. The compositional range (relative proportion of the different layer types) of these phases is limited and smectite Illitization proceeds essentially as relative proportions of the different phases vary. In addition, reaction kinetics and stability of the different intermediate products also need to be reconsidered.
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Diagenetic smectite-to-illite transition in clay-rich sediments: A reappraisal of X-ray diffraction results using the multi-specimen method
American journal of science, 2009Co-Authors: Bruno Lanson, Boris A. Sakharov, Francis Claret, Victor A. DritsAbstract:Smectite Illitization is a common mineralogical reaction occurring during the burial diagenesis of clay-rich sediments and shales, and has thus attracted sustained interest over the last fifty years. Prior studies have concluded that smectite Illitization proceeds through a steady set of homogeneous reactions involving intermediate mixed layers of varying compositions. In these intermediate structures, illite and smectite, or, more generally, expandable layers (I and Exp layers, respectively) coexist among the same crystallites giving rise to non-periodic structures (I-Exp) characterized by specific diffraction effects. Consistent with this model, reaction progress was characterized by the simultaneous increase in the illite content in I-Exp and in their stacking order leading to the following mineralogical sequence: smectite → randomly interstratified I-Exp with high smectite contents (> 50% Exp layers) → ordered I-Exp with high illite contents (> 50% I layers) → illite. Although reaction mechanisms have been extensively debated, this structural characterization has not been challenged, possibly due to a methodological bias. In the present study, X-ray diffraction patterns typical of the diagenetic Illitization of smectite are interpreted using modern approaches involving profile fitting (multi-specimen method). Novel insights into the structure of intermediate reaction products are thus obtained. In particular, original clay parageneses are described including the systematic presence of illite, kaolinite, chlorite and a mixed layer containing kaolinite and expandable layers (K-Exp). In contrast to previous descriptions, the early stages of smectite Illitization are characterized by the coexistence of discrete smectite and of a randomly interstratified I-Exp with a high content of illite layers (>50% I layers). Both the smectite and the I-Exp are authigenic and form under shallow burial, that is at low temperature conditions. With increasing burial depth, the relative proportion of I-Exp increases, essentially at the expense of discrete smectite, and the composition of I-Exp becomes slightly more illitic. In the second stage of smectite Illitization, two illite-containing mixed layers are observed. They result from two parallel reaction mechanisms affecting the randomly interstratified I-Exp present in the shallow section of the series. The first reaction implies the dissolution of this randomly interstratified I-Exp and leads to the crystallization of an ordered I-Exp without significant Illitization, possibly because of the low K-availability. The second reaction affecting the randomly interstratified I-Exp implies the growth of trioctahedral (Mg, Al) hydroxide sheets in Exp interlayers, thus developing di-trioctahedral chlorite layers (Ch layers) in the initial I-Exp to form an I-Exp-Ch. A layer-by-layer mechanism is hypothesized for this reaction. In this scheme, Mg cations released by the dissolution-recrystallization reaction of I-Exp likely represent the source of Mg for the formation of brucite-like sheets in expandable interlayers, and thus of the I-Exp-Ch. The reported structural characterization of smectite Illitization intermediate products contradicts the conventional wisdom of a homogeneous reaction through a series of pure mixed layers of variable composition. In contrast, the coexistence of different phases implies a heterogeneous reaction via a sequence of intermediate phases and requires reassessing the reaction mechanisms proposed in the literature. The compositional range (relative proportion of the different layer types) of these phases is limited and smectite Illitization proceeds essentially as relative proportions of the different phases vary. In addition, reaction kinetics and stability of the different intermediate products also need to be reconsidered.
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Illite-Smectite Mixed-Layer Minerals in the Hydrothermal Alteration of Volcanic Rocks: II. One-Dimensional HRTEM Structure Images and Formation Mechanisms
Clays and Clay Minerals, 2005Co-Authors: Takashi Murakami, Atsuyuki Inoue, Alain Meunier, Bruno Lanson, Daniel BeaufortAbstract:Smectite Illitization was investigated in felsic volcaniclastic rocks from a drill core near the Kakkonda active geothermal system, Japan, using high-resolution transmission electron microscopy (HRTEM) that provided one-dimensional structure images of mixed-layer illite-smectite (I-S) minerals normal to [M0]. Simulated images of a rectorite-like structure revealed that smectite can be distinguished from illite in mixed-layer I-S by HRTEM if the basal spacing of smectite is larger than that of illite. The larger basal spacing of smectite, 1.3 nm under HRTEM, was obtained by intercalation of dodecylammonium ions into smectitic interlayers. In simulated and observed images normal to [ hk 0], tetrahedral ( T ) and octahedral ( O ) cation planes are imaged as dark lines, an illitic interlayer as a bright line, and a smectitic interlayer as a dark line sandwiched between two bright lines. The samples are from depths of 435 m (5% I; R0), 635 m (35% I; R0), 656 m (62% I; R1), and 756 m (85% I; R3) where % I is the percentage of illite layers in a sample and R is the Reichweite parameter. Sample 435 consisted mostly of smectite, and illite layers occurred, though small in amount, as M1 units (module of type 1, defined as consisting of two polar T-O-T silicate layers with one central illitic interlayer and two, half smectitic interlayers at the outermost surface; the number corresponds to that of central illitic interlayers). The M1 units were dominant and isolated and consecutive smectite layers (>2) were present in sample 635. Sample 656 consisted mostly of packets of M1 units of 1 to 5 layers containing M2 to M5 units occasionally. Isolated or consecutive smectite layers (>2) were not present in 656. Illite layers occurred almost entirely as M1 units in samples 435, 635 and 656, and the number of M1 units increased with increase in % I. Sample 756 was characterized by the presence of M2 to M10 units accompanied by smectitic interlayers at the external surface and the absence of M1 units and isolated smectite layers. The HRTEM data strongly suggest that Illitization in a hydrothermal system occurs by precipitation of M1 units for mixed-layer I-S minerals up to 60% I. This does not require the presence of precursor smectite. Illitization of I-S minerals with >60% I proceeds by precipitation of various types of M n ( n ⩾ 2) units. Illite occurs only as M n ( n ⩾ 1) units throughout Illitization.
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Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: II. One- dimensional HRTEM structure images and formation mechanisms.
Clays and Clay Minerals, 2005Co-Authors: Takashi Murakami, Atsuyuki Inoue, Bruno Lanson, A. Meunier, Daniel BeaufortAbstract:Smectite Illitization was investigated in felsic volcaniclastic rocks from a drill core near the Kakkonda active geothermal system, Japan, using high resolution transmission electron microscopy (HRTEM) that provided one-dimensional structure images of mixed-layer illite-smectite (I-S) minerals normal to [hk0]. Simulated images of a rectorite-like structure revealed that smectite can be distinguished from illite in mixed-layer I-S by HRTEM if the basal spacing of smectite is larger than that of illite. The larger basal spacing of smectite, 1.3 nm under HRTEM, was obtained by intercalation of dodecylammonium ions into smectitic interlayers. In simulated and observed images normal to [hk0], tetrahedral (T) and octahedral (O) cation-layers are imaged as dark lines, an illitic interlayer a bright line, and a smectitic interlayer a dark line sandwiched by two bright lines. Samples are from 435 m (5% I; R0), 635 m (35% I; R0), 656 m (62% I; R1), and 756 m (85% I; R3) depths where % I is the percent of illite layers in a sample and R is the Reichweite parameter. Sample 435 consisted mostly of smectite, and illite layers occurred, though small in amount, as M1 units (module of type 1, defined as one consisting of two polar T-O-T silicate layers with one central illitic interlayer and two, half smectitic interlayers at the outermost surface; the number corresponds to that of central illitic interlayers). M1 units were dominant and isolated or consecutive smectite layers of > two were present in sample 635. Sample 656 consisted mostly of packets of M1 units of 1 to 5 containing M2 to M5 units occasionally. Isolated or consecutive smectite layers of > two were not present in 656. Illite layers occurred almost as M1 units in samples 435, 635 and 656, and the number of M1 units increased with increase in % I. Sample 756 was characterized by the presence of M2 to M10 units accompanied by smectitic interlayers at the external surface and the absence of M1 units and isolated smectite layers. HRTEM data strongly suggest that Illitization in a hydrothermal system occurs by precipitation of M1 units for mixed-layer I-S minerals up to 60% I. This does not require the presence of precursor smectite. Illitization of I-S minerals with more than 60% I proceeds by precipitation of various types of Mn(n≥2) units. Illite occurs only as Mn(n≥1) units throughout Illitization.
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Kinetic constraints on Illitization reactions and the effects of organic diagenesis in sandstone/shale sequences
Applied Geochemistry, 1997Co-Authors: Gilles Berger, Daniel Beaufort, Jean-claude Lacharpagne, Bruce Velde, Bruno LansonAbstract:Abstract Based on water-rock interaction modelling and kinetic considerations, the present study is aimed at testing the impact of organic matter maturation on two kinds of diagenetic reactions leading to the alteration of the petrophysical properties of sandstones: (1) Clay mineral conversion to illite with K-feldspar being a local potential source of K; and (2) Dissolution-precipitation processes produced in sandstones by the action of water-soluble organic species derived from adjacent shales. Assuming firstly that the chemical reactions take place in closed systems, the nature and timing of diagenetic reactions in marine sandstone/shale formations were modelled for a 50–120°C temperature range in order to improve understanding of the factors that control the Illitization reaction with K-feldspar coexisting with aluminous clay. Illite is modelled here as a muscovite type mineral. We tested the effects of an energy barrier on illite growth by allowing or preventing the muscovite/illite precipitation reaction to occur, while using several Illitization reaction rates. We also compared the stable mineral parageneses predicted for organic material-free systems with those predicted in the case of organic diagenesis (release of CO 2 , CH 4 , acetic and oxalic acids). Similarities and discrepancies between numerical results and natural mineral assemblages suggest that the Illitization reaction depends on the nature of the reacting clays. Kaolinite conversion to end-member illite involves high-energy conditions (> 2 kcal mol −1 ), which are not met when the pore water equilibrates with the mineral matrix from undersaturated conditions in a closed system. To overcome this barrier, the fluid should be oversaturated with respect to K-feldspar. An external source of K or a pH increase in an open system is necessary for this reaction. No particular effect of organic diagenesis on this reaction was found in the present study. On the other hand, smectite-to-illite conversion involves a lower energy barrier and can operate in closed systems where K-feldspars are the source of K. The maturation of organic matter may speed up the smectite-to-illite conversion rate by increasing the Gibbs Free Energy of illite growth. Interactions between sandstone and shale were examined by modelling the transfer of aqueous species of organic origin from shale to sandstone. Fluid expulsion from shale had little or no effect on diagenesis in adjacent sandstones during each flushing cycle, mainly because organic protons and ligands were neutralized in the source rock. However, the diffusion of shale-derived cations through sandstone as organometallic complexes (Mg > Ca > > Fe > > Al) appears to be an efficient process during carbonate cementation in sandstones, where oxidizing conditions enhance the decomposition of such complexes.
Alain Meunier - One of the best experts on this subject based on the ideXlab platform.
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Microbially induced potassium enrichment in Paleoproterozoic shales and implications for reverse weathering on early Earth
Nature Communications, 2019Co-Authors: Jérémie Aubineau, Jean-yves Reynaud, Olabode M. Bankole, Armelle Riboulleau, Andrea Somogyi, Roberto Macchiarelli, Abderrazak El Albani, Alain Meunier, Andrey Bekker, Kurt O. KonhauserAbstract:Illitisation requires potassium incorporation into a smectite precursor, a process akin to reverse weathering. However, it remains unclear whether microbes facilitate K^+ uptake to the sediments and whether illitisation was important in the geological past. The 2.1 billion-year-old Francevillian Series of Gabon has been shown to host mat-related structures (MRS) and, in this regard, these rocks offer a unique opportunity to test whether ancient microbes induced illitisation. Here, we show high K content confined to illite particles that are abundant in the facies bearing MRS, but not in the host sandstone and black shale. This observation suggests that microbial biofilms trapped K^+ from the seawater and released it into the pore-waters during respiration, resulting in illitisation. The K-rich illite developed exclusively in the fossilized MRS thus provides a new biosignature for metasediments derived from K-feldspar-depleted rocks that were abundant crustal components on ancient Earth.The transition from smectite to illite requires potassium incorporation, yet the role of microbes in facilitating K^+ uptake remains debated, especially during the early Earth. Here, the authors suggest that the Paleoproterozoic microbial mats extracted potassium from sea water and induced localized Illitization during early low-temperature diagenesis.
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From detrital heritage to diagenetic transformations, the message of clay minerals contained within shales of the Palaeoproterozoic Francevillian basin (Gabon)
Precambrian Research, 2014Co-Authors: Lauriss Ngombi-pemba, Alain Meunier, Abderrazak El Albani, Olivier Grauby, François Gauthier-lafayeAbstract:Unmetamorphosed and undeformed marine siliciclastics rocks of the FB, FC and FD of the Francevillian series (Gabon) were deposited in an epicontinental basin. Clay minerals found in black shale, siltstone and sandstone are dominantly illite and chlorite except in two levels of the FB formation, which contain smectite-rich randomly ordered mixed layers. Their survival in a 2.1 Ga old sedimentary series is not related to the abundance of organic matter (total organic carbon or TOC), nor redox conditions at the time of deposition as indicated by the Fe speciation (FeHR/FeT and FePy/FeHR ratios). Rather it results from an incomplete Illitization reaction that reflects potassium deficiency. The K2O/Al2O3 ratio of shale, siltstone and sandstone vary along the series, and appear to conserve the signature of the original chemical composition of the rocks. K-feldspars which are present in the FC and FD formations are missing in the FB formation. Consequently, the smectite layers do not appear to be inherited from a detrital input in the basin but must be considered as representative of an intermediate stage of the Illitization reaction reached during diagenesis.
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2.9 to 1.9 Ga paleoalterations ofArchean granitic basement of the Franceville basin (Gabon)
Journal of African Earth Sciences, 2014Co-Authors: Idalina Moubiya Mouélé, Alain Meunier, Patrick Dudoignon, Abderrazak El Albani, Philippe Boulvais, François Gauthier-lafaye, Jean-louis Paquette, Hervé Martin, Michel CuneyAbstract:The Archean granitoids in the Kiene area, Gabon, are overlained by the Paleoproterozoic sediments of the Franceville basin (2.1 Ga). The basin is known for its high-grade uranium deposits among which some have been forming natural nuclear fission reactors. Most of the studies were dedicated to the FA-FB Paleoproterozoic sediments hosting these uranium deposits. Little is known on the Archean basement itself and specifically on the hydrous alteration events it experienced before and after the sediment deposition. The present work is focused on their petrographical, mineralogical and geochemical characterization. Dating the successive alteration events has been attempted on altered monazite crystals. Rocks in different alteration states have been sampled from eight drill cores crosscutting the Archean - Paleoproterozoic unconformity. The Archean granitoids observed in the deepest levels exhibit typical petrographical features of a propylitic alteration while they are intensely illitized up to the unconformity. The propylitic alteration is mainly pervasive but the original texture of the granitoids is conserved in spite of the formation of new minerals: Mg-chlorite, allanite and epidote forming a typical paragenesis. The illitic alteration is much more invasive near the unconformity. The Illitization process leads to the replacement of feldspars and the corrosion of quartz crysals by an illitic matrix while the ferromagnesian minerals are pseudomorphosed by a Fe-chlorite + phengite + hematite assemblage. The final fluid-rock interaction step is marked by fissural deposits of calcite and anhydrite. The δ13C isotopic data show that the fissural carbonates precipitated from diagenetic fluids enriched carbon products deriving from the maturation of organic matter. The U-Pb isotopic analyzes performed on monazite crystals have dated three distinct events: 3.0-2.9 Ga (magmatic), 2.6 Ga (propylitic alteration) and 1.9 Ga (diagenetic Illitization). The calculation of geochemical mass balances suggests that the water-rock ratio during the propylitic alteration event was weak. On the contrary, it was much higher during the overprinted Illitization which is characterized by an intense leaching of Na, Ca, Mg, Sr, REE and an enrichment in K, Rb,Cs. Neither the petrographic features nor the geochemical data militate for an Archean weathering event (paleosol). In the present case, diagenetic fluids have percolated from the unconformity into the basement where they overprinted the Illitization processes upon the previously propylitized rocks. These fluids were probably oxidant as they are also responsible of the U mobilization which led to the formation of the ore deposits close to the FA-FB interface.
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2.9 to 1.9 Ga paleoalterations ofArchean granitic basement of the Franceville basin (Gabon)
Journal of African Earth Sciences, 2014Co-Authors: Idalina Moubiya Mouélé, Abderrazak El Albani, Alain Meunier, Patrick Dudoignon, Philippe Boulvais, François Gauthier-lafaye, Jean-louis Paquette, Hervé Martin, Michel CuneyAbstract:The Archean granitoids in the Kiéné area, Gabon, are overlained by the Paleoproterozoic sediments of the Franceville basin (2.1 Ga). The basin is known for its high-grade uranium deposits among which some have been forming natural nuclear fission reactors. Most of the studies were dedicated to the FA-FB Paleoproterozoic sediments hosting these uranium deposits. Little is known on the Archean basement itself and specifically on the hydrous alteration events it experienced before and after the sediment deposition. The present work is focused on their petrographical, mineralogical and geochemical characterization. Dating the successive alteration events has been attempted on altered monazite crystals. Rocks in different alteration states have been sampled from eight drill cores crosscutting the Archean - Paleoproterozoic unconformity. The Archean granitoids observed in the deepest levels exhibit typical petrographical features of a propylitic alteration while they are intensely illitized up to the unconformity. The propylitic alteration is mainly pervasive but the original texture of the granitoïds is conserved in spite of the formation of new minerals: Mg-chlorite, allanite and epidote forming a typical paragenesis. The illitic alteration is much more invasive near the unconformity. The Illitization process leads to the replacement of feldspars and the corrosion of quartz crysals by an illitic matrix while the ferromagnesian minerals are pseudomorphosed by a Fe-chlorite + phengite + hematite assemblage. The final fluid-rock interaction step is marked by fissural deposits of calcite and anhydrite. The δ13C isotopic data show that the fissural carbonates precipitated from diagenetic fluids enriched carbon products deriving from the maturation of organic matter. The U-Pb isotopic analyzes performed on monazite crystals have dated three distinct events: 3.0-2.9 Ga (magmatic), 2.6 Ga (propylitic alteration) and 1.9 Ga (diagenetic Illitization). The calculation of geochemical mass balances suggests that the water-rock ratio during the propylitic alteration event was weak. On the contrary, it was much higher during the overprinted Illitization which is characterized by an intense leaching of Na, Ca, Mg, Sr, REE and an enrichment in K, Rb,Cs. Neither the petrographic features nor the geochemical data militate for an Archean weathering event (paleosol). In the present case, diagenetic fluids have percolated from the unconformity into the basement where they overprinted the Illitization processes upon the previously propylitized rocks. These fluids were probably oxidant as they are also responsible of the U mobilization which led to the formation of the ore deposits close to the FA-FB interface.
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Illite-Smectite Mixed-Layer Minerals in the Hydrothermal Alteration of Volcanic Rocks: II. One-Dimensional HRTEM Structure Images and Formation Mechanisms
Clays and Clay Minerals, 2005Co-Authors: Takashi Murakami, Atsuyuki Inoue, Alain Meunier, Bruno Lanson, Daniel BeaufortAbstract:Smectite Illitization was investigated in felsic volcaniclastic rocks from a drill core near the Kakkonda active geothermal system, Japan, using high-resolution transmission electron microscopy (HRTEM) that provided one-dimensional structure images of mixed-layer illite-smectite (I-S) minerals normal to [M0]. Simulated images of a rectorite-like structure revealed that smectite can be distinguished from illite in mixed-layer I-S by HRTEM if the basal spacing of smectite is larger than that of illite. The larger basal spacing of smectite, 1.3 nm under HRTEM, was obtained by intercalation of dodecylammonium ions into smectitic interlayers. In simulated and observed images normal to [ hk 0], tetrahedral ( T ) and octahedral ( O ) cation planes are imaged as dark lines, an illitic interlayer as a bright line, and a smectitic interlayer as a dark line sandwiched between two bright lines. The samples are from depths of 435 m (5% I; R0), 635 m (35% I; R0), 656 m (62% I; R1), and 756 m (85% I; R3) where % I is the percentage of illite layers in a sample and R is the Reichweite parameter. Sample 435 consisted mostly of smectite, and illite layers occurred, though small in amount, as M1 units (module of type 1, defined as consisting of two polar T-O-T silicate layers with one central illitic interlayer and two, half smectitic interlayers at the outermost surface; the number corresponds to that of central illitic interlayers). The M1 units were dominant and isolated and consecutive smectite layers (>2) were present in sample 635. Sample 656 consisted mostly of packets of M1 units of 1 to 5 layers containing M2 to M5 units occasionally. Isolated or consecutive smectite layers (>2) were not present in 656. Illite layers occurred almost entirely as M1 units in samples 435, 635 and 656, and the number of M1 units increased with increase in % I. Sample 756 was characterized by the presence of M2 to M10 units accompanied by smectitic interlayers at the external surface and the absence of M1 units and isolated smectite layers. The HRTEM data strongly suggest that Illitization in a hydrothermal system occurs by precipitation of M1 units for mixed-layer I-S minerals up to 60% I. This does not require the presence of precursor smectite. Illitization of I-S minerals with >60% I proceeds by precipitation of various types of M n ( n ⩾ 2) units. Illite occurs only as M n ( n ⩾ 1) units throughout Illitization.
Victor A. Drits - One of the best experts on this subject based on the ideXlab platform.
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new insights into smectite Illitization a zoned k bentonite revisited
American Mineralogist, 2009Co-Authors: Douglas K Mccarty, B A Sakharov, Victor A. DritsAbstract:The Illitization reaction in a thick K-bentonite bed located in upper Cretaceous marine shale in the Montana disturbed belt was studied by X-ray diffraction, chemical analysis, and thermal gravimetric analysis. Modeling of the experimental XRD patterns from oriented clay specimens in air-dried and glycolated states shows that at each sample location in the bentonite bed a mixture of R0 illite-smectite (I-S) and R1 I-S coexist. Each of these phases in all samples consists of the same or similar content of illite and expandable layers independent on location in the bed. In particular, the illite content in the R0 I-S and the R1 I-S from the <0.5 μm fractions is equal to 30 and 62%, respectively. The main difference between the samples at different locations is the different weight concentrations of the coexisting I-S phases. The R1 I-S content decreases progressively from the lower and upper contacts of the bed to its center. The reverse trend was observed for the R0 I-S. The layer unit-cell parameter b increases from samples located near the middle of the bed toward samples near the bed margins. The DTG patterns of the samples contain two endothermic maxima at about 640 and 470 °C, corresponding to cis -vacant ( cv ) illite and trans -vacant ( tv ) smectite layers coexisting in the R1 I-S and R0 I-S. Analysis of the crystal-chemical features of the R1 I-S and R0 I-S shows that, in the middle of the bed, both phases are characterized by the lowest octahedral Mg and the highest tetrahedral Al contents. In the structural formula of the R1 I-S, the tetrahedral Al content is significantly higher than the (K+Na) content independent of sample location. In contrast, tetrahedral Al in the R0 I-S located near the bed boundaries is lower compared with (K+Na) content. To account for the crystal-chemical features of the coexisting I-S, a first assumption is that the initial volcanic ash was altered into tv smectite having a homogeneous Al-rich composition throughout the bed. Second, along with K, the active role in Illitization was controlled by Mg. Mineralogical zonation of the K-bentonite is explained by the progressive migration of K from the margins toward the bed center with the associated decrease of K cations in the pore fluids. However, the decrease in K concentration was accompanied by a successive increase in R0 I-S content, but not a progressive decrease in illite layer content in a single I-S phase. The main role of Mg was to redistribute octahedral and tetrahedral Al in the 2:1 layers of the R0 I-S and R1 I-S in such a way that the amount of Al in the tetrahedral sheets increased at the expense of the substitution of Mg for Al in the octahedral sheet of the 2:1 layers in the initial smectite. These results demonstrate a new insight into mineralogical sequences of intermediate members of smectite Illitization. Instead of a statistically homogeneous and continuous reaction associated with the increase of illite layers in I-S and the simultaneous increase of order of the layer stacking sequence, the Illitization reaction in the thick K-bentonite consists of the formation of a physical mixture of two I-S having a contrasting content of layer types and their distribution. Factors responsible for the formation of the coexisting R0 I-S and R1 I-S are discussed.
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Diagenetic smectite-to-illite transition in clay-rich sediments: A reappraisal of X-ray diffraction results using the multi-specimen method
American Journal of Science, 2009Co-Authors: Bruno Lanson, Boris A. Sakharov, Francis Claret, Victor A. DritsAbstract:Smectite Illitization is a common mineralogical reaction occurring during the burial diagenesis of clay-rich sediments and shales, and has thus attracted sustained interest over the last fifty years. Prior studies have concluded that smectite Illitization proceeds through a steady set of homogeneous reactions involving intermediate mixed layers of varying compositions. In these intermediate structures, illite and smectite or, more generally, expandable layers (I and Exp layers, respectively) coexist among the same crystallites giving rise to non-periodic structures (I-Exp) characterized by specific diffraction effects. Consistent with this model, reaction progress was characterized by the simultaneous increase in the illite content in I-Exp and in their stacking order leading to the following mineralogical sequence: smectite → randomly interstratified I-Exp with high smectite contents (> 50% Exp layers) → ordered I-Exp with high illite contents (> 50% I layers) → illite. Although reaction mechanisms have been extensively debated, this structural characterization has not been challenged, possibly due to a methodological bias. In the present study, X-ray diffraction patterns typical of the diagenetic Illitization of smectite are interpreted using modern approaches involving profile fitting (multi-specimen method). Novel insights into the structure of intermediate reaction products are thus obtained. In particular, original clay parageneses are described that include the systematic presence of illite, kaolinite, chlorite and a mixed layer containing kaolinite and expandable layers (K-Exp). In contrast to previous descriptions, the early stages of smectite Illitization are characterized by the coexistence of discrete smectite and of a randomly interstratified I-Exp with a high content of illite layers (>50% I layers). Both the smectite and the I-Exp are authigenic and form under shallow burial, that is at low temperature conditions. With increasing burial depth, the relative proportion of I-Exp increases, essentially at the expense of discrete smectite, and the composition of I-Exp becomes slightly more illitic. In the second stage of smectite Illitization, two illite-containing mixed layers are observed. They result from two parallel reaction mechanisms affecting the randomly interstratified I-Exp present in the shallow section of the series. The first reaction implies the dissolution of this randomly interstratified I-Exp and leads to the crystallization of an ordered I-Exp without significant Illitization, possibly because of the low K-availability. The second reaction affecting the randomly interstratified I-Exp implies the growth of trioctahedral (Mg, Al) hydroxide sheets in Exp interlayers, thus developing di-trioctahedral chlorite layers (Ch layers) in the initial I-Exp to form an I-Exp-Ch. A layer-by-layer mechanism is hypothesized for this reaction. In this scheme, Mg cations released by the dissolution-recrystallization reaction of I-Exp likely represent the source of Mg for the formation of brucite-like sheets in expandable interlayers, and thus of the I-Exp-Ch. The reported structural characterization of smectite Illitization intermediate products contradicts the conventional wisdom of a homogeneous reaction through a series of pure mixed layers of variable composition. In contrast, the coexistence of different phases implies a heterogeneous reaction via a sequence of intermediate phases and requires reassessing the reaction mechanisms proposed in the literature. The compositional range (relative proportion of the different layer types) of these phases is limited and smectite Illitization proceeds essentially as relative proportions of the different phases vary. In addition, reaction kinetics and stability of the different intermediate products also need to be reconsidered.
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Diagenetic smectite-to-illite transition in clay-rich sediments: A reappraisal of X-ray diffraction results using the multi-specimen method
American journal of science, 2009Co-Authors: Bruno Lanson, Boris A. Sakharov, Francis Claret, Victor A. DritsAbstract:Smectite Illitization is a common mineralogical reaction occurring during the burial diagenesis of clay-rich sediments and shales, and has thus attracted sustained interest over the last fifty years. Prior studies have concluded that smectite Illitization proceeds through a steady set of homogeneous reactions involving intermediate mixed layers of varying compositions. In these intermediate structures, illite and smectite, or, more generally, expandable layers (I and Exp layers, respectively) coexist among the same crystallites giving rise to non-periodic structures (I-Exp) characterized by specific diffraction effects. Consistent with this model, reaction progress was characterized by the simultaneous increase in the illite content in I-Exp and in their stacking order leading to the following mineralogical sequence: smectite → randomly interstratified I-Exp with high smectite contents (> 50% Exp layers) → ordered I-Exp with high illite contents (> 50% I layers) → illite. Although reaction mechanisms have been extensively debated, this structural characterization has not been challenged, possibly due to a methodological bias. In the present study, X-ray diffraction patterns typical of the diagenetic Illitization of smectite are interpreted using modern approaches involving profile fitting (multi-specimen method). Novel insights into the structure of intermediate reaction products are thus obtained. In particular, original clay parageneses are described including the systematic presence of illite, kaolinite, chlorite and a mixed layer containing kaolinite and expandable layers (K-Exp). In contrast to previous descriptions, the early stages of smectite Illitization are characterized by the coexistence of discrete smectite and of a randomly interstratified I-Exp with a high content of illite layers (>50% I layers). Both the smectite and the I-Exp are authigenic and form under shallow burial, that is at low temperature conditions. With increasing burial depth, the relative proportion of I-Exp increases, essentially at the expense of discrete smectite, and the composition of I-Exp becomes slightly more illitic. In the second stage of smectite Illitization, two illite-containing mixed layers are observed. They result from two parallel reaction mechanisms affecting the randomly interstratified I-Exp present in the shallow section of the series. The first reaction implies the dissolution of this randomly interstratified I-Exp and leads to the crystallization of an ordered I-Exp without significant Illitization, possibly because of the low K-availability. The second reaction affecting the randomly interstratified I-Exp implies the growth of trioctahedral (Mg, Al) hydroxide sheets in Exp interlayers, thus developing di-trioctahedral chlorite layers (Ch layers) in the initial I-Exp to form an I-Exp-Ch. A layer-by-layer mechanism is hypothesized for this reaction. In this scheme, Mg cations released by the dissolution-recrystallization reaction of I-Exp likely represent the source of Mg for the formation of brucite-like sheets in expandable interlayers, and thus of the I-Exp-Ch. The reported structural characterization of smectite Illitization intermediate products contradicts the conventional wisdom of a homogeneous reaction through a series of pure mixed layers of variable composition. In contrast, the coexistence of different phases implies a heterogeneous reaction via a sequence of intermediate phases and requires reassessing the reaction mechanisms proposed in the literature. The compositional range (relative proportion of the different layer types) of these phases is limited and smectite Illitization proceeds essentially as relative proportions of the different phases vary. In addition, reaction kinetics and stability of the different intermediate products also need to be reconsidered.
François Gauthier-lafaye - One of the best experts on this subject based on the ideXlab platform.
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From detrital heritage to diagenetic transformations, the message of clay minerals contained within shales of the Palaeoproterozoic Francevillian basin (Gabon)
Precambrian Research, 2014Co-Authors: Lauriss Ngombi-pemba, Alain Meunier, Abderrazak El Albani, Olivier Grauby, François Gauthier-lafayeAbstract:Unmetamorphosed and undeformed marine siliciclastics rocks of the FB, FC and FD of the Francevillian series (Gabon) were deposited in an epicontinental basin. Clay minerals found in black shale, siltstone and sandstone are dominantly illite and chlorite except in two levels of the FB formation, which contain smectite-rich randomly ordered mixed layers. Their survival in a 2.1 Ga old sedimentary series is not related to the abundance of organic matter (total organic carbon or TOC), nor redox conditions at the time of deposition as indicated by the Fe speciation (FeHR/FeT and FePy/FeHR ratios). Rather it results from an incomplete Illitization reaction that reflects potassium deficiency. The K2O/Al2O3 ratio of shale, siltstone and sandstone vary along the series, and appear to conserve the signature of the original chemical composition of the rocks. K-feldspars which are present in the FC and FD formations are missing in the FB formation. Consequently, the smectite layers do not appear to be inherited from a detrital input in the basin but must be considered as representative of an intermediate stage of the Illitization reaction reached during diagenesis.
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2.9 to 1.9 Ga paleoalterations ofArchean granitic basement of the Franceville basin (Gabon)
Journal of African Earth Sciences, 2014Co-Authors: Idalina Moubiya Mouélé, Alain Meunier, Patrick Dudoignon, Abderrazak El Albani, Philippe Boulvais, François Gauthier-lafaye, Jean-louis Paquette, Hervé Martin, Michel CuneyAbstract:The Archean granitoids in the Kiene area, Gabon, are overlained by the Paleoproterozoic sediments of the Franceville basin (2.1 Ga). The basin is known for its high-grade uranium deposits among which some have been forming natural nuclear fission reactors. Most of the studies were dedicated to the FA-FB Paleoproterozoic sediments hosting these uranium deposits. Little is known on the Archean basement itself and specifically on the hydrous alteration events it experienced before and after the sediment deposition. The present work is focused on their petrographical, mineralogical and geochemical characterization. Dating the successive alteration events has been attempted on altered monazite crystals. Rocks in different alteration states have been sampled from eight drill cores crosscutting the Archean - Paleoproterozoic unconformity. The Archean granitoids observed in the deepest levels exhibit typical petrographical features of a propylitic alteration while they are intensely illitized up to the unconformity. The propylitic alteration is mainly pervasive but the original texture of the granitoids is conserved in spite of the formation of new minerals: Mg-chlorite, allanite and epidote forming a typical paragenesis. The illitic alteration is much more invasive near the unconformity. The Illitization process leads to the replacement of feldspars and the corrosion of quartz crysals by an illitic matrix while the ferromagnesian minerals are pseudomorphosed by a Fe-chlorite + phengite + hematite assemblage. The final fluid-rock interaction step is marked by fissural deposits of calcite and anhydrite. The δ13C isotopic data show that the fissural carbonates precipitated from diagenetic fluids enriched carbon products deriving from the maturation of organic matter. The U-Pb isotopic analyzes performed on monazite crystals have dated three distinct events: 3.0-2.9 Ga (magmatic), 2.6 Ga (propylitic alteration) and 1.9 Ga (diagenetic Illitization). The calculation of geochemical mass balances suggests that the water-rock ratio during the propylitic alteration event was weak. On the contrary, it was much higher during the overprinted Illitization which is characterized by an intense leaching of Na, Ca, Mg, Sr, REE and an enrichment in K, Rb,Cs. Neither the petrographic features nor the geochemical data militate for an Archean weathering event (paleosol). In the present case, diagenetic fluids have percolated from the unconformity into the basement where they overprinted the Illitization processes upon the previously propylitized rocks. These fluids were probably oxidant as they are also responsible of the U mobilization which led to the formation of the ore deposits close to the FA-FB interface.
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2.9 to 1.9 Ga paleoalterations ofArchean granitic basement of the Franceville basin (Gabon)
Journal of African Earth Sciences, 2014Co-Authors: Idalina Moubiya Mouélé, Abderrazak El Albani, Alain Meunier, Patrick Dudoignon, Philippe Boulvais, François Gauthier-lafaye, Jean-louis Paquette, Hervé Martin, Michel CuneyAbstract:The Archean granitoids in the Kiéné area, Gabon, are overlained by the Paleoproterozoic sediments of the Franceville basin (2.1 Ga). The basin is known for its high-grade uranium deposits among which some have been forming natural nuclear fission reactors. Most of the studies were dedicated to the FA-FB Paleoproterozoic sediments hosting these uranium deposits. Little is known on the Archean basement itself and specifically on the hydrous alteration events it experienced before and after the sediment deposition. The present work is focused on their petrographical, mineralogical and geochemical characterization. Dating the successive alteration events has been attempted on altered monazite crystals. Rocks in different alteration states have been sampled from eight drill cores crosscutting the Archean - Paleoproterozoic unconformity. The Archean granitoids observed in the deepest levels exhibit typical petrographical features of a propylitic alteration while they are intensely illitized up to the unconformity. The propylitic alteration is mainly pervasive but the original texture of the granitoïds is conserved in spite of the formation of new minerals: Mg-chlorite, allanite and epidote forming a typical paragenesis. The illitic alteration is much more invasive near the unconformity. The Illitization process leads to the replacement of feldspars and the corrosion of quartz crysals by an illitic matrix while the ferromagnesian minerals are pseudomorphosed by a Fe-chlorite + phengite + hematite assemblage. The final fluid-rock interaction step is marked by fissural deposits of calcite and anhydrite. The δ13C isotopic data show that the fissural carbonates precipitated from diagenetic fluids enriched carbon products deriving from the maturation of organic matter. The U-Pb isotopic analyzes performed on monazite crystals have dated three distinct events: 3.0-2.9 Ga (magmatic), 2.6 Ga (propylitic alteration) and 1.9 Ga (diagenetic Illitization). The calculation of geochemical mass balances suggests that the water-rock ratio during the propylitic alteration event was weak. On the contrary, it was much higher during the overprinted Illitization which is characterized by an intense leaching of Na, Ca, Mg, Sr, REE and an enrichment in K, Rb,Cs. Neither the petrographic features nor the geochemical data militate for an Archean weathering event (paleosol). In the present case, diagenetic fluids have percolated from the unconformity into the basement where they overprinted the Illitization processes upon the previously propylitized rocks. These fluids were probably oxidant as they are also responsible of the U mobilization which led to the formation of the ore deposits close to the FA-FB interface.
Jingong Cai - One of the best experts on this subject based on the ideXlab platform.
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variations and geological significance of solid acidity during smectite Illitization
Applied Clay Science, 2021Co-Authors: Jingong Cai, Mingshui Song, Qian Chao, Xuejun WangAbstract:Abstract Solid acidity is an inherent property of clay minerals and intimately related to the crystal chemistry and structure. Smectite Illitization is a common phenomenon in mudstones, during which the variations of solid acidity are our concern. In present study, we prepared samples with different Illitization extent and conducted a series of measurements, aiming to figure out the evolution of solid acidity during mineral transformation and draw some inspirations for the impacts of solid acidity on hydrocarbon generation approach and mechanism during Illitization. In the early stage of Illitization from T-25 to T-250, the morphology is irregular and clay minerals are composed of randomly interstratified mixed-layered mineral (MLM) with total illite layer less than 54%. Bronsted (B) acid sites increase slightly even if smectite layer reduces. The slight reduction of interlayer water and increase of interlayer charge are considered to improve the proton-donating ability of interlayer water. Meanwhile, H3O+ adsorbed by the net structural negative charge also contributes to the increase of B acid sites. For Lewis (L) acid sites, the reduction of octahedral Al, which is caused by incomplete crystallization of illite layers as proved by amorphous particles consisting of Si and Al, controls the decline of L acid sites in early stage. In the late stage from T-300 to T-350, R1 and R2 ordered MLMs are predominant with around 80% total illite layers and regular morphology. B acid sites decrease progressively while L acid sites increase at first and then decrease. The match of reductions of interlayer water and B acid sites indicates the significant reduction of smectite layer turns into the main factor controlling the reduction of B acid sites. Meanwhile, the improvement of the ordering, dramatic increase of total illite layer, and absence of amorphous particles render L acid sites increase. However, further increased crystallinity makes Al bond with hydroxyl groups and thus the decrease of L acid sites. T-450 represents another scenario in which decrease of clay minerals and formation of quartz dilute both B and L acid sites. In summary, the amount, strength, and type of solid acidity are varying constantly in the process of Illitization. Random MLMs are favorable for B acid sites while ordered MLMs prefer L acid sites. The acid strength peaks when random MLMs are dominated. B and L acid sites could promote clay-organic matter (OM) interactions via different mechanisms, in this way, variation of solid acidity determines priority of two reaction mechanisms of clay-OM interactions and the composition of the hydrocarbons. In conclusion, the variation of solid acidity during Illitization connects the mineral transformation and hydrocarbon generation, affecting the mechanisms of organic-inorganic interactions during hydrocarbon generation and providing implications for further investigation of the organic-inorganic interactions in nature
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A contrastive study of effects of different organic matter on the smectite Illitization in hydrothermal experiments
Applied Clay Science, 2019Co-Authors: Jingong Cai, Zewen Chen, Tianzhu Lei, Shoupeng Zhang, Xie ZhonghuaiAbstract:Abstract Although influence of organic matter (OM) on smectite Illitization has received considerable attention in previous studies, the effect of different OM types on smectite Illitization mechanism has rarely been examined. In the present study, hydrothermal experiments were conducted with N,N-dimethylhexadecylamine (16DMA)-smectite (M2) and lysine-smectite (M3) complexes to explore the effect of OM types on smectite Illitization mechanisms. X-ray diffraction (XRD) analysis showed that mineral contents, illite percentage in mixed-layer illite-smectite (I% in I Sm), the average number of layers (Nave), and stacking mode of I Sm changed weakly at 300 °C. The changes varied obviously between M2 and M3 at >300 °C, indicating characteristics of solid-state transformation Illitization mechanism in M2 and dissolution crystallization mechanism in M3. Moreover, the mid-infrared and thermo-XRD analyses indicated that the OM was mainly adsorbed in smectite interlayers at
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Hydrothermal experiments reveal the influence of organic matter on smectite Illitization
Clays and Clay Minerals, 2018Co-Authors: Jingong Cai, Zewen Chen, Tianzhu Lei, Xiaojun ZhuAbstract:Smectite Illitization is an important diagenetic phenomenon of mudstones, but only rarely has the influence of organic matter (OM) on this process been examined. In the present study, hydrothermal experiments were conducted with smectite (M1, total organic carbon (TOC) 1%). X-ray diffraction (XRD), infrared, X-ray fluorescence (XRF), and organic carbon analyses were employed to characterize the mineralogy and OM of the samples and the effect of OM on smectite Illitization. The XRD patterns showed changes in clay mineral parameters with increased temperature. These changes varied in both M1 and M2 and indicated a difference in the degree of smectite Illitization. Moreover, the OM in M2 was mainly adsorbed in smectite interlayers, the OM was largely desorbed/decomposed at temperatures above 350°C, and the OM was the main reason for differences in the degree of smectite Illitization between M1 and M2. Bulk mineral composition, elemental content, and infrared absorption band intensities were changed with increased temperature (especially above 350°C). This indicated the formation of new minerals (e.g., ankerite). Overall, OM entered the interlayer space of smectite in M2 and delayed the exchange of K+ by interlayer cations, and thus, suppressed the transformation of smectite to illite and resulted in differences in smectite Illitization of M1 and M2. In particular, the formation of CO2 after the decomposition of OM at temperatures above 300°C led to the formation of ankerite in M2. This demonstrated the effect of organic-inorganic interactions on smectite Illitization and mineral formation. The disparities in smectite Illitization between M1 andM2, therefore, were linked to differences in the mineral formation mechanisms of a water-rock system (M1) and a water-rock-OM system (M2) in natural environments. The insights obtained in the present study should be of high importance in understanding organic-mineral interactions, hydrocarbon generation, and the carbon cycle.
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Smectite-Illitization difference of source rocks developed in saline and fresh water environments and its influence on hydrocarbon generation: A study from the Shahejie Formation, Dongying Depression, China
Marine and Petroleum Geology, 2017Co-Authors: Jingong Cai, Xuejun Wang, Yunqing Hao, Qing LiuAbstract:Abstract Samples of argillaceous source rocks from three sub-members of the Shahejie Formation (Es) in the Dongying Depression, China, were collected to investigate the differences in hydrocarbon generation among the sub-members, which developed in fresh (Es32, Es33) and saline (Es41) water environments. Pyrolysis, XRD and thermo-XRD analyses were used to compare the characteristics of organic matter (OM), clay minerals and OM occurrences. Total organic carbon and hydrocarbon potential proxies suggest that the samples from Es33 were much better than the other two intervals, which agrees with previous studies. The characteristics of clay minerals suggest that the samples from Es41 have the most abundant illite, with a maximum illite percentage in mixed-layer illite-smectite (I Sm), and the best crystallinity of I Sm with a main stacking mode of R1.5. However, the stacking modes of I Sm in Es32 and Es33 were primarily R0 and R1, respectively, and the crystallinity was relatively poor. Thus, the smectite Illitization process was faster in Es41 than in the other two intervals, and a saline environment was a primary cause for the acceleration of the process. Moreover, OM occurrence indicates that the samples from Es41 had the lowest amount of interlayer OM, whereas Es33 had the largest amount. Therefore, the rapid Illitization in Es41 caused abundant interlayer OM to be desorbed and discharged, which in turn caused the amount of residual interlayer OM in Es41 to be less than that in the other two intervals. Thus, the source rocks of Es41 made a more significant contribution to hydrocarbon generation than those of the other two units. In conclusion, the inconsistent Illitization among these intervals was a major cause of the differences in hydrocarbon generation.
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influence of organic matter on smectite Illitization a comparison between red and dark mudstones from the dongying depression china
American Mineralogist, 2016Co-Authors: Jingong Cai, Mingshui Song, Yujin BaoAbstract:Interactions between organic matter (OM) and clay minerals have received considerable attention in previous studies. The influence of OM on smectite Illitization has been analyzed primarily in simulation experiments rather than in diagenetic studies. The present study explores the influence of OM on smectite Illitization during diagenesis. Thirty red and dark mudstone samples from the Dongying Depression were analyzed. X-ray diffraction (XRD) analyses revealed that the illite percentages in mixed-layer illite-smectite (I-S) of both types of samples were dispersive above 3100 m and more convergent below this depth. The stacking mode of I-S in dark mudstones above 3100 m remained primarily at R0–R0.5 ordering with the average number of layers (Nave) dispersively distributed between 2 and 4.5. In red mudstones, the I-S changed from the R0 to R0.5 mode with the Nave increasing from 2 to 5. Over this range, the smectite Illitization in dark mudstones was slower than that in red mudstones. Below 3100 m, the I-S stacking mode of dark mudstones changed from R0.5 to R3 ordering with the Nave increasing sharply from 4 to 8. In red mudstones, the I-S displayed R1.5 and R3 ordering with the Nave varying between 4.5 and 6.5. Over this range, the smectite Illitization in dark mudstones accelerated rapidly, whereas the process in red mudstones was retarded. Additionally, the red mudstone samples contained little OM, whereas the dark mudstone samples contained abundant total organic carbon (0.17–4.43%). Thermo-XRD, near-infrared (NIR) as well as mid-infrared (MIR) spectroscopy analyses suggested that the OM in dark mudstones exhibited a significant transition at 3100 m, coincident with the Illitization change. Above 3100 m, the smectite Illitization in dark mudstones was delayed due to the OM pillar effect in the interlayer spaces of smectite. Below 3100 m, the interlayer OM became varied and desorbed, discharging organic acid. This led to the dissolution of smectite structural layers. Consequently, Illitization in the dark mudstone was accelerated. This study revealed that the existence and occurrence of OM could influence the smectite Illitization in diagenesis. Further study on the interactions between OM and clay minerals is needed to facilitate our understanding on the mechanism of smectite Illitization as well as its geological applications.
