Variations In Clay Mineralogy Research Articles

Effects of the Types and Amounts of Clay Minerals on Durability of Lime-Stabilized Clay Soils

Although the interaction between clay minerals and lime is the most effective factor in lime stabilization techniques, it has not been deeply evaluated. This research study investigated the microstructural characteristics of lime-stabilized Bentonite and Kaolin soils using X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive X-ray (EDX) analyses. To consider the variation in clay mineralogy, these soils were mixed at varied ratios, stabilized, and then subjected to a durability process. The microstructural findings showed that the pozzolanic reactions with lime did not occur or occurred at a low level for Bentonite soil. However, they occurred at a very high level for Kaolin soil. The durability test confirmed the microstructural results and showed that the samples in which Bentonite soil had a share of 40 to 100% by dry weight of the soil did not last with any percent of lime. When the Kaolin soil content reached 100% by dry weight of the soil, the specimens lasted in the best possible way, even with 4% of the lime. This study concluded that the determination of optimum lime content based on the amounts and types of soil clay minerals is an important innovation for geotechnical projects and may be very cost-effective.

Differential effects of clay mineralogy on thermal maturation of sedimentary n-alkanes

Clay minerals serve as important catalysts for the degradation of a range of biomolecules during diagenesis, yet their effects are seldom considered when interpreting organic molecular and isotopic data in the context of paleoenvironmental reconstruction. We investigated the thermal maturation of immature sedimentary organic matter in the presence of two clay minerals (kaolinite and montmorillonite) using a series of laboratory-based anhydrous pyrolysis experiments. Organic matter was extracted from a modern terrestrial sediment, mixed with each clay mineral, and heated at 100–300 °C for up to 30 days – in this way, initial organic matter was identical in each system. Abundances, molecular distributions, and paired compound-specific stable isotopic compositions (δDn-alkane and δ13Cn-alkane) of n-alkanes were measured as a function of heating time. For both kaolinite and montmorillonite amended experiments, the shifts in molecular distributions are highly controlled by significant secondary production of n-alkanes from molecularly large lipid precursors, as evidenced by significant increases in the total amount of n-alkanes accompanying increased thermal maturation. Significant decreases in carbon preference index (CPI) are observed before minor decreases in average chain length (ACL), leading to behavior in CPI-ACL space that is fundamentally different from identical systems free of clay minerals. Furthermore, distinct distributions of secondary n-alkanes between the clay mineral systems indicate differential mechanisms and sites of precursor degradation. In the presence of clay minerals, δ13Cn-alkane of immature bituminous organics initially decreases with increasing thermal maturation, opposite most data reported from experimental alteration of relatively mature sedimentary matter. This shift is similarly driven by secondary production of n-alkanes, arising from kinetic fractionation associated with preferential cleavage of 12C12C bonds from parent molecules – the kinetics of this process vary per clay mineralogy but are controlled by the same process. Conversely, we see an enrichment in δDn-alkane with increasing maturation, indicating control of the two isotopic systems by separate mechanisms. Moreover, kaolinite and montmorillonite exhibit differential control on δDn-alkane, leading to separate domains in dual isotopic space. Overall, the changes in δ13Cn-alkane and δDn-alkane are relatively small over the timescales studied, even at temperatures as high as 300 °C (less than ∼0.6–1‰ for δ13Cn-alkane and ∼8–15‰ for δDn-alkane). Considering these factors on a geological timescale, variations in clay mineralogy through a buried sedimentary sequence may impart matrix effects on molecular and isotopic signatures that can bias interpretation of paleoenvironmental conditions.

X-ray fluorescence (XRF) fingerprinting of Palaeogene deposits in Denmark

In this study, we test if cost-efficient X-ray fluorescence (XRF) analyses can be used to fingerprint Palaeogene clay and marl deposits in Denmark. A total of 67 samples from key sites in Denmark have been analysed. Our preliminary results indicate that it is possible locally within 10–30 km to distinguish between most of the Palaeogene units, but on a regional scale across Denmark, the units are not unique, and this probably reflects variations in clay mineralogy, grain size and calcareous content. Accordingly, we suggest that a comprehensive reference database is now needed if the full potential of the method is to be utilised, and this will ultimately result in more reliable geological models.

Experimental Study on the Impact of Clay Distribution and Mineralogy on NMR T2 and Internal Gradient: Insights From Well-Controlled Physical Samples

Summary Nuclear magnetic resonance (NMR) is a reliable tool for petrophysical evaluation and the characterization of pore structures. Compared to conventional carbonate reservoirs, sandstone reservoirs contain higher amounts of ferromagnetic and paramagnetic ions (such as iron, nickel, or manganese) usually found in microporous clay aggregates. The interpretation of petrophysical data in sandstone formations can be complicated by variations in clay mineralogy and distribution patterns (laminated, structural, and dispersed). Nevertheless, the impact of clay distribution patterns on NMR signals is not well understood. This study aims to investigate the impact of clay mineralogy and distribution patterns on the T2 relaxation times and internal gradient (i.e., inhomogeneity in the magnetic field). Glass beads were mixed with three different clay minerals characterized by no iron content (kaolinite) to higher iron content (illite and nontronite). The bead-clay mixtures were prepared at a fixed clay content but with variable clay distribution patterns to examine the impact of clay distribution alone. NMR T2 measurements at several echo times were performed on the pure glass beads and the mixtures to evaluate how clay mineralogy and distribution patterns affect the T2 and the internal gradient of the host glass beads. At a given clay distribution pattern, a more significant decrease in T2 relaxation times and a larger increase in the internal gradient of glass beads were observed when adding clays with higher iron content. This is explained by the higher surface relaxivity, and magnetic susceptibility caused when introducing clay with higher iron content. Such an impact can complicate the characterization of NMR-derived pore sizes as similar pore size distribution (PSD) can have very different T2 distribution and the logarithmic mean of T2 relaxation time distribution (T2LM) values. Micro-computed tomography (µCT) images were acquired to compute the PSD to compare it with ones obtained from the NMR measurements. The PSD for the three clay minerals showed almost the same distribution using µCT; however, they showed totally different T2 relaxation times distributions. That is due to the significant impact of the internal gradient causing a distortion in the magnetic field. Thus, careful consideration must be taken before converting the NMR data into PSD. The introduction of iron-free kaolinite resulted in a negligible impact on the internal gradient of glass beads regardless of the clay distribution pattern. On the other hand, the addition of dispersed iron-rich clays (illite and nontronite) results in up to two orders of magnitude increase in internal gradients, compared to mixtures with laminated and structural clays (at the same clay mineralogy and content). Moreover, dispersed clay mixtures display larger changes in T2LM and porosity as a function of increasing echo time. The results from this study suggest that changes in T2LM and the logarithmic mean of the effective internal gradient distribution geff,LM, compared to the clean host sand, can provide insight into iron-rich clay distribution. Larger changes in any given clay content and mineralogy would suggest a more dominant dispersed clay distribution while negligible changes would suggest a laminated distribution.

Terrestrial record of cyclic early Eocene warm-humid events in clay mineral assemblages from the Salta basin, northwestern Argentina

The Eocene continental sequence investigated in this study belongs to the Salta Group; it was deposited in an intracontinental rift, the Salta Basin (NW Argentina) that evolved from the Lower Cretaceous to middle Paleogene. The Salta Group contains the Maíz Gordo and Lumbrera Fms, spans the Paleocene-early Eocene, and shows excellent exposures in the region of the Valles Calchaquíes. This research is focused on the continental facies of the Lumbrera Fm, which correspond to the early Eocene. We studied the mineralogy of the fine-grained beds of the Lumbrera Fm in five locations (Valle Encantado, Tonco, Tin Tin, Luracatao, and Obelisco) by X-ray diffraction and scanning electron microscopy in order to examine the vertical variations in clay mineralogy and their relations with global paleoclimatic changes registered during the Eocene. The microtexture of the authigenic smectite-type clays (Sm to I/Sm R0) suggests that they mainly originated by crystallization from glassy volcanic materials. The high reactivity of the glass precludes the use of smectite-type-clay formation as an indicator of specific paleoclimatic conditions in the studied sediments. In contrast, the formation of kaolinite in sections with very low smectite proportions and a strong degree of weathering in crystalline silicates reflects intense weathering conditions and is a useful paleoclimatic proxy in terrestrial sediments. Significant variations in kaolinite/muscovite ratios at the base and in an intermediate bed in the Lumbrera Fm at Valle Encantado suggest the presence of cyclic hyperthermals through the Ypresian stage that may be tentatively correlated with the Eocene Thermal Maxima 2 and 3, which are the largest events that have been identified at a global scale in early Eocene marine sediments.

Control of coal-bearing claystone composition by sea level and redox conditions: An example from the Upper Paleozoic of the Datong Basin, North China

In order to investigate the genesis of different types of claystones and facies-related variations in clay mineralogy, a mineralogical and geochemical investigation of the coal-bearing Upper Carboniferous Taiyuan Formation of North China was undertaken in the Wangping and Xiaoyu mines of the SE Datong Basin. This formation contains both kaolinitic and mixed-layer illite/smectite (I/S) claystones having distinctly different compositions. The former is composed of kaolinite with minor illite, quartz, calcite, and boehmite, whereas the latter consists mainly of R1 and R3 mixed-layer I/S clay and kaolinite with minor illite, siderite, quartz, dolomite, and anatase. The two types of claystone formed from the same parent materials, as shown by Al2O3/TiO2 ratios and rare earth element (REE) patterns. Paleosalinity indices (B/Ga and Sr/Ba) and redox proxies (Mo and U enrichment factors and Corg/P ratios) indicate that the kaolinitic claystones formed under freshwater, oxic-anoxic conditions, whereas the I/S claystones formed under brackish, oxic-suboxic conditions. Chemical index of alteration (CIA) values show that the kaolinitic claystones experienced more intense leaching than the I/S claystones, even though both formed in a warm and humid climate, and this result is consistent with late Carboniferous-early Permian global climate and weathering intensity. The kaolinite developed through recrystallization and alteration of terrigenous clastics and an aluminosilicate colloidal solution in a reducing, freshwater peaty swamp environment. The formation of smectite was linked to marine transgressions that altered the salinity and oxygen content of sedimentary porewaters, with subsequent deep-burial conversion of smectite to mixed-layer I/S. The frequent changes in salinity and redox conditions that caused mineralogical variations in Taiyuan Formation claystones were driven by glacio-eustatic fluctuations of the Late Paleozoic Ice Age (LPIA), which led to episodic marine incursions into the Datong Basin.

Past Hydrological Conditions in a Fluvial Valley: Records from C-O Isotope Signatures of Holocene Sediments in the Loire River (France)

Multi-proxy indices (grainsize distribution, mineralogy, δ18O, δ13C) in sediments from a meander infill in the Middle Loire alluvial plain of central France are used to highlight some aspects of the basin evolution over the period from 0 to 10,000 years BP. During the Late-Glacial and Holocene period, the lacustrine carbonate substratum of the alluvial plain was incised by the Loire River, creating numerous oxbows and channels related to meander migration. The channel fills consist mainly of fine clayey sediments deposited during flooding of the river, with an almost total absence of coarse-clastic and sandy material, except in the basal part. The record of isotope ratio variations together with the distribution of particle sizes allows the evolution of the river dynamics to be constrained. The strong decrease of carbonate δ13C in the upper part of the record is ascribed to a progressive closure of the meander and, thus, an increasing control of the C-isotope signature by biological activity in a local environment. Variations in carbonate δ18O, rather, reflect paleohydrological/paleoclimatic changes at the basin scale. The isotope record of the river dynamics also agrees with the variations in clay mineralogy.

Coupled measurements of hydraulic permeability and full stiffness tensor compaction trends in artificial shales

Understanding of compaction trends of elastic and hydraulic properties in anisotropic shales is crucial for exploration of energy resources, ecological disposal of nuclear waste, and hydrogeological applications. However, complexity of the natural shale mineralogy and lack of quality data available for analysis results in poor knowledge of these compaction trends. Careful control over the shale mineralogy, pore fluid composition and applied stresses allows us to simulate the natural environments and acquire quality data on the properties of artificial shales. Here for the first time we present methodology and describe a setup that allows simultaneous acquisition of all five independent elastic constants and extremely low hydraulic permeability values of transversely isotropic artificial shale samples during mechanical compaction experiments (porosity decrease from 40% to 10%). Hydraulic permeabilities of artificially compacted samples are comparable to the ones of natural shales. Permeability drops exponentially with compaction. Silt fraction and clay mineralogy are the two key parameters that are responsible for broad variations of permeability in shales with the same porosity. We provide analytical equations that allow calculating permeability if porosity and silt fraction are known. Elastic constants of clay matrix exhibit positive linear trends with the porosity decrease. Small variations in clay mineralogy have a minor effect on absolute values of elastic coefficients or anisotropy but lead to noticeable increase of the compressional (VP) to shear (VS) velocities ratio at the same porosity. Finally, strong correlations (R2 above 0.95) of the hydraulic permeability with acoustic impedance and VP/VS ratio are observed for all the prepared samples.

Geochemical pattern recognition through matrix decomposition

This study compares the use of joint singular value decomposition and semi-discrete decomposition (JSS) and non-negative matrix factorisation with univariate analysis of raw data, to detect multi-element patterns in soils related to geochemical dispersion from Mississippi Valley-Type Pb-Zn deposits in the Irankuh area of central Iran. Joint singular value decomposition and semi-discrete decomposition clustering of the data identified a suite of mineralisation-related variables and corresponding sample clusters that are spatially associated with mineralisation or variations in parent lithology. Non-negative matrix factorisation generated three main factors and sample clusters that relate to the main zones of sulphide mineralisation, variations in clay mineralogy and the composition of unaltered host rocks. These two matrix decomposition techniques deliver similar results, though JSS delivers a significantly higher sample classification accuracy. Both methods result in delineation of a contiguous cluster of samples above an extensive zone of blind mineralisation at the Tappe Sorkh Pb-Zn deposit and define new potentially mineralised targets between this deposit and the Gushfil Pb-Zn deposit. More conventional univariate methods to detect anomalous populations of Pb and Zn in the soil geochemical data, such as number-size fractal analysis, were less effective at defining these targets. Joint application of practical matrix decomposition techniques, utilising noise removal and modelling of the underlying geochemical patterns associated with ore-forming processes, has led to more reliable prospectivity mapping of the Pb-Zn deposits in Irankuh.

Human impact overwhelms long-term climate control of weathering and erosion in southwest China

During the Holocene there has been a gradual increase in the influence of humans on Earth systems. High-resolution sedimentary records can help us to assess how erosion and weathering have evolved in response to recent climatic and anthropogenic disturbances. Here we present data from a high-resolution (similar to 75 cm/k.y.) sedimentary archive from the South China Sea. Provenance data indicate that the sediment was derived from the Red River, and can be used to reconstruct the erosion and/or weathering history in this river basin. Accelerator mass spectrometry C-14 dating provides direct age control and reveals coherent variations in clay mineralogy, geochemistry, and terrigenous flux, indicative of strong chemical weathering and physical erosion during the mid-Holocene warm period (6400-4000 cal [calibrated] yr B.P.), followed by weakening from ca. 4000-1800 cal yr B.P., and renewed intensification since 1800 cal yr B.P.. Comparison with climatic records from China indicates that precipitation and temperature controlled both physical erosion and chemical weathering intensity before 1800 cal yr B.P.. However, weathering proxies in the offshore sediment indicate recent increased soil erosion. We suggest that enhanced human activity (deforestation, cultivation, and mining) since the end of the Chinese Han Dynasty (220 CE) has overwhelmed the natural climatic controls on erosion in the Red River.

Influence of organic matter, clay mineralogy, and pH on the effects of CROSS on soil structure is related to the zeta potential of the dispersed clay

The high proportion of adsorbed monovalent cations in soils in relation to divalent cations affects soil structural stability in salt-affected soils. Cationic effects on soil structure depend on the ionic strength of the soil solution. The relationships between CROSS (cation ratio of soil structural stability) and the threshold electrolyte concentration (TEC) required for the prevention of soil structural problems vary widely for individual soils even within a soil class, usually attributed to variations in clay mineralogy, organic matter, and pH. The objective of the present study was to test the hypothesis that clay dispersion influenced by CROSS values depends on the unique association of soil components, including clay and organic matter, in each soil affecting the net charge available for clay–water interactions. Experiments using four soils differing in clay mineralogy and organic carbon showed that clay dispersion at comparable CROSS values depended on the net charge (measured as negative zeta potential) of dispersed clays rather than the charge attributed to the clay mineralogy and/or organic matter. The effect of pH on clay dispersion was also dependent on its influence on the net charge. Treating the soils with NaOH dissolved the organic carbon and increased the pH, thereby increasing the negative zeta potential and, hence, clay dispersion. Treatment with calgon (sodium hexametaphosphate) did not dissolve organic carbon significantly or increase the pH. However, the attachment of hexametaphosphate with six charges on each molecule greatly increased the negative zeta potential and clay dispersion. A high correlation (R2 = 0.72) was obtained between the relative clay content and relative zeta potential of all soils with different treatments, confirming the hypothesis that clay dispersion due to adsorbed cations depends on the net charge available for clay–water interactions. The distinctive way in which clay minerals and organic matter are associated and the changes in soil chemistry affecting the net charge cause the CROSS–TEC relationship to be unique for each soil.

Influence of lixiviation by irrigation water on residual shear strength of weathered red mudstone in Northwest China: Implication for its role in landslides' reactivation

Irrigation in loess tableland areas of northwest China has induced a large number of landslides around their edges, of which loess–red mudstone landslide is one of the most frequent types. Salt sinters commonly observed at the toes of these landslides, where the weathered red mudstone (WRM) outcrops, have raised the possibility that removal of soluble salts in the WRM by irrigation water could be crucial in the reduction of its shear strength, and thus in the occurrence of these landslides. In order to investigate the effects of lixiviation by irrigation water on residual shear strength of the WRM, samples of the WRM close to two landslides in Lanzhou city, one of the largest cities in northwest China, were tested after being treated under different conditions. These included: (1) saturation with the Yellow River water, the only source of irrigation, (2) saturation with groundwater, and (3) saturation and leaching with deionized water. Based on the results of these tests, specific mechanisms of residual shear strength reduction of the WRM due to lixiviation by irrigation water were discussed in terms of physico-chemical interactions between the WRM and water. It was found that lixiviation reduced residual shear strength and residual friction angle φr′ of the WRM by up to 65% and 62% respectively, after being leached 6 times. This coincided with a dramatic dissolution and leaching of soluble salts from the WRM. Particle size distribution showed that the more the WRM was leached, the more the clay fraction and the lesser coarser fractions. This suggested that reduction in residual shear strength of the WRM could be attributed to mainly disaggregation of particles coarser than clay, and to a lesser degree to weakening of interparticle forces among individual clays. It was further postulated that variation in clay mineralogy may influence performance of particle disaggregation.

Radionuclide Interaction with Clays in Dilute and Heavily Compacted Systems: A Critical Review

Given the unique properties of clays (i.e., low permeability and high ion sorption/exchange capacity), clays or clay formations have been proposed either as an engineered material or as a geologic medium for nuclear waste isolation and disposal. A credible evaluation of such disposal systems relies on the ability to predict the behavior of these materials under a wide range of thermal-hydrological-mechanical-chemical (THMc) conditions. Current model couplings between THM and chemical processes are simplistic and limited in scope. This review focuses on the uptake of radionuclides onto clay materials as controlled by mineral composition, structure, and texture (e.g., pore size distribution), and emphasizes the connections between sorption chemistry and mechanical compaction. Variable uptake behavior of an array of elements has been observed on various clays as a function of increasing compaction due to changes in pore size and structure, hydration energy, and overlapping electric double layers. The causes for this variability are divided between "internal" (based on the fundamental structure and composition of the clay minerals) and "external" (caused by a force external to the clay). New techniques need to be developed to exploit known variations in clay mineralogy to separate internal from external effects.

Increased contribution of terrigenous supply from Taiwan to the northern South China Sea since 3 Ma

Seismic profiles provide evidence that there has been strong transport by deep-water bottom currents and drift deposition on the northern slope of the South China Sea. Earlier geochemical studies suggest that the drift sediments originated primarily from Taiwan. However, the transport process, history and origin of the deep-water bottom deposition in the northern South China Sea, on both glacial–interglacial and tectonic time scales, remain unclear. Here, we show new high-resolution records of clay minerals, grain size and mass accumulation rate (MAR) of terrigenous materials from Ocean Drilling Program (ODP) Site 1144, together with trace element concentrations in siliciclastic sediments from ODP Site 1146. Combined with other published data, we find that the primary source for sediments at ODP Sites 1144–1148 since 3 Ma is from Taiwan, and not from Pearl River as previously thought. Before 3 Ma, however, sediment source to ODP Sites 1146 and 1148 was mainly from the Pearl River. Increased contribution of terrigenous supply from Taiwan to the northern South China Sea since ~ 3 Ma may be related to the formation of the Taiwan orogen and strengthening of deep-water bottom current transport in the northern South China Sea. Variations in clay mineralogy and sedimentology at ODP Site 1144, located on a sediment drift, shows strong glacial–interglacial cyclicity. This suggests that bottom current deposition is highly dependent on sea-level fluctuations, which control the terrigenous supply to the deep sea.

Clay Mineralogy and Zonation in the Campana Mahuida Porphyry Cu Deposit, Neuquen, Argentina: Implications for Porphyry Cu Exploration

Analysis of clay mineralogy and illite crystallinity in the clay-size fraction (<2 μm) of 102 whole-rock samples from 33 drill cores of the Campana Mahuida porphyry copper deposit has identified (1) a central chlorite-rich zone superimposed on the potassic core, (2) a peripheral chlorite-rich zone in the outermost propylitic halo, (3) an illite-rich zone within the south and southwestern quadrangles of the phyllic halo that has overprinted the outer potassic core and inner propylitic zone, and (4) a smectite-rich zone within the northeast and northwest quadrangles of the phyllic halo. Lateral and vertical variations in clay mineralogy and illite crystallinity are attributed to alteration and mineralization such that surficial or remotely sensed mineralogy can be used to see through superimposed supergene alteration. In the chlorite zones, the chlorite octahedral cations vary between 11.62 and 11.99 per O20(OH)16 formula unit and the tetrahedral cation composition is between (Si7Al1) and (Si4.6Al3.4) with an overall composition of (Al2.3–2.85Mg5.8–7.9Fe2+1.5–3.1)(Si5.5–5.9Al2.1–2.5)O20(OH)16. Thus, all are trioctahedral chlorites and, based on Fe2+/(Fe2+ + Mg) and Si contents, can be classified as Fe-Al clinochlore and Fe clinochlore. The chlorite zone in the potassic core formed mostly by destruction of hydrothermal biotite during sulfide precipitation as temperature decreased and f S2 increased. The chlorite contains more Mg than the biotite it replaces and has higher Fe2+/(Fe2+ + Mg) but lower Al2O3 than chlorite formed in the outer propylitic halo. The phyllic zone that surrounds the potassic core is dominated by illite (K1.25–1.84Na0.0–0.15)(Al3.24–3.98Fe0.01–0.35 Mg0.06–0.64)(Si6.04–6.78Al1.22–2.07)O20(OH)4. Illite from the centers and selvages of veins has a composition close to muscovite, whereas disseminated illite in altered protoliths contains more Mg and Fe cations in the octahedral sites, although all belong to the 2 M 1 polytype. This hypogene alteration resulted from fluids with lower pH, temperature, and salinity than fluids responsible for the potassic alteration, propylitic alteration, chlorite formation after biotite, and Cu mineralization. The K content of illite decreases outward from the potassic core and from early, disseminated illite to late crosscutting veinlets. A smectite-rich zone occurs in the northeast and northwest quadrangles of the phyllic halo. Based on X-ray diffraction analysis and a structural formula of (Ca,K,Na)0.18–1.2(Al2.8–3.8Fe0.11–0.78Mg0.1–1.04Mn0–0.01Ti0–0.07)(Si6.9–7.9 Al0.1–1.07)O20(OH4), the analyzed smectites are of the aluminum montmorillonite-beidellite dioctahedral series. Smectites from the deepest parts of the porphyry copper system, either from the potassic core or the phyllic halo, are montmorillonite-type smectite, with high total MgO and interlayer cation contents, and are probably hypogene, whereas shallow samples above the potassic core and phyllic halo contain beidellite-type smectite that has the highest FeOtotal and Al2O3 contents. This likely resulted from intense leaching at low temperatures as is typical of a supergene environment. Illite is widespread in the Campana Mahuida porphyry Cu deposit and in the unaltered Tordillo Formation that hosts the deposit. The Kubler index (illite crystallinity) in the clay-size fraction of fresh sandstone (0.7° 2𝛉) corresponds to the highest grade of diagenesis, whereas in the altered rocks in the uppermost level of the deposit, the Kubler index ranges from 0.5 to 0.2° 2𝛉, values that are indicative of high temperature and/or high fluid/rock ratios. Within the deposit, illite crystallinity increases from the outer limit of the phyllic zone toward the center of the deposit, above the potassic core, and with depth, from the leached cap to the hypogene sulfide copper zones. Contours of illite crystallinity from the leached cap roughly delineate the outer limits of the ore zones at depth.

Origin of the sedimentary deposits of the Naracoorte Caves, South Australia

The origin of the sediments located in the Naracoorte Caves (South Australia) was investigated via the analysis of strontium isotope ratios ( 87Sr/ 86Sr), elemental geochemistry, and mineralogy. Sedimentary deposits located in Robertson, Wet, Blanche and several other chambers in Victoria Cave are all variable mixes of fine sand and coarse silts, which display similar and consistent strontium isotope ratios (0.717–0.725). This suggests that over the 400 ka time frame that these deposits span there has been minimal variation in the source of the clastic sediments. Increased strontium concentrations for these cave sediments correspond with increasing silt content, yet there is no correlation between 87Sr/ 86Sr ratios and silt content. This implies that the silt-sized component of the sediments is the main contributor of strontium to the cave sediments. Comparisons of 87Sr/ 86Sr with regional surficial deposits show a significant correlation between the cave sediments (avg: 0.7228; n = 27), the fine silt lunettes of the Bool Lagoon area (avg: 0.7224; n = 4), the sandy A horizons of the Coonawarra Red Brown Earths (RBEs; avg: 0.726; n = 5), and Holocene age podsolic sand deposits (0.723). These data suggest that there has been substantial flux from this group of deposits to the caves, as would be expected considering prevailing winds. This relationship is further supported by a strong correlation between many trace elements, including Ti, Zr, Ce, and Y; however, variations in clay mineralogy suggest that the fine silt-dominated lunettes and Padthaway RBEs were not significant contributors to the cave deposits. Hence, the detritus entering the caves was more than likely from areas proximal to the cave entrance and was dominated by medium grain-sized materials. Major regional deposits, including the coarser-grained, calcite-rich Bridgewater Formation sands, basalts from the lower SE, Padthaway Horst granites, Gambier limestone, and metamorphics from the Adelaide geosyncline show minimal correlation in 87Sr/ 86Sr ratios, elemental geochemistry, and mineralogy with the cave sediments, and are discounted as significant sources. In comparison, 87Sr/ 86Sr ratios for the Coorong silty sands (0.717–0.724), Lower Murray sands (0.727–0.730), and the medium size silt component of the Murray–Darling River system (0.71–0.72), compare favourably with the cave sediments. This relationship is further supported by similarities in elemental chemistry and mineralogy. Thus, much of the strontium-rich silt that is now located in the Naracoorte Cave sediments likely originated from the Murray–Darling basin. Over time, this material has been transported to the SE of South Australia, where it mixed with the medium sand component of the regressive dune ridge sequence, locally derived organic matter, limestone fragments, and fossil material to produce the unique deposits that we see evident in many of the chambers of the Naracoorte Cave system today.

Late Cretaceous palaeoenvironments expressed by the clay mineralogy of Cenomanian–Campanian chalks from the east of the Paris Basin

The clay fraction of Cenomanian–Campanian chalks cored at Poigny and Sainte-Colombe, close to Provins (east-south-east of Paris), includes variable proportions of smectitic minerals, illite and kaolinite. The smectitic sediments (which constitute the background of low-terrigenous supply throughout the stratigraphic interval) resulted mainly from the warm, humid climate and high sea level that prevailed during Late Cretaceous in this area. During the Late Turonian, the smectitic sedimentation was interrupted by significant detrital inputs of illite and kaolinite. This reflected tectonic rejuvenation of landmasses coeval with an explosive volcanism expressed by the occurrence of bentonite layers. Comparison with clay assemblages occurring at equivalent stratigraphic intervals in the western part of the Paris Basin reveals great variation in clay mineralogy reflecting either local detrital input or complex basin morphology. Bentonites identified in the Poigny and Sainte-Colombe boreholes include the Southerham, Caburn, Bridgewick and Lewes marls. Cropping out from England to Germany, these clay-rich beds usually show a smectitic clay fraction originating from the submarine weathering of volcanic glass shards. Surprisingly, at Poigny and Sainte-Colombe, the occurrence of authigenic kaolinite characterizes these bentonite layers. Authigenesis of kaolinite in volcanic layers deposited in marine environments is unusual. The formation of kaolinite, which requires a low pH, usually occurs in organic matter-rich continental settings. At Poigny and Sainte-Colombe, kaolinite probably formed shortly after burial in reducing microenvironments. The systematic Cr, V, U and organic carbon enrichment of kaolinite-rich bentonites by comparison with smectite-rich bentonites suggests deposition in oxygen-depleted environments in the deepest part of the Paris Basin.

Heterogeneous mixed-layer clays from the Cretaceous Greensand, Isle of Wight, southern England

AbstractThe sea-cliffs of the Isle of Wight were deposited during a period of overall sea-level rise starting in the Barremian (Lower Cretaceous) and continuing into the Aptian and Albian. They consist of fluvial, coastal and lagoonal sediments including greensands and clays. Numerous episodes of erosion, deposition and faunal colonization reflect condensation and abandonment of surfaces with firmgrounds and hardgrounds. This study focused mainly on shallow marine cycles where variations in clay mineralogy would not be expected, because overall system composition, sediment source, and thermal history are similar for all the samples in the studied section. Instead we found a wide variety of clay assemblages even in single samples within a 200 m interval.In this interval, distinct clay mineral assemblages were found and can be described as consisting of Al-rich, Fe-richand intermediate Fe and Al compositions withrespect to 2:1 and 1:1 layers in mixed-layer arrangements. Nearly pure glauconite-nontronite clays exist in the &lt;2 µm fraction only when the bulk rock is free of K- and plagioclase feldspar. Conditions favorable to glauconite-nontronite formation are interpreted to result from a hiatus in volcanoclastic sedimentation, thus providing a stable substrate for glauconitization.The Fe-bearing mixed-layer clay assemblages consist of glauconite, nontronite and berthierine-like layers in various proportions with several mixed-layer clays often coexisting in the same sample. In different samples, Al-richand Fe-Mg-rich mixed-layer clays are similar in their content and distribution of 1:1 and 2:1 layers. This suggests that the original clay assemblages were similar and later diagenesis affected certain horizons resulting in substitution of Al by Fe + Mg while preserving the original layer structure and arrangement.Structural formulae for the berthierine-like phase and berthierine-like layers in these mixed-layer clays show their layer cation composition is intermediate between odinite and standard berthierine. The total sum of octahedral cations varies from 5.26 to 5.55 whereas the amount of Fe2+ cations varies from 2.12 to 2.22 per O10(OH)8. A feature of the berthierine-like phase as well as of berthierine-like layers is that they are di-trioctahedral and Fe2+ and Fe3+ are the prevalent cations. Moreover, in these berthierine-like components, the amount of Fe2+ is greater than that of Mg (in contrast to odinite) and Fe3+ cations prevail over Al (in contrast to berthierine). The presence of authigenic ferrous Fe clays and the relationship between glauconite-nontronite and bulk mineralogy has implications for sedimentological processes and geochemical conditions during and shortly after deposition.

Triassic–Jurassic weathering and clay mineral dispersal in basement areas and sedimentary basins of southern Sweden

The basement rocks of southern Sweden were deeply weathered in Late Triassic through Cretaceous times, and possibly even earlier. Sub-Mesozoic saprolites formed, presumably due to long-term warmth and humidity, in conjunction with decomposition of abundant organic matter and subsequent infiltration of acidic water. The upper parts of the weathering profiles were subjected to intense leaching and kaolinitisation whereas the less evolved deep weathering front is enriched in smectite. Thus, any erosion prior to full regolith development, or extraordinary deep erosion, released less developed, smectite-rich weathering material to the Late Triassic–Jurassic receiving basins. Mesozoic sedimentary successions in Southern Sweden show high detrital smectite and/or kaolinite contents, moderate illite contents and low chlorite contents, compatible with expected mid-latitude warm equable greenhouse conditions. Norian and older Triassic haematite-cemented continental arkoses and smectite-dominated clays (smectite≫kaolinite) were clearly formed in arid settings. From the Rhaetian and throughout the Jurassic, humid onshore conditions sustained abundant plant and peat accumulation, early diagenetic meteoric flushing, and intense chemical weathering. Kaolinite is therefore a dominating detrital clay mineral in these sedimentary successions (kaolinite≈illite≫chlorite and smectite (I/S)). Minor quantitative variations in clay mineralogy within the “humid climate” Rhaetian–Jurassic sedimentary successions are difficult to interpret. In deltaic coal-bearing successions, the detrital clay mineral composition is conspicuously constant (kaolinite≈illite≫chlorite), possibly due to post-depositional clay mineral transformation. Conversely, in shallow marine deposits, some quantitative variation is notable (kaolinite≈illite>variable smectite≫chlorite). This may reflect that the original detrital composition was better preserved in shallow marine settings. Smectitic clay minerals associated with gypsum and caliche nodules ambiguously indicate increased aridity towards the end of the Jurassic.

Paleoenvironmental evolution of the Pliocene Villarroya Lake, northern Spain. A multidisciplinary approach

Abstract The Pliocene Villarroya basin fill consists of lacustrine and alluvial deposits over 100 m thick. The lacustrine deposits, up to 30 m thick, comprise three sequences. The two lower sequences consist of profundal, laminated deposits formed by mostly terrigenous facies that are overlain by charophytic carbonates and varves. The uppermost, third sequence made up of massive bioturbated mudstones corresponds to a very shallow lacustrine–palustrine environment. Bulk mineralogy comprises carbonates, clay minerals, quartz and feldspars. The carbonates are: exogenic, biogenic calcite and biogenic aragonite. Biogenic calcite constitutes ostracod shells and charophyte stem encrustations, whereas aragonite forms gastropod shells and, in some varve intervals, charophyte encrustations. The clay minerals are illite, kaolinite, palygorskite, chlorite and paragonite, which are inherited from source rocks, and smectite and interstratified clays, which are probably formed in the soils of the source area. Variations in clay mineralogy reflect the interplay between the climate, tectonic rejuvenation and the paleogeography of the basin. The pollen assemblages indicate significant variations in climate: a first phase of varying climate was followed by cool-temperate and sub-humid phases that culminated in a temperate and dry episode, which is recorded in the lower part of the second lacustrine sequence. This was succeeded by a marked warm and humid episode, which may be correlated with the Mid-Pliocene warmth about 3.0 Ma and the subsequent cooling phase, which is recorded in the upper, third sequence. In the two lower sequences (profundal lacustrine deposits), ostracods indicate oligosaline to mesosaline waters with salinity variations and some warm and Na–Cl episodes. Molluscs are characterized by hydrobiids, suggesting a salinity stress. In the upper, palustrine deposits, the ostracods and aquatic mollusc assemblages indicate fresh, cold waters. Faunal records indicate that changes in salinity do not correlate with climate, suggesting phases of significant saline inputs to the lake which were recorded in the lower first sequence and in the upper part of the second sequence. The lacustrine sequence of Villarroya record pronounced variations in climate and in the lacustrine environment. However, there are no consistent relationships between the climate conditions inferred from the pollen data and the sedimentary record of the environmental changes, showing that other factors, especially tectonics, have exerted an influence on the paleoenvironmental evolution, and on the sedimentary record of the Pliocene Villarroya lake.