Trioctahedral smectite and regularly interstratified chlorite/smectite in strata of the East Berlin Formation of the Connecticut Valley are largely restricted to black shale and gray mudstone deposited in alkaline, perennial lakes. The precursor of the mixed-layer clay appears to have been a smectite. Alkaline lake waters and inherited pore waters rich in magnesium favored the transformation of smectite to mixed-layer chlorite/smectite by fixation of brucitic interlayers into the smectite unit structure. Gray mudstones containing the mixed-layer chlorite/smectite are invariably underlain by magnesium-rich black shale—a possible source of Mg for the clay mineral transformations. The black shale is composed predominantly of Mg-rich trioctahedral smectite of probable authigenic origin.

X-Ray powder diffraction and thin section analyses indicate that marginal lacustrine mudstones of the Green River Formation in the south-central Uinta basin, Utah, contain abundant analcime. The analcime has a low Si/Al ratio (<2.31) and occurs as very fine grained disseminated crystals and, to a lesser extent, as coarser-grained pore-filling cement. Analcime-rich mudstones and associated sandstones, siltstones, and carbonates lack volcanic detritus and zeolites other than analcime, thus making it difficult to support the concept that the analcime formed from precursor zeolites derived from volcanic glass altered in saline, alkaline-lake water. Abundant dolomite, syneresis cracks, and the absence of freshwater pelecypods and gastropods suggest that the lake (Lake Uinta) was moderately saline and alkaline. The restricted illite-illite/smectite clay mineral suite in the analcime-rich mudstones suggests that detrital clays significantly altered in a moderately saline and alkaline environment, thereby providing a source of Si and Al for the formation of analcime.

Red mudstones contain twice as much analcime as green mudstones (14 vs. 7 wt. %). Green mudstones have a day mineral suite consisting of illite (44 wt. %), mixed-layer illite/smectite (35 wt. %), smectite (12 wt. %), and minor kaolinite (4 wt. %) and chlorite (5 wt. %), whereas red mudstones have a more restricted day mineral suite consisting ofillite (68 wt. %) and mixed-layer illite/smectite (26 wt. %) with very minor smectite, chlorite, and kaolinite. Periodic minor fluctuations in lake level probably exposed large areas of shallow lacustrine-interdistributary green mud. Evaporative pumping on the exposed mudflats concentrated the moderately saline and alkaline-lake water, thereby producing Na-rich brines that enhanced the formation of analcime by accelerating the alteration of detrital clays and, perhaps, other minerals. Oxidation of iron from altered iron-bearing minerals stained the analcime-rich mud red with iron hydroxide or oxide (perhaps hematite). The overall reaction from green to red mud (mudstones) was probably: detrital phyllosilicates + Na-brine + iron-bearing minerals + oxygen → analcime + iron hydroxide or iron oxide.

We report on how the effects of mechanical compaction and clay mineral diagenesis have affected the alignment of phyllosilicates in a suite of Miocene-Pliocene mudstones buried to sub-seabed depths of between 1.8 and 5.8 km in the deep-water Gulf of Mexico. Mechanical compaction has reduced the porosity of the samples to 15% at 5 km, with modal pore sizes between 10 and 20 nm. High-resolution X-ray texture goniometry data show that the intense mechanical compaction has not resulted in a strongly aligned phyllosilicate fabric. The muds were apparently deposited with a weak or isotropic phyllosilicate fabric which was not substantially realigned by mechanical compaction. Unusually, X-ray diffraction of <0.2 µm separates shows that: (1) there is no illitization trend between 90 and 120°C; and (2) discrete smectite persists to ∼120°C, coexisting with R1 I-S or R0 I-S with 30–40% expandable layers. Between 120 and 130°C, discrete smectite disappears and the expandability of I-S decreases to ∼25–30%. We propose a two-stage diagenetic process involving (1) the alteration of volcanic glass to smectite and (2) the illitization of smectite and I-S; the alteration of glass results in smectite without a preferred orientation and retards the illitization reaction. We suggest that the lack of a strongly aligned phyllosilicate fabric reflects the apparently limited extent of illitization, and thus recrystallization, to which these mudstones have been subjected.

We used high-resolution X-ray texture goniometry to quantify changes in the mm-scale orientation of phyllosilicate minerals in a suite of Paleogene mudstones from the Podhale Basin in southern Poland. The sample set covers an estimated range of burial depths between 2.4 and 7.0 km, corresponding to a temperature range of 60–160°C. Although mechanical compaction has reduced porosities to ∼10% in the shallowest samples, the phyllosilicate fabric is only modestly aligned. Coarser-grained (>10 µm) detrital chlorite and mica appear to be more strongly aligned with (001) parallel to bedding, suggesting their deposition as single grains rather than as isotropic flocs or aggregates. From 2.4 to 4.6 km, R0 illite-smectite with 40–50% illite layers changes to R1 illite-smectite with 70–80%) illite layers. At the same time kaolinite is lost and diagenetic chlorite is formed. The mineralogical changes are accompanied by a strong increase in the alignment of illite-smectite, chlorite, and detrital illite, parallel to bedding and normal to the presumed principal effective stress. We propose that the development of a more aligned I-S fabric results from the dissolution of smectite and the growth of illite with (001) normal to the maximum effective stress. Water released by illitization may act as a lubricant for the rotation of all platy minerals into nanoporosity transiently formed by the illitization reaction. At greater depths and temperatures, further illitization is inhibited through the exhaustion of K-feldspar. After the cessation of illitization, a further 2.4 km of burial only results in a small increase in phyllosilicate alignment. At such small values for porosity and pore size, increasing stress does not substantially reorient phyllosilicates in the absence of mineralogical change.

The Upper Miocene Mustafapas-a member of the Ürgüp Formation in the Cappadocia region consists predominantly of mudstones, sandstone, and conglomerate lenses with ignimbrite and basalt intercalations. The mudstones are an important source of raw materials for the ceramics industry in Turkey. A detailed mineralogical, geochemical, and genesis study of these materials has not been performed previously and the present study aims to fill that gap. The characteristics of mudstones of the Mustafapas-a member were examined using X-ray diffraction, scanning and transmission electron microscopy, energy dispersive spectroscopy, and chemical analyses. Weathering products of ophiolitic and pyroclastic rocks were transported into the tectonically subsided zone where they accumulated as fluvial and lacustrine deposits.Weathering in the mudstones is evidenced by smectite flakes associated with relict pyroxene, rod-like amphibole, feldspar, and volcanic glass. The chemical composition of mudstones and their distribution suggest that the depositional basin was supplied with ophiolitic material in the south and ignimbrite material in the north. This interpretation is based on an increase in the quantity of feldspar and opal-A and a decrease in the Fe2O3+MgO/Al2O3+SiO2 ratio from south to north in the study area. The northward increases in Light Rare Earth Elements/Heavy Rare Earth Elements, La/Yb, Zr/Ni and Zr/Co ratios and Nb, Ba, Rb, Sr, and Eu in the mudstones of the Mustafapas-a member with positive Eu anomalies suggest that the Fe, Mg, Al, and Si required to form smectite were supplied mainly through the decomposition of amphiboles, pyroxenes, feldspars, and volcanic glass during weathering processes. After the deposition of mudstones, relative increases in evaporation-controlled Ca, K, and Al in pore water favored the partial dissolution of Ca-bearing minerals and smectite flakes and in situ precipitation of calcite and traces of illite fibers under alkaline micro-environmental conditions during early diagenesis.

Despite the high-grade diagenesis experienced by the Skoorsteenberg Formation mudstones, Tanqua Karoo basin, South Africa, geochemical data have been interpreted to reveal primary mineralogy and so help understand provenance evolution. The geochemical signatures show systematic variations related to stratigraphy. The main changes in mudstones from the lower to the upper part of the section include: (1) an increase in the feldspar content of the primary sediment and a decrease in the content of Al-rich clay (probably dioctahedral smectite); (2) a decrease in the degree of chemical weathering of the sediment, representing a change to a dryer and/or cooler climate; (3) an increase in TiO2/Al2O3 representing increasing mafic sources; (4) an increase in CaO/(K2O+CaO) also possibly representing increasing mafic sources. Mass flux and differential diagenesis are unlikely to be responsible for the depth-related changes since the rocks have undergone the same degree of high-grade diagenesis and the mudstones are interrupted by other lithologies, so disturbing any sort of diffusion gradient. These variations could plausibly be the result of one or more of differential weathering, evolving provenance characteristics or variable hydrodynamic fractionation of the sediment. The ratio of Zr/Y, a possible indicator of hydrodynamic fractionation, increases only slightly and irregularly up-section. There is no relationship between the silica content, representative of the quartz-silt content of the sediment, and TiO2/Al2O3 showing that the amount of quartz, and so the degree of hydrodynamic fractionation, has not controlled mudstone geochemistry. The stratigraphic increase in feldspar content, the decrease in Al-rich clay content and increase of both TiO2/Al2O3 and CaO/(K2O+CaO) must be due to a combination of marginally evolving provenance characteristics (more mafic and felsic rocks exposed to weathering with time) and changes in the degree of rock weathering (less chemical weathering with time).

The excavation of a tunnel through a mudstone formation provides an opportunity to examine the effects of the modification of the physical and chemical environment on the rock. The mineralogical and chemical consequences of hydration-dehydration cycles and of oxidation have been evaluated in the case of the Toarcian mudstone formation at the Tournemire experimental site (France). Studies by X-ray diffraction and tansmission electron microscopy of both altered and preserved samples show that the introduction of air and condensed water causes the oxidation of pyrite and the subsequent generation of acid and sulphate-rich waters at the micron scale, in the local environment of pyrites. The fluid-clay particle interactions around the oxidized pyrites induce: (1) a statistical enrichment in Si of the I-S clay minerals; (2) an increase in the Fe(III)/Fe total ratio in some of the I-S particles; and (3) the dissolution of illite layers in mixed-layer I-S. These evolutions are consistent with the results of numerical modelling which reproduced the interaction between the clay particles and the acid water.

The significance of the potential impacts of microbial activity on the transport properties of host rocks for geological repositories is an area of active research. Most recent work has focused on granitic environments. This paper describes pilot studies investigating changes in transport properties that are produced by microbial activity in sedimentary rock environments in northern Japan. For the first time, these short experiments (39 days maximum) have shown that the denitrifying bacteria, Pseudomonas denitrificans, can survive and thrive when injected into flow-through column experiments containing fractured diatomaceous mudstone and synthetic groundwater under pressurized conditions. Although there were few significant changes in the fluid chemistry, changes in the permeability of the biotic column, which can be explained by the observed biofilm formation, were quantitatively monitored. These same methodologies could also be adapted to obtain information from cores originating from a variety of geological environments including oil reservoirs, aquifers and toxic waste disposal sites to provide an understanding of the impact of microbial activity on the transport of a range of solutes, such as groundwater contaminants and gases (e.g. injected carbon dioxide).

The role of the microbial activity on the transport properties of host rocks for geological repositories, particularly in the far-field, is an area of active research. This paper compares results from experiments investigating changes in transport properties caused by microbial activity in sedimentary rocks in Japan (mudstones) and sandstone (UK).

These experiments show that both Pseudomonas denitrificans and Pseudomonas aeruginosa appear to survive and thrive in pressurized flow-through column experiments which utilized host rock materials of relevance to radioactive waste disposal. Indeed, despite there being a difference in the numbers of organisms introduced into both biotic experiments, numbers appear to stabilize at ∼105 ml−1 at their completion. Post experimental imaging has highlighted the distinct differences in biofilm morphology, for the chosen rock types and bacteria, with Pseudomonas aeruginosa derived biofilms completely covering the surface of the sandstone host and Pseudomonas denitrificans forming biofilament structures. Regardless of substrate host or choice of microbe, microbial activity results in measurable changes in permeability. Such activity appears to influence changes in fluid flow and suggests that the transport of radionuclides through the far-field will be complicated by the presence of microbes.

An understanding of the nature and scales of diagenetic variability within organic-rich mudstones is critical to the accurate assessment of shale-gas reservoir properties, as well as for elucidating chemical evolution pathways within mudstones. Here we integrate field observations with thin section descriptions (optical and electron optical techniques) and mineralogical data for the Blackhawk Member time-equivalent Mancos Shale in Book Cliffs, Utah, to determine the impacts of early and burial diagenesis on this mudstone succession.

The detrital assemblage in the Mancos Shale comprises quartz-silt, feldspar, clay minerals, dolomite and organic matter (TOC of 1 to 2.5%). Biogenic silica is negligible. Field mapping reveals laterally continuous (km scale), ferroan dolomite cemented units up to 0.3 m thick, are present. These cemented units cap both coarsening-upward units (1 to 3 m thick), and stacked successions of coarsening-upward units (5 to 15 m thick). These upward-coarsening sediment packages, capped by dolomite cemented strata, correlate to bedsets and parasequences in updip settings. Pervasive cementation in these dolomite-cemented units is likely to have occurred prior to compaction as a result of bacterially mediated respiratory processes. Cementation at these levels is particularly evident because cement precipitation occurred during breaks in sediment accumulation below marine flooding surfaces. The abundance of dolomite cements highlights the importance of macroscopic-scale diagenetic carbonate mobility in these mudstones.

In addition to carbonate-cements, diagenetic alteration and precipitation of quartz and alumina-silicate minerals are also important in these mudstones. Kaolinite is present both in uncompacted test of organisms and as vein fills in septarian concretions. Kaolinite precipitation is interpreted to have occurred prior to significant compaction and indicates that both silicon and aluminium were mobile during early diagenesis. We interpret the abundance of early diagenetic kaolinite cement to be the result of Al-mobilization by organic acids generated during organic matter oxidation reactions, with the Al sourced from poorly crystalline detrital aluminium oxides and clay minerals. There is also indirect evidence for burial diagenetic kaolinitization of feldspar grains. Quartz cement takes the form of quartz overgrowths and microcrystalline quartz crystals. Textures and CL spectra for the quartz microcrystalline cement suggests that recrystallization of biogenic silica (opal-A) was likely to have been an important source for quartz cements, although smectite-to-illite transformation may have contributed some. These mineral phases highlight that microscopic-scale diagenetic mobility of silica is important, even within mudstones lacking obvious sources of biogenic silica and is likely to be an important processes in a wide range of mudstones.

This paper addresses the mineralogy, geochemistry, and genesis of palygorskite and associated calcretes in the Kırşehir region. In this area, the Quaternary calcretes are present within and/or on lacustrine mudstones of the Kızılırmak Formation (Late Miocene-Pliocene). The calcretes occur in the form of nodules, tubes, fracture-infills, and hard laminated crusts (hardpans). Calcrete samples are predominantly composed of calcite associated with smectite, quartz, feldspar, and minor palygorskite. The lacustrine mudstone samples consist mainly of smectite and palygorskite with minor calcite, quartz, feldspar, illite, kaolinite and amorphous material. Petrographic studies revealed that the calcrete samples are predominantly micrites that were partially converted to microsparites by recrystallization. Scanning and transmission electron microscopy images indicate that palygorskite fibres formed authigenically on and between calcite rhombs in the calcrete samples and at the edges of smectite flakes of host rocks adjacent to the calcretes. The oxygen and hydrogen stable isotope values of the calcite suggest that it precipitated from meteoric water under arid or seasonally arid climatic conditions. Calcite precipitation occurred in the vadose zone, as evidenced by desiccation cracks and vadose pisolites. The enrichment of Rb+Ba±Sr and Light Rare Earth Elements (LREE) relative to Heavy Rare Earth Elements (HREE)with a distinct negative Eu anomaly are responses to the fractionation of volcanogenic feldspar and hornblende under the influence of meteoric water. The palygorskite associated with calcretes was precipitated from alkaline water rich in Si, Al and Fe and low in Mg. The palygorskite precipitation occurred at increased temperatures, characterising the advanced stage of calcrete formation which lowers the Ca content in the soil water. The required chemical elements for palygorskite and calcite were most likely derived from the dissolution of smectitic clays, ignimbrites and carbonate minerals.

Geological disposal facilities (GDF) are intended to isolate and contain radioactive waste within multiple protective barriers, deep underground, to ensure that no harmful quantities of radioactivity reach the surface environment. The last line of defense in a multi-barrier GDF is the geosphere, where iron is present in the host rock mineralogy as either Fe(II) or Fe(III), and in groundwater as Fe(II) under reducing conditions. The mobility of risk-driving radionuclides, including uranium and technetium, in the environment is affected significantly by their valence state. Due to its low redox potential, Fe(II) can mediate reduction of these radionuclides from their oxidized, highly mobile, soluble state to their reduced, insoluble state, preventing them from reaching the biosphere. Here a study of five types of potential host rocks, two granitoids, an andesite, a mudstone and a clay-rich carbonate, is reported. The bulk rocks and their minerals were analysed for iron content, Fe(II/III) ratio, and for the speciation and fine-grained nature of alteration product minerals that might have important controls on groundwater interaction. Total iron content varies between 0.9% in clays to 5.6% in the andesite. X-ray absorption spectroscopy reveals that Fe in the granitoids and andesite is predominantly Fe(II), and in mudstones, argillaceous limestone and terrestrial sandstone is predominantly Fe(III). The redox reactivity of the potential host rocks both in the presence and absence of Fe(II)-containing 'model' groundwater was investigated using an azo dye as a probe molecule. Reduction rates as determined by reactivity with the azo dye were correlated with the ability of the rocks to uptake Fe(II) from groundwater rather than with initial Fe(II) content. Potential GDF host rocks must be characterized in terms of mineralogy, texture, grain size and bulk geochemistry to assess how they might interact with groundwater. This study highlights the importance of redox reactivity, not just total iron and Fe(II)/(III) ratio, when considering the host rock performance as a barrier material to limit transport of radionuclides from the GDF.

The spatial arrangement of gypsum veins as preserved natural hydraulic fractures have been characterized in the Triassic Keuper Marl Formation (UK), a caprock for hydrocarbon reservoirs and CO2 sequestration. The marls cropping out are subdivided into five discrete fracture units based on the presence and abundance of gypsum veins. The nodular gypsum in evaporite horizons provides excess gypsum for nodule-rooted horizontal gypsum veins. Our petrographic observations demonstrate that the development of gypsum veins in beds lacking macroscopic evaporites is closely associated with disseminated gypsum cement in the marls. We interpret that the gypsum veins in marl are sourced from disseminated gypsum cements in the host rocks, based on stratigraphic correlations, and much lower Sr concentrations than gypsum nodules. Gypsum was transported to adjacent veins mainly through diffusion in the low-permeability marls. The localization of gypsum veins and varied Sr concentrations of veins and nodules indicate that the diagenetic fluids are a mix of connate water with meteoric water rather than brines transported from evaporite beds along faults to non-evaporite beds. This results in the absence of gypsum fillings in fractures in rocks without primary gypsum cements. The study implies that the cementation of natural fractures in low-permeability rocks can highly depend on the presence of cement minerals in the host rock.

The Ordovician and Silurian rocks of the Southern Uplands of Scotland have been interpreted as sediments deposited on the northern margin of the Iapetus Ocean. Trace fossils are abundant at many localities in ocean-floor turbidites and mudstones that usually lack all other evidence of life. Twelve ichnogenera are present, and they are mainly meandering locomotion and feeding trails and burrow networks: Dictyodora, Caridolites, Helminthoida, Neonereites, Nereites, Protovirgularia, Gordia, Megagrapton, Paleodictyon, Chondrites, Plano-lites and Skolithos. The trace fossils occur in at least five distinct assemblages and the composition of these was probably controlled by the frequency and nature of the turbidity currents, and possibly by the oxygen content of the mudstones. Where turbidity currents were weak, abundant Dictyodora, together with Caridolites, Neonereites, Nereites, Protovirgularia and Gordia occur in various combinations. Where currents were stronger, traces such as Gordia, Paleodictyon and Megagrapton may be exhumed and cast on turbidite soles, and the sand may contain Skolithos. The ‘deep-sea’ Nereites trace fossil facies is divisible into several assemblages, presumably environmentally controlled.