Salinity,temperature and pressure effect on hydrogen wettability of carbonate rocks

Hydrogen had been injected into the geologic formations, and the geologic formation wettability would influence the hydrogen storage. Hydrogen wettability of sandstone reservoirs (quartz), mica and other rocks have been explored in the previous study. However, the research on hydrogen wettability of carbonate rocks was lacked. In this study, we studied the carbonate rock wettability alteration when exposed to the hydrogen environments. Salinity, temperature and pressure effect on H₂/carbonate rock/brine wettability were explored. When the solutions ions concentration increased, the advancing/receding contact angle would increase, and divalent ions could make the contact angle higher than monovalent ion, which was because ions could compress the electric double layer. The carbonate rock powder in brine showed negative charge, and the zeta potential increased with higher ions concentration. When temperature increased and the pressure decreased, the contact angle would decrease, which was related to the H₂ gas density and molecular interactions.     

Thermodynamic characterization of H2-brine-shale wettability: Implications for hydrogen storage at subsurface

Large-scale underground hydrogen storage (UHS) appears to play an important role in the hydrogen economy supply chain, hereby supporting the energy transition to net-zero carbon emission. To understand the movement of hydrogen plume at subsurface, hydrogen wettability of storage rocks has been recently investigated from the contact angles rock-H₂-brine systems. However, hydrogen wettability of shale formations, which determines the sealing capacity of the caprock, has not been examined in detail. In this study, semi-empirical correlations were used to compute the equilibrium contact angles of H₂/brine on five shale samples with various total organic content (TOC) at various pressures (5–20 MPa) and at 343 K. The H₂ column height that can be securely trapped by the shale and capillary pressures were calculated. The shale's H₂ sealing capacity decreased with increasing pressure, increasing depth and TOC values. The CO₂/brine equilibrium contact angles were generally higher than H₂/brine equilibrium, suggesting that CO₂ could be used as favorable cushion gas to maintain formation pressure during UHS. The utmost height of H₂ that can be safely trapped by shale 3 (with TOC of 23.4 wt%) reduced from 8950 to 8750 M while that of shale 5 (with TOC of 0.081 wt%) reduced slightly from 9100 M to 9050 M as the pressure was increased from 5 to 20 MPa. The capillary entry pressure decreased with increasing depth and shale TOC, implying that the capillary trapping effect, as well as the over-pressure required to move brines from the pores by hydrogen displacement, reduces with increasing depth, and shale TOC. However, the shales remained at strongly water-wet conditions, having an equilibrium contact angles of not more than 17° at highest pressure and TOC. The study suggests that the increasing contact angles with increasing pressure and shale TOC, as well as decreasing column height and capillary pressure with increasing depth for H₂-brine-shale systems might not be sufficient to exert significant influence on structural trapping capacities of shale caprocks due to low densities of hydrogen.     

The effect of clay on initial and residual saturation of hydrogen in clay-rich sandstone formation: Implications for underground hydrogen storage

Understanding wettability in rock-brine-hydrogen systems is essential for dependable predictions of capillary/residual trapping in clay-rich sandstone formations. Despite being the most used technique, wettability assessment based on contact angle measurements is confronted with inherent uncertainties that limit its reliability. In contrast, core flooding techniques provide a more direct and realistic picture of wettability and its time evolution. Nuclear Magnetic Resonance (NMR) allows us to evaluate the initial and residual hydrogen saturations and distribution along the core specimen. It is a fast, reliable, and effective way of inferring the impact of wettability on hydrogen migration, and residual trapping in prospective geo-storage rock formations. Recent publications have reported the evaluation of wettability in a brine-hydrogen-rock system where the rock is a clean sandstone (no clays). Here we evaluate the impact of the presence of clays in a sandstone, which has not been reported yet. NMR monitoring was employed to characterize the initial and residual hydrogen saturations in the Bandera Grey (BG) sandstone. To investigate the impact of clay minerals on hydrogen saturation, same rock sample was characterized in its natural state, and after heating it to 700 °C for 12 h in an air environment to burn off clay minerals, During the NMR core flooding experiments, ten pore volumes (PVs) were injected/withdrawn during the drainage/imbibition cycles at a fluid injection rate of 2 mL/min under room temperature and 1000 psi confining pressure. Due to the hydrophilicity of quartz and clay, the tested BG sandstone (clay-rich sandstone) shows a significant residual/trapped saturation (～3.5% can be reproduce); therefore, clay-rich sandstone may not be ideal for hydrogen storage unless cushion gas is used.The results show that initial and residual hydrogen saturations were slightly changed after firing (from 16% to 18% for initial and from 14% to 13% for residual). This also suggests that the wettability of the BG sandstone-brine-hydrogen system is slightly impacted by clay content and type. We also observed that clay firing at 700 °C has little effect on the porosity and gas permeability of the BG sandstone. Moreover, X-ray powder diffraction (XRD) results showed that quartz content increases from 68.1% to 76.2%, Kaolinite transformed into illite and clinochlore disappeared. The disappearance of chlorite after firing suggests that it is transformed into another clay type.     

Importance of clay-H2 interactions for large-scale underground hydrogen storage

Underground hydrogen storage is considered an option for large-scale green hydrogen storage. Among different geological storage types, depleted oil/gas fields and saline aquifers stand out. In these cases, hydrogen will be prevented from leaking back to the surface by a tight caprock seal. It is therefore essential to understand hydrogen interactions with shale-type caprocks. To this end, natural pure montmorillonite clay was exposed to hydrogen gas at different pressures (0–50 bar) and temperatures (77, 195, 303 K) to acquire data on its adsorption capacity related to UHS and caprock saturation. Montmorillonite was chosen because of its large specific surface area enabling quantification of the adsorption process. Hydrogen adsorption was successfully fitted with a Langmuir isotherm model and yielded small partition coefficients indicating that hydrogen does not preferentially adsorb to the clay surface. Adsorption on montmorillonite goes back to weak physisorption as inferred from minor negative changes in the enthalpy of reaction (−790 J/mol), derived from an Excel Solver approach to the van't Hoff equation. Based on own as well as literature values, adsorption capacities, which were originally reported as mol/kg or wt%, are recast as hydrogen volume adsorbed per specific surface area (μL/m²). The acquired range is surprisingly narrow, with values ranging from 3 to 6 μL/m², and indicates the normalised volume of hydrogen that can be expected to remain in the shale-type caprock after injected hydrogen migrated upwards through the porous reservoir. This ‘residual’ caprock saturation with hydrogen can be further restrained by considering the geothermal gradient and its effect on the molar volume of hydrogen. The experimental results presented here recommend injecting hydrogen deeper rather than shallower as pressure and temperature work in favour of increased storage volumes and decreased hydrogen loss through clay adsorption in the caprock.     

Use of clays as liners in solar ponds

An alternative to synthetic materials for use in solar pond liners is to select clayey soils as hydraulic barriers. This option reduces the cost of construction and the risk of contamination of subsoil and groundwater by hot brines. This paper deals with the physical, chemical and hydraulic properties of different soils tested mainly as compacted clay liners. The underdeveloped nations have the option to use this type of liner, but before doing so several tests are recommended, including those for soil and water composition, permeability, plasticity and X-ray diffraction analysis. In this investigation the following samples are analyzed: native clayey soils with illite, montmorillonite and halloysite, treated and non-treated bentonites in powder and granulated form, a mixture of zeolite and sodium bentonite, and industrial minerals composed largely of halloysite, kaolinite and attapulgite selected clays. Neutral salt aqueous solutions (NaCl and KCl) at different concentrations and under temperature gradients were used for compatibility testing conducted on these specimens. Experiment setup and particular testing procedures are also discussed.     

Hydrogen tightness evaluation in bedded salt rock cavern: A case study of Jintan,China

Hydrogen is regarded as one of the most important energy sources for the future. Safe, large-scale storage of hydrogen contributes to the commercial development of the hydrogen industry. Use of bedded salt caverns for natural gas storage in China provides a new option for underground hydrogen storage (UHS). In this study, the physical properties of multicomponent gases in UHS and salt rock are reviewed and discussed, along with the flow of hydrogen in the surrounding salt rock. Mathematical models of the two-phase multicomponent flow of the gas–brine system in the UHS were established. A numerical model of a simplified elliptical salt cavern was built to simulate the migration of the gas–brine system in the UHS. The hydrogen tightness of the UHS was evaluated through simulation with different storage strategies, salt rock and interlayer permeabilities, and gas components. The results indicate that: (1) Cyclic injection and withdrawal facilitate hydrogen leakage, which is accelerated by increasing the frequency. (2) The huff-n-puff of hydrogen gas in the injection and withdrawal cycles forces the gas into pore space and enhances the relative permeability of the gas phase. The migration of hydrogen and brine weakens the hydrogen tightness. Brine saturation is an important index for evaluating the hydrogen tightness of UHS. (3) The leakage rate of UHS increases with an increase in the permeability of the salt rock and interlayer and the total thickness of the interlayers. The average permeability K_wa weighted by the thickness of layers for the bedded salt formation is proposed to integrate three variables to facilitate field application of the simulation results. The critical K_wa is less than 3.02 × 10⁻¹⁷ m² if the recommended annual hydrogen leakage rate is less than 1%. (4) The difference between hydrogen and other gas species is another important factor in the leakage rate and should be considered. This study provides theoretical guidance for evaluating the feasibility of UHS in salt caverns and site selection in China.     

Measurements and predictive models of high-pressure H2 solubility in brine (H2O+NaCl) for underground hydrogen storage application

In the context of Underground Hydrogen Storage (UHS), the stored gas is in direct contact with brine (residual brine from the cavern or formation water of deep aquifers). Therefore, knowledge of the phase equilibria (solubility of hydrogen in brine and water content in the hydrogen-rich phase) in the geological reservoir is necessary for the study of hydrogen mobility and reactivity, as well as the control, monitoring and optimization of the storage. The absence of measured data of high-pressure H₂ solubility in brine has recently led scientists to develop predictive models or to generate pseudo-data using molecular simulation. However, experimental measurements are needed for model evaluation and validation. In this work, an experimental apparatus based on the “static-analytic” method developed and used in our previous work for the measurement of gas solubility in brine was used. New solubility data of H₂ in H₂O+NaCl were measured more or less under the geological conditions of the storage, at temperatures between 323 and 373 K, NaCl molalities between 0 and 5m, and pressures up to 230 bar. These data were used to parameterize and evaluate three models (Geochemical, SW, and e-PR-CPA models) tested in this work. Solubility and water content tables were generated by the e-PR-CPA model, as well as a simple formulation (Setschenow-type relationship) for quick and accurate calculations (in the fitting range) of H₂ solubility in water and brine was proposed. Finally, the developed models estimate very well the water content in hydrogen-rich phase and capture and calculate precisely the salting-out effect on H₂ solubility.     

Hydrogen wettability in carbonate reservoirs: Implication for underground hydrogen storage from geochemical perspective

Hydrogen has been considered as a promising renewable source to replace fossil fuels to meet energy demand and achieve net-zero carbon emission target. Underground hydrogen storage attracts more interest as it shows potential to store hydrogen at large-scale safely and economically. Meanwhile, wettability is one of the most important formation parameters which can affect hydrogen injection rate, reproduction efficiency and storage capacity. However, current knowledge is still very limited on how fluid-rock interactions affect formation wettability at in-situ conditions. In this study, we thus performed geochemical modelling to interpret our previous brine contact angle measurements of H₂-brine-calcite system. The calcite surface potential at various temperatures, pressures and salinities was calculated to predict disjoining pressure. Moreover, the surface species concentrations of calcite and organic stearic acid were estimated to characterize calcite-organic acid electrostatic attractions and thus hydrogen wettability. The results of the study showed that increasing temperature increases the disjoining pressure on calcite surface, which intensifies the repulsion force of H₂ against calcite and increases the hydrophilicity. Increasing salinity decreases the disjoining pressure, leading to more H₂-wet and contact angle increment. Besides, increasing stearic acid concentration remarkably strengthens the adhesion force between calcite and organic acid, which leads to more hydrophobic and H₂-wet. In general, the results from geochemical modelling are consistent with experimental observations that decreasing temperature and increasing salinity and organic acid concentration increase water contact angle. This work also demonstrates the importance of involving geochemical modelling on H₂ wettability assessment during underground hydrogen storage.     

Structural and textural control of high-pressure hydrogen adsorption on expandable and non-expandable clay minerals in geologic conditions

Both natural and anthropogenically-generated hydrogen gas occurs in sedimentary rocks and geotechnical barriers. Due to high-pressure conditions, a significant portion of H₂ can be physisorbed in available micropores of clay minerals, which have been proven to control the gas adsorption properties of rocks. This study investigates H₂ adsorption on clay minerals naturally occurring in rocks, by combining the high-pressure H₂ experiments with sample characterization analyzed using low-pressure adsorption of N₂ and CO₂, and structural analysis using X-ray diffraction. We found that H₂ adsorption depends strongly on the mineral texture, which is not related directly to its structure. Most of H₂ is adsorbed in the micropores accessible to CO₂. H₂ intercalates in the smectitic interlayers with a basal spacing larger than 10.8 Å. The density of adsorbed H₂ is about double that of free H₂ gas under given pressure and temperature, effectively increasing the gas storage capacity of rocks.     

Hydrogen adsorption by microporous materials based on alumina-pillared clays

Hydrogen adsorption capacities of various alumina-pillared clays have been studied. The starting material for the preparation of the samples was a natural montmorillonite, which was intercalated with solutions of hydrolysed aluminium, at various Al concentration/clay weight ratios, and then calcined at various temperatures. The results of the adsorption at 77 K indicate that there is a relation between the hydrogen uptake and the microporous volume of the pillared clays. Equilibrium adsorption data were analysed using the Freundlich, Langmuir and Toth isotherm models, in order to investigate the heterogeneity of the materials.     

Mineralogy and K–Ar geochronology of mixed-layered illite/smectite from The Geysers coring project,California, USA

Clay fractions for K–Ar dating, separated from Franciscan-Assemblage (late Mesozoic) metagraywacke and argillite from The Geysers Coring Project corehole SB-15-D, and a “background” outcrop consist of illite, mixed-layer illite/smectite, mixed-layer chlorite/smectite, and chlorite with minor calcite and quartz. Except for the chlorite/smectite, these phases were formed during both subduction-related Franciscan regional metamorphism and late Cenozoic (post-1.1 Ma) hydrothermal alteration and mineralization related to The Geysers hydrothermal system. The chlorite/smectite is exclusively hydrothermal in origin. In spite of careful efforts to physically separate the metamorphic and layer silicates prior to analysis, all samples contained variable contributions from both sources. The proportion of illite plus illite/smectite clays in the <5 μm fractions of the samples ranges widely from about 41 to 97 wt.%. K₂O contents of the dated clay fractions (⩽3 μm) also ranged widely, from 1.66 to 8.0 wt.%. Despite having been heated to at least 300°C, matrix illite and illite/smectite do not show depth- and temperature-dependent trends of downward-increasing illite interlayers as is the case in many old sedimentary basins (e.g. Texas Gulf Coast and North Sea oil fields) and other geothermal systems (e.g. Valles caldera and Salton Sea). Fourteen mixed-layer illite/smectites of different size fractions (0.1–0.35, 0.35–1 and 1–3 μm) from the SB-15-D metagraywackes and argillites (and a nearby “background” sample of the same rock type) yielded K–Ar dates ranging widely from 105.5 Ma (the outcrop sample) to 1.5 Ma. Although there is a general decrease in apparent age with depth, most of the clays yielded middle Tertiary dates (35.4–19.4 Ma). The outcrop age is consistent with published data for Franciscan regional metamorphism. The youngest of the core dates, 2.3 and 1.5 Ma, are slightly older than the earliest Geysers felsite intrusions (1.1 Ma), and substantially older than vein adularia from the corehole (0.57 Ma). Thus, even these youngest illite/smectite dates for the SB-15-D cores likely reflect minor contamination from metamorphic phases. These K–Ar dates do, however, indicate that pre-existing detrital and metamorphic clays have not been totally degassed even though the host rocks have resided in a high-temperature geothermal system for more than a million years. We conclude that the partially reset metamorphic-hydrothermal illite/smectite dates probably reflect the effects of low host-rock permeabilities, of sluggish reaction kinetics of the metamorphic layer silicates, and of “fines migration” of Franciscan Complex clays, along fractures, induced by hydrothermal fluid circulation.     

Li-decorated Pmmn8 phase of borophene for hydrogen storage. A van der Waals corrected density-functional theory study

We performed standard and van der Waals-corrected density functional theory calculations to investigate the hydrogen storage capacity of a phase of borophene with Pmmn symmetry and nonzero thickness. This borophene sheet (Pmmn8) has 8 atoms in its unit cell and is more stable than the planar α sheet and that the corrugated Pmmn2 sheet (2 atoms in the unit cell). Our results show that, in pristine form, the Pmmn8 sheet is not suited for hydrogen storage applications. However, decoration with Li atoms and strain increase the hydrogen storage ability of the sheet. We performed also a detailed quantum chemical topological analysis that shows that the BLi interaction in the hydrogenated Li-decorated Pmmn8 sheet is ionic. Our results for the adsorption of H₂ on the Li-decorated Pmmn8 sheet are compared with those obtained for the adsorption of H₂ on Ti-decorated zigzag graphene nanoribbons.     

Experimental simulations of hydrogen migration through potential storage rocks

In the framework of future decarbonization of the energy industry, the safe and effective storage of hydrogen is an important approach to add to a climate-friendly energy system. Until the development of sufficiently large electrical storage systems, the storage of hydrogen in the order of GWh to TWh is envisaged in salt caverns or porous geological formations with a gas-tight covering of salt or claystone. In order to calculate gas losses from these H₂ storage facilities, the H₂ diffusivity of the storage and cap rocks must be known. To determine the hydrogen diffusion rates in these rocks, an experimental set-up was designed, constructed and tested. The set-up comprises two gas chambers, separated by the rock sample under investigation with an exposed area of approximately 7 cm². The driving force for gas migration through the rock sample from the hydrogen-containing feed gas chamber to the hydrogen-free permeate chamber is the chemical potential (concentration) gradient. With respect to hydrogen migration behaviour, hydrogen breakthrough times and hydrogen diffusion coefficients were determined for dry and wet Bentheimer sandstone, Werra rock salt and Opalinus clay samples. Breakthrough times varied between less than 1 h and 843 h. Based on concentration changes at the permeate side, hydrogen diffusion coefficients were derived ranging from 10⁻⁹ to 10⁻⁸ m²/s. The differences between the materials and the effect that wetted or water-saturated samples have higher hydrogen retention due to closed pores and microcracks were clearly shown. The experimental set-up proves to be a suitable approach to determine site-specific rock characteristics such as hydrogen diffusion coefficients and breakthrough times for natural geomaterials.     

In-situ hydrogen wettability characterisation for underground hydrogen storage

Hydrogen storage in subsurface aquifers or depleted gas reservoirs represents a viable long-term energy storage solution. There is currently a scarcity of subsurface petrophysical data for the hydrogen system. In this work, we determine the wettability and Interfacial Tension (IFT) of the hydrogen-brine-quartz system using captive bubble, pendant drop and in-situ 3D micro-Computed Tomography (CT) methods. Effective contact angles ranged between 29° and 39° for pressures 6.89–20.68 MPa and salinities from distilled water to 5000 ppm NaCl brine. In-situ methods, novel to hydrogen investigations, confirmed the water-wet system with the mean of the macroscopic and apparent contact angle distributions being 39.77° and 59.75° respectively. IFT decreased with increasing pressure in distilled water from 72.45 mN/m at 6.89 MPa to 69.43 mN/m at 20.68 MPa. No correlation was found between IFT and salinity for the 1000 ppm and 5000 ppm brines. Novel insights into hydrogen wetting in multiphase environments allow accurate predictions of relative permeability and capillary pressure curves for large scale simulations.     

Molecular dynamics simulation of hydrogen diffusion in water-saturated clay minerals; implications for Underground Hydrogen Storage (UHS)

A suitable option for Underground Hydrogen Storage (UHS) is aquifers composed of porous and permeable media. However, the successful execution and long-term safety of the UHS process require further understanding of hydrogen transport behavior in the pathway of caprock to eliminate the possibility of hydrogen leakage. In this regard, we employ Molecular Dynamics simulation to predict the hydrogen diffusion coefficient in five different water-saturated clay minerals, major components of caprock, and to assess the effects of clay mineral characteristics in terms of pore size (1–8 nm), layer charge distribution (octahedral sheet and tetrahedral sheets), and counterion type (Na⁺ and Ca²⁺) on hydrogen diffusion. The results show that the diffusion of hydrogen in negatively charged water-saturated clay minerals significantly increases for pore size increments of less than 2 nm (critical pore size) based on layer charge distribution and interlayer cations. However, no considerable changes in the hydrogen diffusion coefficients are observed for pore sizes larger than 2 nm. While the hydrogen diffusion coefficient in pyrophyllite (no charge, no interlayer cation) experiences a monotonous rise as pore size increases. Our findings in this study assist in better experimental test design to examine the possibility of leakage in actual field implementation by focusing on the clay mineral characteristics.     

Intrinsic affinity of acid-activated bentonite towards hydrogen and carbon dioxide

The effect of acid activation on bentonite affinity toward carbon dioxide (CO₂) and hydrogen (H₂) was investigated at ambient conditions. Characterization through X-ray diffraction and fluorescence, thermal gravimetric analysis, nitrogen adsorption-desorption isotherms, Fourier Transform Infrared spectroscopy and differential scanning calorimetry allowed correlating newly induced textural and structural features with adsorptive properties. Optimum acid treatment improved the specific surface area and porosity. The resulting decrease in dehydration temperature indicates decay in hydrophilic character. The affinity improvement towards hydrogen was due to Brønsted acidity suppression and surface basicity attenuation, which are essential requirements for adsorption on aluminosilicates (AS) via weak Lewis Acid-Base interactions, but excessive acid attack was detrimental. Low Si/Al surfaces should be suitable for CO₂ capture, while moderately acid-treated clays should be interesting candidates as hydrogen adsorbents. This allows envisaging promising prospects for low-cost AS-based materials intended for selective CO₂-free capture and storage of hydrogen without energy and safety constraints.     

Behavior of basic elements during coal combustion

X-ray absorption fine structure (XAFS) spectroscopy, Mossbauer spectroscopy and computer-controlled scanning electron microscopy (CCSEM) have been used to investigate the reactions of Ca, Fe and alkalies in combustion systems. Ca may either transform to a CaO fume that reacts with SO₂ to form CaSO₄, or may react with clays, quartz and other minerals to form slag droplets, or flyash. Similarly, pyrite may devolatilize and oxidize exothermically to form molten or partially molten iron sulfide-iron oxide mixtures, or may react with other minerals to become part of the slag. Alkalies in lignites (principally Na) volatize and may react with either SO₂ to form sulfates or with clay minerals (principally kaolinite) to form aluminosilicate slag droplets. K in bituminous coal is contained in illite which melts and becomes part of the slag phase. The calcium and alkali sulfates and the iron-rich species are observed to be concentrated in the initial layers of deposits, while the complex aluminosilicate slag droplets collect to form an outer glassy layer.     

Effects of charging,strain, and doping on the interaction between H2 and nitrogen-rich Penta-CN2 sheet

The development of advanced materials for the safety and efficiency of hydrogen storage media is necessary. We computationally explored the hydrogen storage properties of penta-CN₂ sheet. The hydrogen adsorption properties of neutral, negatively charged, externally strained, and metal-doped penta-CN₂ sheets were investigated in detail. Here, for the first time, the effect of the strain of two-dimensional nonmetallic materials on hydrogen adsorption is investigated. We found that the hydrogen binding energy increases to ?0.20 eV and achieves storage capacities up to 9.00 wt % on the negatively charged substrate, and to ?0.14 eV at 18% stretching. Moreover, metal doping causes hydrogen adsorption energy to increase to ?0.25–0.82 eV. The hydrogen storage capacity of Li-doped defective CN₂ sheet is up to 10.90 wt%. Our study may provide new insights into the search for advanced materials for reversible hydrogen storage.     

Hydrogen storage on calcium-decorated BC7 sheet: A first-principles study

The adsorption behavior of hydrogen molecules on the calcium-decorated BC₇ sheet has been investigated using first-principles calculations. Our calculations demonstrate that the van der Waals interactions are crucial for the hydrogen storage in the calcium-decorated BC₇ sheet. We find that the average adsorption energy per hydrogen molecules decreases with the number of adsorbed hydrogen molecule increasing. When six hydrogen molecules adsorb, the average adsorption energy is 0.26 eV. In this case, the gravimetric density for hydrogen storage on two sides of calcium-decorated BC₇ sheet is about 4.96 wt%. These features indicate that the calcium-decorated BC₇ sheet has potential application in hydrogen storage.     

Lithium,a preliminary survey of its mineral occurrence in flint clay and related rock types in the United States

Maximum concentrations of lithium found in samples of flint clay and associated rocks of Pennsylvanian age in different States, in parts per million (ppm), are: Missouri, 5100; Pennsylvania-Maryland, 2100; Kentucky, 890; Ohio, 660; Alabama, 750; and Illinois, 160. Lithium-bearing kaolin deposits are distributed in the Coastal Plain province from New Jersey to Texas, and one occurs in Idaho; maximum lithium concentrations in samples from these deposits range from 64 to 180 ppm. The maximum concentration found in the Arkansas bauxite region is 460 ppm and that in flint clay in Colorado is 370 ppm. Samples from areas other than Pennsylvania, Maryland, Kentucky and Missouri are relatively few in number, represent mostly commercially valuable clays, and represent only a part of the refractory clay deposits in the United States. Data are not available on the clays associated with these deposits that may be unusable because they contain too much lithium as well as other deleterious elements. In both Pennsylvania and Missouri, lithium contents vary regionally between districts and locally between deposits.In samples containing more than 2000 ppm lithium, the lithium occurs in a dioctahedral chlorite mineral very similar to cookeite, which previously has not been recognized in sedimentary clays. The associated clays consist chiefly of well-crystallized kaolinite. The dioctahedral chlorite, however, seems to be most abundant where diaspore and boehmite occur along with the kaolinite. Barium, chromium, copper, phosphorus and strontium are present in some samples in amounts of several hundred pans per million or more, and may contribute to the failure of some clays to perform satisfactorily in firing tests.Lithium-rich clays could serve as a significant lithium resource in the very distant future. Clays that contain as much as 1% lithium may be common enough in Missouri or in Pennsylvania to be produced as a by-product to help support benefication costs for refractory clays. Sufficient amounts of lithium-rich clay may be found in deposits that have been explored, found unsatisfactory for normal refractory uses, and not developed. The lithium-rich clay in some deposits presently being worked may be worth stockpiling for eventual use.