Expandable Clay Minerals Research Articles

Ecotoxicological evaluation of dredged sediments from Rodrigo de Freitas Lagoon (Rio de Janeiro State, Brazil) using bioassays with earthworms and collembolans

The toxicity and metal bioavailability were studied in dredged sediments from Rodrigo de Freitas Lagoon (Rio de Janeiro, RJ, Brazil) using acute and avoidance tests with Eisenia andrei, and reproduction tests with Folsomia candida. The sediment was mixed with an artificial soil, and two natural soils (ferralsol and chernosol—representative Brazilian tropical soils) to obtain the following doses: 1, 3, 6, 12, 18, 24 and 30%. Total metal concentrations were determined in the sediment to support the interpretation of ecotoxicological data. Metal concentrations in the mixtures were in agreement with the threshold limits established by Brazilian law. However, significant avoidance responses were found on doses ≥ 3% and were the most sensitive endpoint. Earthworm mortality found in artificial soil mixtures (LC50 = 3.9) suggests higher toxicity levels than those obtained in ferralsol (LC50 = 7.6%) and chernosol (11.0%) treatments. Earthworm mortality, avoidance responses and collembolan reproduction levels found in ferralsol mixtures (LC50 = 9.2; avoidance EC50 = 2.3%; reproduction EC50 = 2.8%) were higher compared to chernosol treatments (LC50 = 11.0%; avoidance EC50 = 4.3%; reproduction EC50 = 4.9%). The reduction of toxicity levels in chernosol mixtures is probably due to the abundance of expansive clay minerals in chernosols with capacity of adsorbing metals and other xenobiotic substances from soil pore water, decreasing metal bioavailability. Finally, threshold limits defined by Brazilian legislation for soil quality and land disposal of dredged sediments are not sufficient to prevent noxious effects on soil fauna and should be complemented with a preliminary ecotoxicological evaluation.

Competitive Adsorption of H2O and CO2 in 2-Dimensional Nanoconfinement: GCMD Simulations of Cs- and Ca-Hectorites

The intercalation of H2O, CO2, and other fluid species in expandable clay minerals (smectites) may play a significant role in controlling the behavior of these species in geological carbon sequestration and enhanced petroleum production and has been the subject of intensive study in recent years. This paper reports the results of a computational study of the effects of the properties of the charge-balancing, exchangeable cations on H2O and CO2 intercalation in the smectite mineral, hectorite, in equilibrium with an H2O-saturated supercritical CO2 fluid under reservoir conditions using grand canonical molecular dynamics methods. The results show that the intercalation behavior is greatly different for the cations with relatively low hydration energies and high affinities for CO2 (here Cs+) than for cations with higher hydration energies (here Ca2+). With Cs+, CO2 intercalation occurs in a 1-layer structure and does not require H2O intercalation, whereas with Ca2+, the presence of a sub-monolayer of H2O is ...

Mineralogy and facies variations of Devonian and Carboniferous shales in the Ukrainian Dniepr-Donets Basin

Abstract The mineralogy of Devonian to Carboniferous shales from the Ukrainian Dniepr-Donets Basin (DDB) was investigated during this study. These shales show a high compositional variability in vertical and lateral directions. Furthermore, stratigraphic trends were found to be controlled both by climatic factors as well as by changing detrital input from the hinterland. High kaolinite contents and predominance of kaolinite over illite in the Tournaisian and partly in the lower Visean units are likely a result of intense chemical weathering related to the Hangenberg climatic event at the Devonian/ Tournaisian boundary. In contrast, abnormally high kaolinite contents in upper Visean and Serpukhovian samples at the basin center might be caused by different transport properties of kaolinite and illite, leading to selective concentration of small detrital kaolinite particles, which are often in the sub-micrometer range according to scanning electron microscopy observations. K/Al elemental ratios correlate well with illite/kaolinite ratios for samples in which significant amounts of both clay minerals are present, which enables a pre-evaluation of the relative kaolinite content based on bulk geochemical data. As kaolinite is suggested to decrease the fraccability of shales and to have a great influence on their wetting behaviour, this is useful information for explorational purposes. Higher feldspar contents in Devonian and Tournaisian samples, especially along the NE basin margin and in the shallow NW part of the DDB, are likely related to increased detrital input from magmatic precursors (e.g. in the Voronezh Massif ) during (and shortly after) the active rift stage of the DDB. In general, feldspar contents are higher in proximal positions compared to the basin center, which is likely a result of shorter transport distances of the comparably large feldspar grains. Finally, the presence of expandable clay minerals down to depths of 6 km and the fact that no thermal maturity trend is visible down to these depths, proves, that a low post-depositional heat flow was present in the DDB. This is in good agreement with vitrinite reflectance measurements and thermal modelling results from previous studies, which suggest a low Mesozoic heat flow.

Enhanced irreversible fixation of cesium by wetting and drying cycles in soil.

The retention of radioactive cesium (Cs) in soil is significantly related to the types of clay minerals, while the weathering process affects the irreversible adsorption sites in clay minerals. In this study, the effect of weathering (exposure duration of Cs and repeated wetting and drying cycles) on fractionation of Cs in soils was investigated using fractionation analysis by the sequential extraction. The residual fraction of Cs increased slowly with exposure time but increased rapidly by repeated wetting and drying cycles. XRD analysis shows that a 1.43nm of interlayer size for vermiculite is shortened to 1.00nm, i.e., similar to that of illite. The change implies the potential that the structure of expandable clay minerals is transformed to the non-expandable structure by weathering process after Cs retention. Based on the result, the residual fraction of Cs, most stable form of Cs in the soil, reached relatively rapidly to a maximum. However, the process is much slower kinetically in the field because the bench-scale weathering process used in this study is more aggressive. This study implies that Cs fractionations in the soil are converted into a more stable fraction by weathering processes in the soil. Therefore, Cs removal should be conducted as soon as possible after accidental release of Cs in an environmental side.

Microstructural strength of four subtropical soils evaluated by rheometry: properties, difficulties and opportunities

The structural strength of soils has been extensively described by physical and mechanical properties evaluated on the mesoscale for different soils and management conditions. However, changes in the organization of the soil matrix at the microstructural level, which can be obtained by rheometry, are still seldom used in soil mechanics. Our aim was to use the amplitude sweep test, a rheometry technique, to investigate the microstructural strength of four subtropical soils (two Oxisols, an Ultisol and a Vertisol) and to discuss difficulties with respect to the samples, water content, soil density and vertical force. The various rheological properties which reveal the soil microstructural strength were determined: deformation at the end of the linear viscoelastic range, LVE range (γLVE), shear stress at the end of the LVE range (τLVE), deformation at yield point, YP (γYP), storage and loss moduli at YP (G′G″YP), maximum shear stress (τmax), and integral z. In general, soil elasticity (γLVE and γYP) and microstructural strength (τLVE and τmax) were greater in the Oxisols and the Vertisol, which both possess high clay content, while the latter also contains expansive clay minerals. The lowest structural strength was observed in the Ultisol which had a high sand content. As rheological properties are related to soil properties such as particle size distribution and carbon content, they can be applied in the evaluation of the microstructural strength of clayey and sandy soils and allows for inferences regarding inter-particle shear strength. However, the test is not applicable to very dry soil samples and sample preparations can affect the results. We suggest a number of approaches to find solutions for these difficulties/problems.

Smectite formation upon lime stabilization of expansive marls

Abstract Marl was treated with Mg-rich lime to evaluate the effectiveness of lime stabilization for this expansive, clay- and carbonate-rich material. XRD and TEM analyses revealed very limited dissolution of expandable clay minerals and neoformation of Mg-enriched smectites. C-(A)-S-H phases formed and initially cemented clay particles. However, they experienced decalcification and decomposition over time, leading to disaggregation of clay particles and deterioration of some of the marl's geotechnical properties. Results question the effectiveness of lime stabilization for marls, especially using Mg-rich lime. Modifications to commonly used treatment protocols in order to overcome these shortcomings are discussed.

Changes in technological properties and microstructure of clayey raw materials from the Corumbataí Formation upon drying: Relevance to dry route tilemaking process

Abstract The Ceramic District of Santa Gertrudes (CDSG) is the most important producer of ceramic floor and wall tiles in Brazil. Industries that produce ceramic tiles by dry route process have researched new technologies to dry the Corumbatai Formation (Fm) raw materials. It is due to environmental problem related to the dust emission in the air, produced by beneficiation areas where the raw ceramic materials have been dried. Laboratory experiment and technological test, simulating the current tile production processes in the CDSG, were performed in the present study in order to better understand the role of temperature on clayey rocks from Corumbatai (Fm) and to avoid losses in the current tile production process. Three different illitic raw materials of different degree of weathering and compactness (hard, intermediate and soft) already used to compose the ceramic batch were dried by sunlight and forced drying at 100 °C, 200 °C and 300 °C. Results after forced drying have shown changes at the raw material morphology and size distribution after grinding; also the progressive dehydration of the expandable clay minerals which have caused the plasticity and consistency loss of the raw material and consequently affected the quality of the product. Ceramic bodies presented decreasing linear shrinkage values, loss in their bending strength, and horizontal cracks in some bodies whose raw materials were forced to dry at 200 °C and 300 °C.

Impact of Salinity and Mineralogy on Slick Water Spontaneous Imbibition and Formation Strength in Shale

Spontaneous imbibition is one of the mechanisms proposed to reveal the fate of fluids used during fracturing operations in shale reservoirs. However, the influence of salinity and mineralogy on the imbibition kinetics is not yet well understood. We performed imbibition experiments on samples collected from Woodford and Caney shale outcrops with slick water having salinities up to 15 wt % KCl. The impact of salinity on the imbibition rate and capacity shows a different trend for samples of varying mineralogy. A correlation between the carbonate/clay ratio and the imbibition rate is observed. This ratio and the illite content were linked to be factors affecting the impact of salinity on imbibition. This correlation suggests a maximum volume of imbibition at a ratio of 1.5. The impact of salinity on the formation strength seems to correlate with the amount of siderite and anhydrite present in the rock. It is worth noting that the swelling of the expansive clay minerals could mask the imbibition signature of ...

Spatial Microstructural Ordering of Expansive Clay Minerals

The volumetric change within soils containing expanding clays depends upon their spatial ordering and their mineralogy. So this work studies the spatial microstructural ordering and mineralogy of different expanding clays in order to correlate them with some of their geotechnical properties (plasticity, classification, and expansion). The clay microstructure was obtained using X-ray diffraction, and sweeping electronic microscopy. Physical characterization includes pH analysis. Clays with greater swelling (12.1–14.6% expansion) present greater agglomeration (gathering) (20–50 microns) in a flocculated structure that mainly contained sodic montmorillonite with 14–22 Å openings and 8.28 average pH, which indicates high electron concentrations with border-face interactions. Clays with the least expansion (7.6%) present lower agglomerations (5–10 microns) with face-to-face contacts within a dispersed structure that mainly contained calcium montmorillonite with openings lower than 15 Å and pH 7.56, which indicates a low electron concentration and face-to-face interaction. Major clay expansion corresponds to a greater size agglomerates, greater opening between particles, and greater pH presenting border-face and face-to-face interactions of a flocculent structure. Yet, the soils with lower expansion only present face-to-face interactions corresponding to a dispersed structure.

A review on the geotechnical and engineering characteristics of marine clay and the modern methods of improvements

Marine clay is a soft soil that could be found widely at the coastal and offshore areas. This type of soil is usually associated with high settlement and instability, poor soil properties that are not suitable for engineering requirements and low unconfined compressive strength of less than 20 kPa. Considerable failure could occur even with light loads and it shows flat or featureless surface. This kind of soil is considered as problematic due to the existence of high moisture content and usually exists as slurry with noticeable percentage of expandable clay minerals. In this paper, the geotechnical, micro-structure and engineering properties of marine clay are thoroughly reviewed and discussed. The properties include moisture content, particle size distribution, specific gravity, Atterberg limits, mineral compositions and shear strength. Moreover, due to the increasing demand of construction at coastal and offshore areas involving the marine clay, many attempts have been made to stabilize this kind of soil in order to solve the geotechnical related problems. Some of the common stabilization methods used to improve the properties of marine clay such as cement grouting, chemical additives and some environmental friendly additives are discussed. In long term, marine clay treatment using cement was found to be the best method. In addition, this paper serves as a guideline for the design and construction of projects on marine soils.

Mechanisms of stabilization of expansive soil with lignosulfonate admixture

Abstract This study investigated and identified the mechanisms by which a remoulded expansive soil was modified or altered by a non-traditional admixture, lignosulfonate (LS). To achieve this objective, untreated and LS treated samples of expansive soil were examined microscopically using X-ray Diffraction (XRD), a Scanning Electron Microscope coupled with Energy Dispersive Spectroscopy (SEM/EDS), Fourier Transform Infrared (FTIR), Computed Tomography (CT-Scan), Nuclear Magnetic Resonance (NMR), Cation Exchange Capacity (CEC), and the role of Specific Surface Area (SSA). The interest was to identify and compare any physical and chemical changes between the untreated and treated samples and then propose the most likely reaction modes between the admixture and the soil minerals. The results indicated that the percent swell is intimately related to the amount of water that is adsorbed by the expansive clay minerals. Furthermore, the amount of moisture in an expansive soil is influenced by a small addition of organic (cationic) compound such as LS. The adsorption of LS on the mineral surfaces provided waterproofing effect on soil due to the hydrophobic nature of LS, which in turn contributed to a decrease in the extent of swelling of the otherwise expansive soil. The basal and peripheral adsorption of LS led to smearing and subsequent agglomeration of soil particles restricting water ingress into the soil body. In addition, the cationic exchange between the admixture and the soil particle surfaces (i.e. replacing the negative surfaces on clay lattices) prompted flocculation, which further decreased the soil’s affinity to water.

膨潤性粘土鉱物の特徴 : 鉱物としての見方(地盤工学会・技術者交流特別セッション)

膨潤性粘土鉱物の特徴 : 鉱物としての見方(地盤工学会・技術者交流特別セッション)

Field level digital mapping of soil mineralogy using proximal and remote‐sensed data

AbstractPrimary (e.g., quartz) and secondary (clay) minerals are key factors determining the physical and chemical characteristics of soil. Understanding spatial distribution of minerals at the field scale would, therefore, be of potential benefit for soil management. However, current analysis requires time‐consuming laboratory procedures and computational quantification analysis (e.g., SIROQUANT). Furthermore, mineral composition (e.g., quartz, kaolinite, illite and expandable clay minerals) must sum to 100. We aimed to add value to laboratory data by developing multiple linear regression (MLR) relationships between mineralogy and ancillary data such as digital numbers (DNs) (i.e., Red [R], Green [G] and Blue [B]) acquired from remotely sensed air‐photographs and soil apparent electrical conductivity (ECa – mS/m) measured from proximal sensing electromagnetic (EM) instruments (i.e., EM38 and EM31). To account for composition, we compare results from the MLR approach with those from additive log‐ratio (ALR) transformation of mineralogy prior to MLR modelling. This approach together with various ancillary data and trend surface parameters (i.e., scaled Easting and Northing) has greater precision and less bias of prediction than the MLR approach using untransformed data. Our approach also enables predictions to sum to 100. We conclude that the most useful ancillary data to predict the abundance of quartz, kaolinite and illite are B DNs and EM31, while expandable clays are best predicted with R DNs, EM38 and scaled Northing. The use of ancillary data to map mineralogical components combined with ALR‐MLR is an effective approach, with resulting maps providing insights into soil and water management issues consistent with farmer experience.

Artificially induced clay mineral authigenesis in an underground gas storage field, North Alpine Foreland Basin, Austria

ABSTRACT Secondary processes within reservoir sandstones during and after hydrocarbon production are poorly understood. This study looks at the effect of secondary water fill on a sandstone reservoir within a time span of 8 yr. The reservoir rocks consist of medium-grained litharenites with large clasts of shales and carbonates. They originate from a depleted gas reservoir that has been converted into an underground storage field for natural gas. Gas production resulted in a rise of the gas–water contact of approximately 30 m (98 ft). Based on their initial and final gas and water saturations, four zones can be identified. Observed diagenetic changes in all four zones include carbonate cementation, K-feldspar overgrowths, authigenic quartz overgrowths, pyrite formation, and poorly crystallized authigenic clay minerals. However, the authigenic clay mineral fraction differs significantly within the zones. Total clay mineral content and crystallinities of smectite, chlorite, kaolinite, and illite increase from the gas-bearing to the initial water zone. Additionally, expandable clay minerals and kaolinite were not identified in the gas-bearing zone. This is different in the secondary watered zones, where smectites and kaolinites are developing. The study shows that within a maximum of 8 yr from the start of water influx into the gas zone, new clay minerals are forming. The porosity and permeability reduction caused by this artificially induced process might continue and could also be of relevance within producing reservoirs, where water saturation increases during production.

Stabilization of Algerian Clayey Soils with Natural Pozzolana and Lime

Cohesive soils with a high plasticity index present difficulties in construction operations because they usually contain expansive clay minerals. However, the engineering properties of soils can be improved by different techniques. The aim of this paper is to study the effect of using lime, natural pozzolana or a combination of both lime and natural pozzolana on plasticity, compaction and shear strength of two clayey soils classified as CH and CL according to the unified soil classification system (USCS). The obtained results indicated that for CH class clay soil, the plasticity index decreased significantly for samples stabilized with lime. On the other hand, for the soil classified as CL class clay, a high decrease in the plasticity index value was observed for samples stabilized with natural pozzolana compared to those stabilized with lime. Also, both the cohesion and internal friction angle in lime added samples were demonstrated to increase with time. The combination of lime and natural pozzolana exhibits a significant effect on the enhancement of both the cohesion and internal friction angle at later stages. The lime-natural pozzolana combination appears to produce higher shear strength parameters than lime or natural pozzolana used alone.

Mechanism of the Mudstone Tunnel Failures Induced by Expansive Clay Minerals

Expansive clay minerals can be a serious threat to underground rock structure because of their swelling behavior when absorbing water. Roof and wall collapse as well as large deformation were observed in Xiaotun Coal Mine, Guizhou Province, China. This paper studies the characteristics of expansive clays in order to understand the mechanism of the mudstone tunnel failures in the mine. The physical and mechanical properties of the tunnel surrounding rock samples, including mudstone, silty mudstone, argillaceous siltstone and fine sandstone were determined. X-ray diffraction analysis was conducted to determine the mineralogical composition while the scanning electron microscope was used to examine the internal structure of the different samples. The results indicate that the illite–smectite and the montmorillonite are the main minerals composing the rock samples. A series of micro-cracks and pores occur in the samples which suggest a good hydraulic conductivity. The results indicate that the mudstone has poorer mechanical properties in comparison to the fine sandstone. Also, the Flac3D numerical simulations were conducted and it was shown that the large deformations were consistent with the field observation due to weak mechanical properties of the surrounding rock under seepage action especially with the increase of water head and porosity. It is concluded that internal structure and mechanical strength of the mudstone is weakened by the illite–smectite and the montmorillonite mineral content as well as the combined action of underground water causing physical disintegration.

Evaluation of the particle sizes of four clay minerals

The particle size distributions (PSD) of four clay minerals from the US Clay Minerals Society, kaolinite, illite, Ca2+-montmorillonite, and Na+-montmorillonite, were analyzed using different dispersion techniques to evaluate their PSD variability and intrinsic particle sizes. Both dry dispersion in pressurized air and wet dispersion in water and ethyl alcohol with and without an interlayer expanding agent, glycerol, were employed, along with mechanical ultrasound disaggregation and chemical dispersion. Results show that all four clay minerals exhibit multimodal PSD consisting of superimposed subordinate lognormal distributions from submicron and 1–2μm primary particles, 10–20μm flocculi (and trace impurities). Microflocs of 50–500μm occur only in the dry dispersion mode. Owing to the presence of relatively strong flocculi that are not prone to breakdown to smaller primary particles, the intrinsic PSD of these clay minerals cannot be readily obtained by the adopted dispersion methods. The two swelling clay minerals, Na+-montmorillonite and Ca2+-montmorillonite, are more sensitive than the non-swelling ones, kaolinite and illite, to different dispersion solvents. The variability of these clay minerals' PSD is further discussed in terms of the complex interactions among clay mineral particles, polar or less polar solvent, and dispersant, such as swelling, cation exchange, exfoliation, and electrical double layer repulsion. A significant practical implication is that most existing dispersion methods for PSD analysis tend to overestimate the silt-sized fraction but underestimate the clay-sized fraction, which may yield misleading classification for clay mineral-rich soils. For expansive clay minerals, appropriate dispersants should be selected to avoid cation exchange between the sample and dispersants, which can change their intrinsic PSD.

Dual liquid porosimetry: A porosity measurement technique for oil- and gas-bearing shales

Measuring and differentiating effective and ineffective porosity in oil- and gas-shale remains a challenging step in the calculation of hydrocarbon reserves. Shale composition, typically dominated by clay minerals and organic matter (OM), forms a complex pore system, making the proportion between effective and ineffective porosity variable and difficult to determine in the course of porosity measurement.In this study, total porosity was measured from a set of rock chips using both light kerosene (kerosene immersion porosimetry-KIP) and deionized (DI) water (water immersion porosimetry-WIP) as saturation-immersion fluids. Both KIP and WIP measurements were performed on samples equilibrated at 40% and 80% relative humidity (RH) and combined with water adsorption isotherms to estimate the range of clay-bound water (CBW). A combination of these procedures, and the use of kerosene and water as the saturation-immersion fluids, is hereafter called dual-liquid porosimetry (DLP). To test the method, two different burial diagenetic sequences of mudstone with different mineralogy and OM content were used.The results demonstrate that all the porosity values show a consistent declining trend with depth regardless of sample origin and type of saturation-immersion fluid used in the measurement process. Samples saturated with DI water were susceptible to swelling and the swelling decreased with sample depth, maturation and cementation, and increased with illite-smectite mineral content and degree of expandability. A consistently lower calculated grain density for KIP samples compared with WIP samples reflected incomplete pore saturation by kerosene. Kerosene saturation can be restricted by the presence of residual water that blocks kerosene pathways to small pores. This limitation, however, reflects geologic formation conditions making KIP measurement a good approximation for maximum liquid hydrocarbon-available porosity. Equilibrating the rock chips at 40% and 80% RH provides a rapid evaluation of the range of CBW that may represent ineffective porosity under various formation conditions. CBWmin (40% RH) constitutes roughly 1W–2W smectite equivalent layers and gives an estimation of bound water close to high hydrocarbon saturation. CBWmax (80% RH) represents the maximum possible bound water content in formations with higher water saturation. For samples, with low porosity and a significant content of expandable clay minerals and cation exchange capacity, CBW can constitute up to 100% of rock porosity.In order to avoid the limitations and assumptions of WIP (swelling) and KIP (incomplete saturation of nanopores), a combination of grain density and bulk density measured by WIP and KIP, respectively, was proposed. The paper further tests and discusses the pitfalls of saturation-immersion techniques.

Shale gas/shale oil potential of Upper Visean Black Shales in the Dniepr-Donets Basin (Ukraine)

Upper Visean Rudov Beds (V-23) are considered as one of the main source rocks within the northwestern Dniepr Donets Basin (DDB). Average TOC values within the Srebnen Bay (northwestern DDB) exceed 4% (thickness up to 100 m). Average TOC contents north and northwest of the Srebnen Bay (up to 60 m) are significantly lower (2–4%) but still sufficient for unconventional hydrocarbon production, whereas wells along the northeastern basin margin do not hold a shale oil/gas potential (<2% TOC). Insufficient information is available from positions along the basin axis in the central DDB, mainly because of considerable burial depths >5.5 km.Although hydrogen index (HI) values of the most immature samples are ≤300 mgHC/gTOC, basinal Rudov Beds host a potential to generate Paraffinic-Naphthenic-Aromatic (P-N-A) low wax oil according to pyrolysis experiments. A basinward maturity trend is visible, but the maturity cut-offs for shale oil/gas production (>0.8%Rr/>1.2%Rr) are reached at great depths (>4.5 km/5.5 km) only. Furthermore, results of kinetic experiments do not argue for presence of type IIS kerogen potentially triggering earlier hydrocarbon generation.The mineralogy of Rudov Beds varies strongly in lateral and vertical directions, limiting the predictability of mechanical behaviour. Samples from the siliceous (basinal) facies partly contain more than 60% of brittle minerals (quartz, feldspar, carbonates, etc.) suggesting a good fraccability. This value is rarely reached by samples from the transitional and lagoonal (calcareous and clayey) facies. The clay mineral fraction within many investigated Rudov samples is dominated by kaolinite, which is suggested to diminish the fraccability of shales. Apart from that, expandable clay minerals (ECM) are present in samples from more than 5 km depth, a result of low Mesozoic heat flow.

Unearthing the Antibacterial Mechanism of Medicinal Clay: A Geochemical Approach to Combating Antibiotic Resistance.

Natural antibacterial clays, when hydrated and applied topically, kill human pathogens including antibiotic resistant strains proliferating worldwide. Only certain clays are bactericidal; those containing soluble reduced metals and expandable clay minerals that absorb cations, providing a capacity for extended metal release and production of toxic hydroxyl radicals. Here we show the critical antibacterial components are soluble Fe2+ and Al3+ that synergistically attack multiple cellular systems in pathogens normally growth-limited by Fe supply. This geochemical process is more effective than metal solutions alone and provides an alternative antibacterial strategy to traditional antibiotics. Advanced bioimaging methods and genetic show that Al3+ misfolds cell membrane proteins, while Fe2+ evokes membrane oxidation and enters the cytoplasm inflicting hydroxyl radical attack on intracellular proteins and DNA. The lethal reaction precipitates Fe3+-oxides as biomolecular damage proceeds. Discovery of this bactericidal mechanism demonstrated by natural clays should guide designs of new mineral-based antibacterial agents.