Showing papers on "Clay minerals published in 2000"

Weathering of rocks induced by lichen colonization — a review

Abstract: The evidence presented by numerous investigations of the interface between lichens and their rock substrates strongly suggests that the weathering of minerals can be accelerated by the growth of at least some lichen species. The effects of lichens on their mineral substrates can be attributed to both physical and chemical processes. The physical effects are reflected by the mechanical disruption of rocks caused by hyphal penetration, expansion and contraction of lichen thallus, swelling action of the organic and inorganic salts originating from lichen activity. Lichens also have significant impact in the chemical weathering of rocks by the excretion of various organic acids, particularly oxalic acid, which can effectively dissolve minerals and chelate metallic cations. As a result of the weathering induced by lichens, many rock-forming minerals exhibit extensive surface corrosion. The precipitation of poorly ordered iron oxides and amorphous alumino-silica gels, the neoformation of crystalline metal oxalates and secondary clay minerals have been frequently identified in a variety of rocks colonized by lichens in nature. For a better understanding of the impacts of lichens on environments, further work on the comprehensive involvement of the lichen effects on weathering of natural rocks, deterioration of building stones and stonework, and formation of primitive soils should be carried out.

Adsorption, desorption and oxidation of arsenic affected by clay minerals and aging process

Abstract: Adsorption/desorption and oxidation/reduction of arsenic at clay surfaces are very important to the natural attenuation of arsenic in the subsurface environment. Although numerous studies have concluded that iron oxides have high affinities for the adsorption of As(V), very little experimental work has addressed the arsenic attenuation capacities of different clay minerals and aging process affecting the transformation of arsenic. The abundance of clay minerals in a variety of geochemical environments and their influence on adsorption of contaminants suggests a need for more experimental work to characterize the adsorption desorption, and oxidation of arsenic on clay minerals. In this investigation three types of clay mineral were studied: the 1 : 1 layer clays [halloysite (IN), sedimentary M-kaolinite, and weathered EPK-kaolinite]; the 2 : 1 layer clays [illite (MT) and illite/montmorillonite (MT)]; the 2 :>: 1 layer clay [chlorite (CA)]. The halloysite and the chlorite had much greater As(V) adsorption (25–35 folds) than the other clay minerals. The clay minerals had lower As(III) adsorption than As(V) adsorption, and the adsorption was affected by pH. Desorption of arsenic from the clay minerals was significantly influenced by the aging process. The quantities of extractable As(III) and As(V) decreased with increasing aging time. The results demonstrated that oxidation of As(III) to As(V) occurred on the clay surfaces, whereas reduction of As(V) to As(III) was not found in any of the clay minerals studied. The oxidation of As(III) was affected by the types of clay and aging time.

Hydrothermal Minerals and Precious Metals in the Broadlands-Ohaaki Geothermal System: Implications for Understanding Low-Sulfidation Epithermal Environments

Abstract: The Broadlands-Ohaaki geothermal system is a boiling hydrothermal system hosted by a sequence of Quaternary felsic volcanic rocks and Mesozoic metasediments. More than 50 wells have been drilled (400 to >2,600 m deep) to assess the geothermal potential for the production of electricity. Fluids and precipitates sampled from wells, along with descriptions of the alteration minerals in more than 500 drill cores, provide a three-dimensional picture of the distribution of fluid types and secondary minerals. Interpretation of these features and the distribution of gold and silver highlight the relationship between alteration and mineralization in an active, low-sulfidation epithermal environment. Quartz, illite, K feldspar (adularia), albite, chlorite, calcite, and pyrite are the main hydrothermal minerals that occur in the deep central upflow zone at >250°C and> 600 m depth. These minerals form through recrystallization of the volcanic host rocks and incorporation of H 2 O, CO 2 , and H 2 S in the presence of a deeply derived chloride water containing ~1,000 mg/kg Cl and ~26,400 mg/kg CO 2 . At the same time, and on the periphery of the upflow zone, illite, smectite, calcite, and siderite form through hydrolitic alteration in the presence of CO 2 -rich steam-heated waters that contain 2 . Upward and outward from the deep central upflow zone, mineral patterns reflect the shift from rock-dominated to fluid-dominated alteration and the prevailing influence of boiling, mixing, and cooling on fluid-mineral equilibria. Accordingly, the abundance of quartz and K feldspar increase toward the upflow zone, whereas clay abundance increases toward the margin of the upflow zone (with smectite dominating at 200°C); the abundance of chlorite, pyrite, and calcite varies here, but albite is absent. Geothermal production wells with high fluid fluxes are the main sites of precious-metal mineralization. The deep chloride water (with or without minor amounts of vapor) enters the well at depths >500 m and undergoes a pressure drop that causes boiling. As a result, precious metals precipitate and accumulate as scales on back-pressure plates or as detritus in surface weir boxes; these deposits contain 1,000 mg/kg Au, 10,000 mg/kg Ag and ~10 to ~1,000 mg/kg As and Sb, each. Within production wells, platy calcite deposits as a scale at the site of first boiling near the fluid feed point, while crustiform-colloform-banded amorphous silica deposits in surface pipe work. By contrast, the hydrothermally altered host rocks contain low concentrations of gold, ranging from <0.01 to 1.0 mg/kg Au (68 analyses), and these correlate positively with arsenic (<100 to ~5,000 mg/kg) and antimony ( Reaction path modeling using SOLVEQ and CHILLER shows that calcite, K feldspar, gold, and amorphous silica deposit in sequence from a chloride water that cools along an adiabatic boiling path (300° to 100°C), analogous to fluid flow in a production well. By contrast, calcite, quartz, K mica, and pyrite deposit from a chloride water that cools due to mixing with CO 2 -rich steam-heated waters; dilution prevents precipitation of precious metals. Thus field observations and reaction path modeling demonstrate that boiling is the main process influencing the deposition of precious metals. The results of this study show how peripheral hydrolytic alteration by CO 2 -rich steam-heated waters relate to propylitic and potassic alteration by chloride waters in the epithermal environment of a hydrothermal system. Both the distribution of alteration mineral assemblages associated with the different water types and the broad-scale distribution of temperature-sensitive smectite and illite reflect the location of the upflow zone. On a local scale, the occurrence of platy calcite, crustiform-colloform silica, and K feldspar in veins indicates the existence of boiling conditions conducive to precious-metal deposition.

Geology and characterization of two hydrothermal nontronites from weathered metamorphic rocks at the uley graphite mine, south australia

Abstract: Mining operations during the early 1990s at Uley Graphite Mine near Port Lincoln on southern Eyre Peninsula, South Australia, uncovered abundant nontronite veins in deeply weathered granulite facies schist, gneiss, and amphibolite of Palaeoproterozoic age. Two types of nontronite are present: a bright yellowish-green clay (NAu-1) distributed as veinlets and diffuse alteration zones within kaolinized schist and gneiss, and a massive to earthy, dark-brown clay (NAu-2) infilling fracture networks mainly in amphibolite or basic granulite. The nontronites are the product of low-temperature hydrothermal alteration of primary minerals, biotite, and amphibole. The principal chemical difference between NAu-1 and NAu-2 is a higher alumina content in NAu-1, which was either inherited during hydrothermal alteration of biotite in the host rock or acquired through recrystallization of nontronite during subsequent weathering and associated kaolinization. Sufficient bulk samples of both NAu-1 and NAu-2 were collected to supplement reference nontronite of the Source Clay Repository of The Clay Minerals Society. The clay fraction of the bulk samples is typically >85%. NAu-1 contains minor kaolin and quartz which are easily removed to give a high purity nontronite of composition M + 1.05 [Si 6.98 Al 1.02 ][Al 0.29 Fe 3.68 Mg 0.04 ]O 20 (OH) 4 , similar to that of nontronite from Garfield, Washington. NAu-2 contains fewer total impurities but the presence of trace amounts of submicron carbonate and iron oxyhydroxide requires additional chemical treatment to produce a nontronite of purity comparable to NAu-1. Composition of NAu-2 was calculated as M + 0.72 [Si 7.55 Al 0.45 ][Fe 3.83 Mg 0.05 ]O 20 (OH) 4 , although infrared data indicate that at least some Fe is in tetrahedral coordination.

Conformation heterogeneity and mobility of surfactant molecules in intercalated clay minerals studied by solid-state NMR

Abstract: The conformation heterogeneity and mobility of surfactant molecules in intercalated montmorillonite clay minerals have been investigated using variable-temperature solid-state 13C NMR and two-dimensional proton wide-line separation (2D WISE) 1H−13C NMR spectroscopy. Previous FTIR studies by Vaia et al. for the first time revealed the existence of a gauche conformation of surfactant molecules in clay minerals and further illustrated the transition from an ordered conformation, to a liquid crystalline state, to completely liquidlike behavior. The NMR study reported in this paper clearly demonstrates the coexistence of order and disordered chain conformations. Two main resonance peaks are resolved and associated with the backbone alkyl chains: the resonance at 33 ppm corresponds to the ordered conformation (all-trans), and the resonance at 30 ppm corresponds to the disordered conformation (mixture of trans and gauche). The NMR technique allows detailed characterization and quantification of the conformation...

Formation of Layered Single- and Double-Metal Hydroxide Precipitates at the Mineral/Water Interface: A Multiple-Scattering XAFS Analysis

Abstract: Spectroscopic and microscopic studies have shown that Ni and Co sorption by clay minerals may proceed via formation of surface precipitates. Several studies employing X-ray absorption fine structure (XAFS) spectroscopy suggested the formation of turbostratic, alpha-type metal hydroxides, of layered double hydroxides (LDH) with Al-for-metal substitution, and of 1:1 or 2:1 phyllosilicates. Distinction of these phases is difficult because they have low crystallinity and/or a small mass compared to the sorbents, and because they have similar metal-metal distances in their hydroxide layers/sheets. Distinction of these phases is crucial, however, because they have substantially differing solubilities. In this paper we show that an XAFS beat pattern at about 8 A(-1) can be used as a fingerprint to unequivocally distinguish LDH from the alpha-type hydroxides and phyllosilicates. Full multiple-scattering simulations and experimental spectra of model compounds indicate that the beat pattern is due to focused multiple scattering at Me/Al ratios between 1 and 4 (Me=Ni, Co). By applying the fingerprint method to new and to already published XAFS data on Ni and Co surface precipitates, we found that LDH preferentially forms in the presence of the Al-containing sorbents pyrophyllite, illite, kaolinite, gibbsite, and alumina above pH 7.0. However, alpha-type metal hydroxides form in the presence of the Al-free sorbents talc, silica, and rutile, and in the presence of the Al-containing clay minerals montmorillonite and vermiculite. We believe that the high permanent charge of these latter minerals prevents or retards the release of Al. When Al is available, the formation of LDH seems to be thermodynamically and/or kinetically favored over the formation of alpha-type hydroxides. Copyright 2000 Academic Press.

'Clay hutches': a novel interaction between bacteria and clay minerals

Abstract: Biofilm formation on a low-energy substratum floating on the surface of a water column overlying a polychlorinated biphenyl (PCB)-contaminated sandy clay soil was followed by light and electron microscopy. The biofilms that developed consisted of a dense lawn of clay aggregates, each one of which contained one or more bacteria, phyllosilicates and grains of iron oxide material, all held together by bacterial extracellular polysaccharides (EPS). The clay leaflets were arranged in the form of 'houses of cards' and gave the aggregates the appearance of 'hutches' housing the bacteria. Interestingly, although the soil is poor in carbon, and the weakly bioavailable PCBs constitute the principal source of carbon in this system, the bacteria contained electron-transparent structures presumed to be carbon storage granules. These, and the EPS material present in the hutches, indicate that carbon is not limiting in this system and, as PCBs have been found associated with the clay mineral fraction of the floating substratum, the clay particles may serve as carbon shuttles. The interesting possibilities that the 'clay hutches' may represent a 'soil microhabitat', a 'minimal nutritional sphere' and an 'effective survival unit' for autochthonous bacteria are noted. The formation of clay hutches by bacteria would seem to merit further investigation, particularly regarding their roles in bacterial processes in soil and in geological processes.

Adsorption of Cesium by Synthetic Clay−Organic Matter Complexes: Effect of the Nature of Organic Polymers

Abstract: The strong correlation observed in situ between the bioavailability of radiocesium and soil organic matter content is paradoxical given the lack of coordination chemistry of cesium and its strong affinity for clay minerals. The aim of this work is to determine whether the adsorption of soil polymers, a protein and a polysaccharide, on reference clay minerals decreases cesium adsorption. Measurements were carried out in dilute suspensions of montmorillonite or illite, varying the initial polymer concentration, pH, and ionic strength. Planar clay surfaces were also blocked to elucidate the nature of the Cs adsorption sites affected. Both polymers caused a decrease in Cs adsorption, by up to an order of magnitude, particularly on illite and for trace concentrations of Cs. The effect increased with increasing polymer adsorbed on the clay surface but varied with experimental conditions so that there is no correlation between the quantity of polymer adsorbed and decreased Cs adsorption. Polymers appear to effec...

Estimation of soil weathering stage and acid neutralizing capacity in a toposequence Luvisol-Cambisol on loess under deciduous forest in Belgium

Abstract: Soils derived from loess are extensive in Europe and are well suited for forestry. They are suspected to be poor acid buffers, however. We have estimated the weathering stage and acid neutralizing capacity of acid soils under forest in a toposequence on loess in the Belgian silt belt. The soils vary distinctly in morphology and physico-chemical properties according to their topographic position. Dystric Cambisols have developed in colluvial deposits in the dry valley floors, whereas Dystric Luvisols have formed on the slopes in a rejuvenated material. The Cambisols are more acid and less saturated in bases than are Luvisols. They are strongly depleted of clay and contain less weatherable minerals. Easily weatherable minerals are concentrated mainly in the clay fraction of both soil types. Clay minerals of size < 2 mu m therefore act as major sinks for protons in these soils. A simplified expression taking into account the total reserve in bases, total aluminium and iron occluded in silicates is used to estimate acid neutralizing capacity. Our estimates confirm that these acid loessic soils are indeed poor acid buffers. They show that the Dystric Cambisols depleted of clay are sensitive to potential acidification, whether natural or man-made.

Calculation of thermodynamic properties of hydrated borates by group contribution method

Abstract: General equations to correlate and predict the thermodynamic properties of hydrated borates were developed based on the experimental results according to their structural types. The thermodynamic properties (ΔH f 0 and ΔG f 0) of a hydrated borate phase are the sum of the contributions of the cations in aqueous solution, the borate polyanions, and the structural water to the corresponding thermodynamic properties. This method is called the group contribution method, and it is extensively used to calculate the thermodynamic properties of many kinds of inorganic compounds, such as silicates and clay minerals.

The Effect of Mechanochemical Treatments on Clay Minerals and the Mechanochemical Adsorption of Organic Materials onto Clay Minerals

Abstract: The present paper reviews investigations on the mechanochemical reactions occurring during the grinding of kaolinite and the mechanochemical adsorption of polar organic compounds by expanding and nonexpanding clay minerals, carried out in our laboratory by IR spectroscopy, supplemented by XRD and TG-DTA techniques Kaolinite was ground in the absence and presence of diluents (alkali halides) and IR spectra were recorded after different grinding periods The mechanochemical reactions can be classified into four groups, diffusion of atoms (mainly protons, "prototropy"), delamination, layer breakdown, and adsorption of atmospheric water by the amorphous product In the mechanochemical adsorption study of phenol and dichlorophenol by smectites (montmorillonite, beidellite, saponite, and Laponite), the mineral was ground with excess phenol Spectra of ground phenol–smectite mixtures were recorded in KBr disks, after leaving the disks for 3 days in a vacuum furnace at 115°C for the evolution of the excess phenol In most cases, the first 5 min of grinding led to an increase in the adsorption of phenol An additional 5 min of grinding resulted in a decrease of the phenol adsorption In the mechanochemical adsorption study of stearic acid by nonexpanding clay minerals (sepiolite, playgorskite, talc, pyrophyllite, and allophane), the mineral was treated with molten acid Disks were prepared with NaCl, KCl, and CsCl Each disk was crushed, reground, and repressed an additional four times Grinding the products converted some of the adsorbed acid into the stearate anion

The role of clay minerals and the effect of H+ ions on removal of heavy metal (Pb2+) from contaminated soils

Abstract: The importance of the surface charge of clay minerals (fixed or variable) and the effect of H+ ions on the adsorption and removal of Pb2+ ions from contaminated soil are investigated using kaolinit...

NMR Relaxation of Clay/Brine Mixtures

Abstract: Effective interpretation of nuclear magnetic resonance (NMR) logs in shaly sands requires an understanding of the NMR contribution of clays Of particular importance is the role of clays in the rapidly relaxing part of the NMR signal In this study we measured the transverse relaxation time spectrum (T 2 ) of brine mixed with four clays (illite, smectite, kaolinite and glauconite) as a function of compaction The Larmor frequency was 2 MHz and the echo spacing 016 msec Mild compaction was achieved by centrifuging the clay slurry at three successive pressures ranging from I to 125 psi Highly compacted samples were produced in a uniaxial press at six sequential pressures ranging from 500 to 16,000 psi Each clay-brine slurry and its associated compacted sample showed a single peak in the T 2 distribution spectrum A second peak, which could be interpreted as the clay-bound water, was never observed The T 2 peak position shifted to faster relaxation times as compaction increased, in proportion to the pore volume-to-surface ratio, V p /S The single peak and V p /S proportionality are consistent with fast diffusion between the pore water and the monolayer of water on the clay surface Surface relaxivity varied among the four clay minerals; glauconite, the clay with the highest magnetic susceptibility and iron content had the largest surface relaxivity These results have important implications for the interpretation of NMR logs in shaly sands Because of the effects of compaction and to a lesser extent the iron content on a clay's T 2 peak position, it is not possible to independently determine clay type from some characteristic relaxation time These data also imply that it is not feasible to estimate the cation exchange capacity from a single time cutoff of the T 2 distribution without additional information such as laboratory measurements or other log data

Experimental hydrothermal alteration of a Martian analog basalt: Implications for Martian meteorites

Abstract: — A number of martian meteorite samples contain secondary alteration minerals such as Ca-Mg-Fe carbonates, Fe oxides, and clay minerals. These mineral assemblages hint at hydrothermal processes occurring in the martian crust, but the alteration conditions are poorly constrained. This study presents the results of experiments that examined the alteration of a high-Fe basalt by CO2-saturated aqueous fluids at 23 and 75 °C and by mixed H2O-CO2 vapors at 200 and 400 °C and water-rock ratios of 1:1 and 1:10. Results indicate that observable alteration of the basalt takes place after runs of only seven days. This alteration includes mobilization of silica into phases such as opal-CT and quartz, as well as the formation of carbonates, oxides, and at some conditions, zeolites and hydrous silicates. The degree of alteration increases with run temperature and, in high-temperature vapor experiments, with increasing water content of the vapor. The degree of alteration and the mineralogy observed in the martian meteorites suggests that none of these samples were exposed to aqueous fluids for long periods of time. Nakhla and Lafayette probably interacted with water for relatively brief periods of time; if so, silica may have been leached from the parent rocks by the altering fluids. Allan Hills 84001 shows possible evidence for very limited interaction with an aqueous fluid, but the overall slight degree of alteration described for this meteorite strongly suggests that it never interacted extensively or at high temperature with any water-bearing fluid. Elephant Moraine A79001 may not have been altered by aqueous fluids at all. The results of this study best support models wherein the meteorite parent rocks were wetted intermittently or for brief periods of time rather than models that invoke long-term reaction with large volumes of water. Our experiments studied alteration of a high-Fe basalt by dilute, CO2-saturated, aqueous solutions at 23 and 75 °C and by mixed H2O-CO2 vapors at 200 and 400 °C. The results suggest that alteration of the parent rock takes place even after very short reaction times of seven days. All experiments produced carbonate minerals, including calcite, and in some cases, magnesite, siderite, and ankerite. A free silica phase, either opal, quartz, or hydrated silica, formed in most experiments. More altered experiments also contained minerals such as zeolites and hydrous phyllosilicates. Clay minerals were not observed to form in any experiments. In aqueous fluids, higher temperature corresponded with a higher degree of alteration, whereas changing fluid composition had no observable effect. In high-temperature vapors, the degree of alteration was controlled by temperature and the proportion of H2O to CO2, with water-rock ratio also playing a role in transport of silica. Application of these results to martian meteorites that contain secondary alteration minerals suggests that none of the martian rocks underwent extensive interaction with aqueous fluids. Nakhla and Lafayette contain clay minerals, which suggests that they interacted with water to some extent, possibly at elevated temperatures. Although ALH84001 shows possible evidence of very limited interaction with aqueous fluids, EETA79001 does not. These results support models for the alteration of these meteorites that do not invoke long-term interaction with water or reaction with large volumes of water. Except for some models for alteration of ALH84001, this conclusion agrees with most of the literature on alteration of martian meteorites.

Synthesis and Characterization of Magnetically Modified Clay Composites

Abstract: The interaction of acetic acid vapors with iron-exchanged montmorillonites leads to iron acetate intercalates which upon pyrolysis transform to magnetic γ-iron oxide nanoparticles dispersed onto the external surface of the clay mineral. As these novel clay composites retain the original swelling and cation-exchange properties of the mineral, the synthesis of various clay derivatives is readily attained. X-ray diffraction, electron spin resonance, Mossbauer spectroscopy, magnetic measurements, and transmission electron microscopy provide evidence for the formation of magnetic iron oxide phase and also for its location in the clay structure.

A further examination of the nature of dispersive soils in Natal, South Africa

Abstract: Dispersive soils are found in the drier areas of Natal, in regions where the annual rainfall is less than 850 mm. They are derived from a variety of rock types. Dispersive soils deflocculate in the presence of relatively pure water to form colloidal suspensions and are therefore highly susceptible to erosion and piping. Dispersive erosion depends on the chemistry of the clay on the one hand, and the dissolved salts in the pore and eroding water on the other. The presence of exchangeable sodium on clay mineral surfaces is one of the main chemical factors contributing towards dispersive behaviour. Susceptibility to dispersion also is influenced by the total content of dissolved salts in the pore water. The fabric of the soils was analysed by means of a scanning electron microscope. Turbostratic and flocculated structures were found in the clay material associated with low and high TDS contents respectively. The mineralogy, particle size distribution and consistency of these soils had little or no influence on dispersivity. A series of physical and chemical tests was carried out in an attempt to assess dispersivity. Then an evaluation of the reliability of the results of these tests was undertaken by means of discriminant analysis. This indicated that the results of the crumb test, the pinhole test, the SAR value and the assessments provided by the CEC vs ESP and TDS vs % Na charts represented the best parameters to use in a rating system. Such a system is proposed to aid the recognition of dispersive soils.

Mineralogical and geochemical characteristics of late Cretaceous bentonite deposits of the Kelkit Valley Region, northern Turkey

Abstract: Late Cretaceous bentonitic clays in the Kelkit Valley region of Turkey are composed of an alternation of limestone lenses and silicified tuff nodule-bearing pyroclastic rocks and their alteration products. Quartz, feldspar, biotite, trace amounts of augite together with pumice and volcanic rock fragments comprise the volcanogenic components. Diagenetic minerals are represented by clay, calcite, opal-CT, zeolite and dolomite. The clay fraction is dominated by smectite and lesser amounts of I-S, illite, chlorite and kaolinite. The d 001 basal spacing of dioctahedral smectites ranges from 12.51 to 12.55 A in Na-smectites and 14.97 to 15.52 A in Ca-smectites. The CaO/Na 2 O ratio of smectites ranges from 0.15 to 19.50, and the interlayer Na and Ca contents are 0.22–0.30 in beidellites and 0.02–0.09 while those in montmorillonites are 0.01–0.13 and 0.03–0.15, respectively. The data obtained indicate that bentonites formed in a marine environment by the alteration of volcanic ash of rhyodacitic/dacitic and intermediate/acidic composition.

Effect of clay minerals and organic matter on the cation exchange capacity of silt fractions.

Abstract: The cation exchange capacity (CEC) and specific surface properties were investigated in four particle-size fractions < 50 μm from three loess (one Kastanozem and two Phaeozems), a holocene (Fluvisol) and a basalt soil (Nitisol) before and after destruction of organic matter. Particle-size fractions were separated by sedimentation after chemical and physical dispersion of the soil samples. Illite, amorphous minerals, mixed layers, smectite and kaolinite were the predominant clay minerals. They were detected in all size fractions. The CEC increased with increasing organic matter contents and this effect was more pronounced in coarser fractions. The organic matter content per unit surface area was two or three times larger in coarse silt than in clay, irrespective of the soil type.

Mineralogy and chemistry of some variable charge subsoils.

Abstract: Colloidal mineralogy is one of the main characteristics of the steady state reached in developed soils. Surface charge and other chemical and physical properties of the soil depend on colloidal mineralogy. It is, therefore, very important to further investigate the clay mineralogy of acidic variable charge subsoils in order to understand better their unusual chemical behavior. The objective of this investigation was to characterize the inorganic colloid mineralogy, chemical (subsoil solution pH and electrical conductivity), and charge properties (PZNC and PZSE) in some variable charge subsoils. Subsoil materials were collected from the southeastern United States and other tropical and subtropical areas around the world. The clay fraction mineralogy in the majority of the subsoils was dominated by the quartet kaolinite, gibbsite, goethite, and hematite. They manifested, however, a significant diversity in their charge and other chemical characteristics because the proportions and contents of miner...