Diffuse reflectance spectra of kaolins have been recorded in samples from different environments. They show the systematic presence of Fe-oxides, even in bleached kaolins, with no contribution from the Fe3+ ions substituted in kaolinite. Second derivative spectra of various Fe-phases (hematite, goethite, lepidocrocite, maghemite, akaganeite, ferrihydrite and Fe-polymer) may be differentiated by the position of a diagnostic band corresponding to the 2(6A1) → 2(4T1(4G)) transition. The systematic comparison of diffuse reflectance spectra of unbleached and bleached kaolins has demonstrated the differences between the Fe-oxides occurring as coatings and as occluded phases. The features observed in second derivative spectral curves are consistent with assignments of crystal field transitions to goethite, hematite, akaganeite, and aged hydrous ferric oxides. The optical determination of the Fe-phases associated to kaolins assists in the interpretation of the formation conditions of these minerals.

This study examines the influence of minerals and amorphous phases associated with kaolin and kaolinitic rocks on kaolinite crystallinity indices (KCI) derived from X-ray diffraction (XRD) data in order to select the best index for systematic studies of commercial kaolins or geological sequences. For this purpose, 8 kaolins of differing structural order were chosen and used to prepare mixtures containing different weight fractions of quartz, feldspar, illite, smectite, chlorite, halloysite and iron hydroxide and silica gels. An additional 17 samples of kaolin were also studied to test the results and evaluate the restrictions. KCIs used included Hinckley (HI), Range and Weiss (QF), Liètard (R2), Stoch (IK), Hughes and Brown (H&B) and Amigó et al. (full width at half maximum, FWHM), and the “expert system” of Plançon and Zacharie.

Based on more than 15,000 KCI determinations, the HI and QF are influenced by quartz, feldspar, iron hydroxide gels, illite, smectite and halloysite. IK can be used in the presence of quartz, feldspar and iron hydroxide and silica gels. Also, R2 is the only KCI that could be measured in the presence of halloysite; FWHM indices should not be used in the presence of chlorite and/or halloysite; and H&B should only be used with pure kaolinite samples. The “expert system” of Plançon and Zacharie is strongly affected by the presence of other mineral phases, particularly with more than 25% of well-ordered kaolinite. Their system is less sensitive to other mineral phases when only disordered kaolinite is present, and it should not be used with kaolinite of medium order-disorder because the well-ordered phase is present in an inappreciable proportion (<10%). KCI is only measurable in kaolinitic rocks if kaolinite is >20 wt% and the precision increases with an increase in the quantity of kaolinite. In all cases, the reliability will depend on the other minerals present. When a KCI can be measured accurately, the others can be obtained by using the empirical relationships reported in this paper.

Offscraped strata within the toe of Nankai accretionary prism display an overall facies pattern of thickening and coarsening upward. Detrital clay minerals within the Quaternary trench-wedge facies are dominated by illite; chlorite is the second-most abundant clay mineral, followed by smectite. Relative mineral percentages change only modestly with depth. The hemipelagic clay-mineral population is virtually identical to clays washed from turbidite matrix, and different size fractions (<2 μm and 2–6 μm) show nominal amounts of mineral partitioning. Smectite content increases beneath the trench-wedge deposits, where the upper subunit of the Shikoku Basin stratigraphy (late Pliocene and early Pleistocene) includes abundance of volcanic ash. Syneruptive, subaerial chemical weathering of volcanic source rocks, together with in situ alteration of disseminated glass shards, caused the increase in smectite. Smectite begins a monotonic transformation to illite/smectite (I/S) mixed-layer clay at ~555 mbsf and an estimated temperature of ~65 °C. Ordered (R = 1) I/S interlayering first appears at ~1220 mbsf (<2 μm size fraction) and ~1100 mbsf (<0.2 μm size fraction). The illitization gradient coincides with a reduction in pore-water chlorinity, but depth-related changes in bulk mudstone geochemistry (K2O, Rb) are subtle. The absolute abundances of discrete smectite and I/S appear to be insufficient to account for the magnitude of pore-water dilution via in situ dehydration reactions. Instead, pore water probably was transported to Site 808, either from sources located deeper in the accretionary prism, where bulk mudstone porosities are lower, or from strike-parallel sources where mudstones originally deposited in the Shikoku Basin might contain higher percentages of smectite.

The structure, chemistry and distribution of hydrothermal alteration and weathering products of feldspars and glass in 3 samples of Yucca Mountain tuffs (GSW G4 borehole at a depth of 1531 ft (464 m) and USW GU3 borehole at 1406 ft (426 m) from the Calico Hills Formation and USW G4 at a depth of 272 ft (82.4 m) from the Topobah Springs Member) were examined by high-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM). Alteration products are of interest because they may influence the form and distribution of contaminants released from the proposed high-level nuclear waste repository. Samples from the Calico Hills Formation contain alkali-bearing aluminosilicate glass and its alteration products. Zeolites appear to have formed from compositionally similar glass by recrystallization, probably under hydrothermal conditions. Crystals are fibrous and frequently no more than a few tens of nanometers in diameter. Porous aggregates of few-nanometer-diameter, poorly crystalline silica spheres (probably opal C-T) develop adjacent to corroded glass surfaces and zeolite crystals. Finely crystalline Fe-rich smectites coat etched glass surfaces, zeolites and feldspar crystals and occur within opal-like silica aggregates. Microstructures in the clay-dominated coatings and details of smectite-glass interfaces suggest that clays grow in orientations controlled by heterogeneously retreating surfaces and from constituents released at associated glass dissolution sites. The alteration assemblage also includes finely crystalline hematite, goethite, Mn-oxide films and illite formed by alteration of muscovite. The zeolitized sample contains abundant opal-like silica whereas glass in the unzeolitized sample is weathered to smectite-like clays. These differences may be attributed to hydrological and consequent geochemical factors resulting from the higher porosity of zeolitized samples. Exsolved alkali feldspar, which occurs as micron-sized crystals in the Calico Hills Formation and as phenocrysts and in the groundmass of the devitrified Topobah Springs Member, are almost unaltered. Feldspar alteration is confined to cracks and grain boundaries, where minor, poorly crystalline, Fe-bearing aluminosilicate alteration products are developed. In these tuffs, most of the porosity, permeability, high surface area and capacity to affect solution chemistry are associated with products of glass alteration.

Progressive alteration by seawater of an andesite in the Aegean Island of Milos and an ignimbrite in the Aegean Island of Kimolos, Greece, formed bentonites with or without zeolites. Both profiles are dominated by migration of alkalis and uptake of Mg, Fe and H2O, while Al and Ti are immobile. The relative removal of alkalis controls the formation of either smectite or zeolites. The behavior of Ca and Si depends on the chemistry of the parent rock. In the rhyolitic profile, alteration is controlled by gain of Mg, Fe2+ and Ca and loss of Na, K and Si, while in the andesitic profile by gain of Mg and Fe2+ and loss of Na, K and Ca. In both profiles, significant uptake of SO4- was not observed. Moreover Zr, Nb, V and Ni are immobile and have been enriched residually, while Sr, Rb and Y are lost in both profiles. Thorium is immobile in the rhyolitic profile but is leached in the andesitic profile. Also, the rare earth elements (REE) display fractionation in both profiles; the degree of fractionation increases with the degree of alteration to bentonite. Fractionation of the REE in both profiles and mobility of Th in the andesitic profile are related to the existence of monazite (rhyolitic profile) and apatite (andesite profile). The REE and Th appear to partition into phosphates rather than smectite.

The mobility of Y coupled with the immobility of Nb increases the Nb: Y ratio with advancing alteration, rendering discrimination diagrams that use this ratio to determine the nature of the protoliths misleading. Mass balance calculations showed that in the smectite-rich zones the water: rock (WR) ratio might be as high as 13:1 in both profiles, while in the zeolite-bearing zones it is about 5.5:1. Such WR ratios explain the observed extensive mass transfer and suggest that the pore fluid chemistry might overprint the chemical characteristics of the parent rocks controlling smectite and bentonite chemistry.

Surface adsorption mechanisms of dissolved inorganic carbon species on soil minerals are not well understood. Traditional infrared (IR) study of adsorbed species of inorganic carbon using air-dried samples may not reveal true species in the solid/water interface in suspension. The purpose of this study was to obtain information on interfacial carbonate speciation between solid and aqueous phases. The interaction of bicarbonate and carbonate ions with X-ray amorphous (am) Al and Fe oxides, gibbsite (γ−Al(OH)3) and goethite (α-FeOOH) was examined by electrophoresis and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The presence of carbonate lowered the electrophoretic mobility and decreased the point of zero charge (PZC) of all minerals, implying specific adsorption. Inner-sphere complexation of bicarbonate and carbonate was supported by a lowering in the anion symmetry due to the interaction with Al and Fe oxide surfaces. Only complexed monodentate carbonate was identified in am-Al(OH)3/aqueous solution at pH 4.1-7.8 when the solid was reacted with either NaHC03 or Na2CO3 solutions. Am-Al(OH)3 was transformed to a crystalline sodium aluminum hydroxy carbonate, dawsonite [NaAl(CO3)(OH)2], and bayerite (α-Al(OH)3) after reacting with 1.0 M Na2CO3 for 24 h. Gibbsite adsorbed much less carbonate than am-Al(OH)3 such that adsorbed carbonate on gibbsite gave weak IR absorption. It is probable that monodentate carbonate is also the complexed species on gibbsite. Evidence suggesting the presence of both surface complexed bicarbonate and carbonate species in the interfacial region of am-Fe(OH)3 in suspension and the dependence of their relative distribution on solution pH is shown. Only monodentate carbonate was found in the interfacial region of goethite in 1.0 M NaHCO3. A ligand exchange reaction was proposed to describe the interaction of bicarbonate and carbonate with the surface functional groups of Al and Fe oxides.

Reduction of structural Fe in Na-exchanged dioctahedral smectites decreases swellability in water, but because clay interlayers also collapse in the process the concomitant effect on surface hydration energy is uncertain. This study examined the hydration behavior of oxidized and reduced dioctahedral smectite clays exchanged with polar (Na) and weakly-polar (organic) cations to determine the nature of the surface before and after Fe reduction, and to determine if clay surfaces are hydrophilic or hydrophobic. The H2O content in various dioctahedral smectites decreased if Na was replaced by tetramethylammonium (TMA), trimethylphenylammonium (TMPA), or hexadecyltrimethylammonium (HDTMA). Among the organo-clays, H2O adsorption decreased with increasing complexity of the cation. For oxidized smectites, those exchanged with TMPA retained less H2O than those exchanged with Na at all pressures. The extent of this difference depended on the clay and decreased with increasing applied pressure. Reduction of Fe(III) to Fe(II) in the octahedral sheets decreased the swelling of Na-saturated smectites, apparently causing some previously swelling interlayers to collapse. If the Na interlayer cation was exchanged to alkylammonium after reduction, but prior to swelling-pressure measurements, the swelling increased or remained near constant, suggesting that the organo-cation disrupted the collapse process of the interlayers associated with the reduced smectite layers. Reduced TMPA-saturated smectite surfaces are more strongly hydrated if the octahedral sheet is reduced than if oxidized. Thus, reduction of structural Fe increases the hydration energy of smectite basal surfaces, but swellability could decrease or increase depending on the extent of interlayer collapse occurring with different exchangeable cations.

Interaction of water with montmorillonite exchanged with Na+, K+, Co2+, and Cu2+ cations as a function of water content was examined using an FTIR/gravimetric cell designed to collect spectroscopic and sorption data simultaneously. Correlation of water desorption isotherms with infrared spectra of the clay-water complex showed that the position of the HOH bending band of water decreased as a function of water content. The largest decreases in frequency were observed for Cu2+ and Co2+; smaller decreases were found for Na+ and K+. In addition, the molar absorptivity of sorbed water increased upon decreasing the water content. The decrease in frequency and the concomitant increase in molar absorptivity were attributed to polarization effects on the sorbed water molecules by exchangeable cations. The interference fringes of a self supporting clay film permitted d-spacings to be determined optically and, therefore, changes in frequency, molar absorptivity, and water sorption behavior to be related directly to changes in interlayer spacing. The d-spacings obtained from the interference fringes were consistently larger by approximately 0.5 Å than those determined using powder XRD.

End members and species defined with permissible ranges of composition are presented for the true micas, the brittle micas and the interlayer-cation-deficient micas. The determination of the crystallochemical formula for different available chemical data is outlined, and a system of modifiers and suffixes is given to allow the expression of unusual chemical substitutions or polytypic stacking arrangements. Tables of mica synonyms, varieties, ill-defined materials and a list of names formerly or erroneously used for micas are presented. The Mica Subcommittee was appointed by the Commission on New Minerals and Mineral Names (“Commission”) of the International Mineralogical Association (IMA). The definitions and recommendations presented were approved by the Commission.

In this paper, we use the unequal radius modified Gouy-Chapman theory to evaluate the effect of the ionic size of the electrolyte on the swelling pressures (II) in different clay systems immersed in electrolytic solutions. First the model is applied to a 1:1 electrolyte to show that the coion size is only important at surface charge densities much lower than those found in typical clay systems. The swelling pressure is calculated and the results are compared with experimental data. Literature ionic radii values are used to show the dependence of the swelling pressure on the specific counterions present. Next the model is applied to a 1:1 and 2:1 electrolyte mixture with unequal-sized counterions to show the swelling pressure is highly dependent on both counterion sizes. The unequal and same-sized cases are compared.

Oil recovery is strongly related to the wettability of reservoir rocks that are formed of quartz grains attached by mineral hydroxides and clay minerals. Illites and kaolinites are the most active due to their high specific surface areas and electrical charge densities. Therefore, these minerals’ relative affinities for oil or water when in contact with a water-oil mix are of great importance. In order to model such a complex system, we used a mix of organic model molecules of the oil constituents and water vapor. Their interactions were estimated by inverse gas chromatography (IGC). IGC experiments were performed using a carrier gas with controlled humidity. By means of IGC at infinite dilution conditions, the dispersive component of the surface energy, γsd, was determined. A strong decrease of γsd, due to water molecules shielding the highest-energy sites, was observed. The energetic surface heterogeneity of the clays was examined using IGC at finite concentration conditions, allowing the determination of organic probe adsorption isotherms in the presence of water. From these isotherms, adsorption energy distribution functions were computed for propanol-2 and pyridine probes. Water mainly modifies the illite distribution functions, whereas practically no change was observed in the case of kaolinite. This observation is related to the higher hydrophilicity of illite as compared with kaolinite, and explains the different behaviors of the 2 clay families in oil reservoirs.

Six kaolin samples from the Lower Tertiary Huber Formation near Wrens, Georgia were analyzed using transmission electron microscopy (TEM), electron diffraction (ED), powder X-ray diffraction (XRD), chemical analysis, and magnetic susceptibility to characterize the Ti-bearing phases. Selected samples were treated with 5 M NaOH to remove kaolinite and concentrate the Ti-bearing phases for additional analysis. TiO2 content in the bulk fraction ranges from 1.2 to 5.4 wt. %. There are at least three Ti-bearing phases, including anatase, rutile, and a poorly defined nanocrystalline form. Anatase is most abundant and is commonly found with {010} faces in association with kaolinite edge and basal faces. The nanocrystalline form occurs at 0–1 wt. %, and rutile occurs in trace amounts. Bulk XRD analysis correlates well with the bulk TiO2 chemical measurements. Average anatase unit-cell parameters are a = 0.37908 ± 0.0002 nm and c = 0.951 ± 0.001 nm. These parameters indicate an approximate chemical formula of Fe3+0.05Ti4+0.95O1.95(OH)0.05.

The distribution of TiO2 content as a function of depth may be useful to obtain original grain-size variations associated with relative sea-level changes responsible for the deposition of the Huber Formation. Evidence for original depositional sediment properties can be seen in the occurrence of pseudomorphic replacement of micas and fecal pellets by kaolinite. Additional evidence for post-depositional changes includes the sub-micrometer euhedral character and low Fe content of the anatase (relative to soil-derived anatase). These observations for the Huber Formation are consistent with a previously published theory for kaolin genesis that includes biomineralization of originally coarser-grained aluminosilicates into a kaolinite-rich ore body.

Complexes formed between montmorillonite, saturated in Li+, Na+, Mg2+, Ca2+, Co2+, Fe3+, Cu2+ and Zn2+, and trimethyl phosphites (TMP) and triethyl phosphites (TEP) were studied. In all of the cases, phosphites penetrate into the interlayer space of the montmorillonite and produce solvates whose basal spacing varies depending on the characteristics of the exchangeable cation. All the complexes with low basal spacing (Li+, Na+, Mg2+, Co2+ and Zn2+) are stable in vacuum, whereas those with high basal spacing, formed by the Ca2+ sample with TMP, and Ca2+ and Fe3+ samples with TEP are transformed into low basal spacing complexes in vacuum. The complexes with high basal spacing (Cu2+ sample with TMP and TEP) are stable in vacuum.

The TMP and TEP complexes stable in vacuum with low spacing are thermally destroyed in one or two stages with two loss maxima, as a result of partial burning of phosphite molecules. Those with high spacing (Cu2+) are destroyed in two stages; the first is probably the result of the transformation process from high to low spacing, as a consequence of the structural reorganization of the molecules which remain in the interlayer space, and the second, could be associated with the destruction of low spacing complexes.

The IR spectra show that the molecule and the cation are linked by the P of the phosphite, which produces a reinforcement of the other bonds in the molecule, caused by an inductive effect. The phosphite intercalation is accompanied by a partial isomerization of phosphite to phosphonate.

The heat of adsorption of phosphites shows that the molecule-cation bond is ion-dipole. In the Cu sample with trimethyl phosphite, this bond seems to be reinforced by retrodonation of electrons from copper to ligand. Finally, the possible disposition of phosphite molecules in the interlayer space is considered. For this purpose, ab initio calculations have been performed on the different conformers of the TMP molecule at 6–31G* and 6–31+G* basis sets.

To study the physico-chemical activity of lichens on micaceous components of granitic rocks, samples covered by thalli of Parmelia conspersa (Ehrht) Ach. and Aspicilia intermutans (Nyl.) Arn. were collected and examined with Scanning Electron Microscopy (SEM) equipped with a Back Scattered Electron (BSE) detector and an Energy Dispersive Spectroscopy (EDS) microanalytical system. The bio-physical activity of both lichen species leads to a deep alteration of biotite, which results in detachment, separation and exfoliation of biotite plates. Chemically, the bioweathering process of biotite in the lichenmineral contact zone involves considerable depletion of potassium (K) from interlayer positions in biotite and removal of several elements, corresponding to a 9.7% loss in matter. The sequence of the loss of elements is: K+ » Fetot > Ti4+ ≅ Mg2+. There are also some gains in the order: Ca2+ > Na+ » Al3+ > Si4+ attributed to dissolution of co-existing Ca and Na rich minerals. Geochemical mass balance results suggest the transformation of K-rich biotite to scarcely altered biotite interstratified with a biotite-vermiculite intermediate phase in the lichen bioweathered contact zones.

The dehydration and migration of the interlayer cation of the synthetic beidellite Na0.7Al4.7Si7.3O20-(OH)4·nH2O, were studied with solid-state 23Na and 27Al MAS-NMR, heating stage XRD, and thermogravimetric analyses (TGA, DTA). The 23Na MAS-NMR of Na-beidellite at 25°C displays a chemical shift of 0.2 ppm, which indicates a configuration comparable with that of Na+ in solution. Total dehydration proceeds reversibly in two temperature ranges. Four water molecules per Na+ are gradually removed from 25° to 85°C. As a result, the basal spacing decreases from 12.54 Å to 9.98 Å and the Na+ surrounded by the two remaining water molecules is relocated in the hexagonal cavities of the tetrahedral sheet. The chemical shift of 1.5 ppm exhibited after the first dehydration stage illustrates the increased influence of the tetrahedral sheet. The high local symmetry is maintained throughout the entire first dehydration stage. During the second dehydration, which proceeds in a narrow temperature range around 400°C, the remaining two water molecules are removed reversibly without any change of the basal spacing.

To characterize the evolution of dioctahedral interstratified clay minerals in the Golden Cross epithermal deposit, New Zealand, hydrothermally altered volcanic rocks containing the sequence smectite through illite-smectite (I-S) to muscovite were examined by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission and analytical electron microscopies (TEM/AEM).

XRD analyses of 30 oriented clay samples show a broad deposit-wide trend of increasing illite content in I-S with increasing depth and proximity to the central vein system. Six representative samples were selected for SEM/TEM study on the basis of petrographic observations and XRD estimates of I-S interstratification. Ca and Na are the dominant interlayer cations in smectite, but as the proportion of illite layers in I-S increases, so does the K content and (IVAl + VIAl)/Si ratio. Layers and packets tend to flatten and form larger arrays, reducing the amount of pore space. Smectite coexists with (R = 1) I-S, rather than being (R = 0) I-S where R is the Reichweite parameter. The highest alteration rank samples contain discrete packets of mica to ∼300 Å thick, but a limited chemical and structural gap exists between illite, which is intermediate in composition between common illite and muscovite, and illite-rich I-S. Selected-area electron diffraction (SAED) patterns of mica show that the 1M polytype dominates, rather than the common 2M1 polytype.

Petrographic, SEM, and TEM data imply that all phyllosilicates formed via neoformation directly from fluids. Relatively mature I-S and micas form simultaneously, without progressing through the series of transformations that are commonly assumed to characterize diagenetic sequences during burial metamorphism in mud-dominated basins. Although the overall distribution of clay minerals is consistent with temperature as a controlling variable, local heterogeneities in the distribution of clay minerals were controlled by water/rock ratio, which varied widely owing to different rock types and fracture control.

Electron spin resonance (ESR) spectroscopy was used to measure the rotational mobility of an uncharged nitroxide spin probe (4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy) in hectorite gels of variable water content. Physical adsorption segregated the probe molecules into two populations: probes in the solution or a solution-like phase, and motionally restricted probes in the adsorbed phase. Although the spectra of these two populations were discrete, indicating that exchange between them was slow on the time scale relevant to ESR, their overlap prevented a straightforward determination of mobility of the adsorbed probes. Even though adsorption was weak, effects of the adsorbed population on the spectral lineshape were detectable for suspensions containing as little as 20 grams clay per liter of water. Orientation of the adsorbed probes on the fully hydrated hectorite surfaces was similar to that of a positively charged nitroxide probe, suggesting that steric factors rather than electrostatic forces control short-range organic molecule interaction with the silicate. The possibility of reaching false conclusions about probe mobility and interlamellar water viscosity when using a weakly adsorbing probe is discussed.

Experimental cation exchange capacities (CEC) of kaolinites were determined and compared to theoretical calculations of CEC. The comparison reveals that the exchangeable cations occur mostly on the edges and on the basal (OH) surfaces of the mineral. It also shows that permanent negative charge from isomorphous substitution of Al3+ for Si4+ is insignificant. The CEC of kaolinite strongly depends on the particle size (both thickness and diameter in the (00l plane) and pH value. Particle size is more important than crystallinity in affecting kaolinite CEC. This study shows that the hydroxyls on the exposed basal surfaces may be ionizable in aqueous solutions. The amount of negative charge on the edges and the exposed basal hydroxyls depends on pH and other ion concentrations. A higher pH value gives rise to more negative charges, which lead to a higher CEC value. This study indicates that charge from broken edges and exposed OH planes rather than charge from Al/Si substitution determines the kaolinite CEC, even at zero point charge. A high CEC in some kaolinites is found to be due to smectite layers on the surface of the kaolinite crystals.

Many soils developed from volcanic rocks in southern Brazil exhibit spontaneous magnetization caused by the presence of fine-grained maghemite (γ-Fe2O3), but few attempts were made to quantify or characterize this important soil component. To that end, clays were separated from freely drained soils derived from acid (≥63% SiO2), intermediate (54–62% SiO2), and basic (≤53% SiO2) igneous rocks produced by the Paraná flood volcanism. The sample set included soils with a wide range of pedogenic development on different landscape positions. The Fe oxide mineralogy of these samples was examined by using a combination of selective dissolution, magnetic susceptibility, and X-ray diffraction (XRD) techniques. Hematite and maghemite were the primary Fe oxides in mature soils (Oxisols, Ultisols, and Alfisols) developed from basic rocks; whereas goethite was dominant in all other soils, especially those formed from acid-intermediate rocks. The association of maghemite with basic rock materials suggests that it was primarily formed by oxidation of lithogenic magnetite. A strong, positive correlation (R2 = 0.89) was obtained between mass specific magnetic susceptibility (χ) of the clay fractions and maghemite contents estimated by XRD. Either method could be used for quantitative analyses, but χ was more sensitive than XRD at low maghemite concentrations (<2 wt. %). The clay-sized maghem-ites were superparamagnetic with an estimated value for the mass specific magnetic susceptibility (χlf) value of 91,000 × 10−8 m3 kg−1 and frequency dependent variations of 10–15%. The maghemites also had low unit cell constants, which, if attributed entirely to replacement of Fe by Al, would correlate with Al substitutions in the range of 5–16 mole %. Selective dissolution of the soil maghemites was achieved by treatment of Fe oxide concentrates with 1.8 M H2SO4 at 75°C for 2 h.