Upper Cretaceous—Danian chalk and interbedded clay-rich layers from wells of the South Arne Field and adjacent wells in the North Sea and from Stevns in Zealand have been investigated to determine the clay mineralogy in the chalk and the origin of the interbedded clay-rich layers. The mineralogy, with emphasis on clay mineralogy, was determined after removal of the calcite by dissolution at pH 4.5–5 in order to preserve the other minerals. Generally, mixed-layer minerals are the dominant clay minerals, except for two wells, Rigs-1 and Rigs-2, where a 3D ordered kaolinite prevails. A detailed structural characterization of the mixed-layer minerals was carried out by modeling of the X-ray diffraction (XRD) patterns. In most of the samples dominated by mixed-layer minerals, two mixed-layer phases, a high-smectite illite-smectite (I-S) and a low-smectite illite-smectite-chlorite (I-S-Ch), prevail, irrespective of depth or location of the samples. However, some samples contain I-S-Ch and ordered S-Ch, and others a chlorite-serpentine (Ch-Sr) phase, and these samples probably formed during diagenesis at higher temperatures. The clay-rich layers and the adjacent chalk have the same or quite similar clay mineralogy, both with respect to kaolinite vs. mixed-layer minerals and with respect to their detailed structure. In conclusion, the kaolinite is detrital and the I-S minerals formed in the chalk from volcanic ash. The main conclusion is that the clay-rich layers in the North Sea chalk formed by dissolution of the calcite in the chalk and that this dissolution took place at burial depths of >1 km, probably through migration of solutions through permeable chalk layers.

Freshwater has become increasingly scarce in many countries. To reduce the consumption of freshwater, the use of saline water resources in industry could provide an opportunity to meet the challenge of water-supply sustainability. However, the presence of electrolytes in saline water causes the coagulation of kaolinite, the colloid stability of which plays a key role in the processing of a number of minerals. Therefore, the dispersion of kaolinite in saline water was studied here. Electrophoretic mobility and colloid stability studies were conducted on a sodium hexametaphosphate-kaolinite system in the presence of NaCl, KCl, CaCl2, and MgCl2, the major electrolytes in saline water resources. The effect of each electrolyte on kaolinite dispersion was studied. Based on the studies of individual electrolytes, a method was developed to disperse kaolinite in 1:1 diluted synthetic seawater with distilled water, which may potentially reduce the consumption of freshwater by 50% when applied in industry.

The adsorption properties of clay minerals (e.g. montmorillonite and palygorskite) have been improved through chemical treatment methods. However, the addition of extra chemicals is often not friendly to the environment and powdered clay-mineral adsorbents are inconvenient for some applications. To overcome these drawbacks in the present study, granular montmorillonite-palygorskite adsorbents (GMPA) were successfully prepared using Na-alginate and thermal treatments to improve heavy metal removal from water. The properties of GMPA samples under different calcination temperatures were examined using thermogravimetric (TG) analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), and specific surface area (BET). The results indicated that loss of mass by GMPA relative to the untreated montmorillonite-palygorskite was due to the loss of water, adsorbed Na-alginate, and mineral decomposition during thermal treatment. Changes in the morphology and crystallinity were significant at calcination temperatures from 500°C to 1000°C. The layered morphology totally disappeared after calcination at 1000°C, while transformation of the montmorillonite and palygorskite to a non-crystalline material was almost complete at 800°C and new crystalline phases appeared. Calcination temperature had a significant influence on the specific surface areas and pore volumes of GMPA. Both the changes in texture and chemical structure affected Pb2+ and Cu2+ removal. The GMPA sample produced at a 600°C calcination temperature was the most promising adsorbent for heavy metal removal from water.

In the production of alumina (Al2O3) from clays by acid leaching, thermal activation by calcination is applied widely as a pre-treatment to improve the dissolution of aluminum. Previous studies have focused only on the thermal behavior of high-purity kaolinite and pyrophyllite, individually. However, thermal activation of complex clay ores containing several silicate minerals and their effect on aluminum extraction have not been studied. The purpose of the present study was to characterize the thermal behavior of a clay ore containing mainly pyrophyllite (Al2Si4O10(OH)2), kaolinite (Al2Si2O5(OH)4), muscovite (KAl2(AlSi3O10)(OH)2), quartz (SiO2), and kyanite (Al2SiO5) from the Pütürge clay deposits (Malatya, Turkey) for possible use in alumina (Al2O3) production by acid leaching. The ore and its calcination products obtained at various temperatures were characterized with respect to their mineral paragenesis, crystal structure, surface morphology, and thermal, calorimetric, and aluminum dissolution properties in order to understand the changes that occurred. Aluminum recovery in the leach solution increased in direct proportion to the dehydroxylation degree (Dtg) of the ore as the calcination temperature increased to 900°C. A maximum aluminum extraction of 90.57% was achieved by leaching of the product from calcination at 900°C. Aluminum extraction decreased sharply above that temperature, even though Dtg increased a little. By increasing the calcination temperature; the structures of pyrophyllite, kaolinite, and muscovite were destroyed by dehydroxylation, resulting in the exfoliation of the mineral layers, thus, a mixture of dehydroxylated phases formed. Depending mainly on the temperature range at which each of the dehydroxylated phases is durable, aluminum could be leached to some extent. The sharp decrease in the extraction of aluminum, iron, and potassium at higher temperatures was attributed to compaction of previously exfoliated layers of the minerals through re-crystallization to form mullite-like structures which seemed insensitive to acid attack during the leaching.

The enthalpy of hydration of natural clinoptilolite was determined by isothermal immersion calorimetry on Ca-, Na- and K-exchanged clinoptilolite (Fish Creek Mountains, Nevada). Heats of immersion of clinoptilolite were determined at initial H2O contents ranging from θ = 0.02 to 0.85 (where θ is the ratio [H2O content]/[maximum H2O content]). The heat of immersion (liquid H2O reference state) of Ca-clinoptilolite ranged from -7.5 (θ = 0.87) to -25.7 kJ/mol-H2O (θ = 0.19); values for Na-clinoptilolite ranged from -6.3 (θ = 0.85) to -21.8 kJ/mol-H2O (θ = 0.11); and values for K-clinoptilolite ranged from -7.7 (θ = 0.80) to -24.6 kJ/mol-H2O (θ = 0.02). Linear regression of the calorimetric data provided the following values for the complete heat of immersion (from θ = 0): Ca-clinoptilolite, -30.3 ± 2.0; Na-clinoptilolite, -23.4 ± 0.6; and K-clinoptilolite, -22.4 ± 0.8 kJ/mol-H2O.

The heat of immersion measurements were compared with the enthalpy of hydration results of Carey and Bish (1996) determined in a thermogravimetric study of the same samples. The heat of immersion data are similar but of smaller magnitude than the values of enthalpy of hydration and are believed to be more accurate because they represent direct measurements of this thermodynamic property.

The effect of dehydration of clinoptilolite on the thermal evolution of the potential high-level radioactive waste repository at Yucca Mountain was considered by comparing the amount of energy consumed by clinoptilolite dehydration with the amount of energy necessary to heat rocks lacking hydrous minerals. The extra energy consumed on heating clinoptilolite from 25 to 200 °C ranges between 70 and 80% in excess of that required for nondehydrating materials (that is, clinoptilolite acts as a heat sink). These results indicate that accurate thermohydrologic modeling of rock units at Yucca Mountain should consider the thermal effect of dehydration/hydration processes in clinoptilolite and other hydrous minerals, in addition to the water produced/adsorbed during heating/cooling.

Some aspects of the crystal structure of illite are not understood properly yet, in spite of its abundance and significance as a component of soils, sediments, and low-grade metamorphic rocks. The present study aimed to explore the role of hydronium cations in the interlayer space of illite in a theoreticalexperimental approach in order to clarify previous controversial reports. The infrared spectroscopy of this mineral has been studied experimentally and by means of atomistic calculations at the quantum mechanical level. The tetrahedral charge is critical for the stability of the hydronium cations, the presence of which has probably been underestimated in previous studies. In the present study, computational studies have shown that the hydronium cations in aqueous solutions are likely to be intercalated in the interlayer space of illite, exchanging for K cations. During the drying process these cations are stabilized by hydrogen bonds in the interlayer space of illite.

A study was conducted to (1) determine the conditions of hydroxy-Mg interlayer formation with respect to type of clay mineral, acidity, and time; (2) evaluate the stability of this interlayer to dissolution treatments; and (3) ascertain the effects of such treatments upon the determination of clay minerals in soils and sediments. Hydroxy-Mg interlayers were formed in montmorillonite and vermiculite by adding MgCl2 and NaOH in amounts to give a wide range of pH. The resulting chloritic intergrades were examined after 10 days, 6 months, and 1 yr.

Alkaline conditions favored the formation of hydroxy magnesium interlayers in phyllosilicates. Hydroxy-Mg interlayered montmorillonite which resulted from 10 days equilibration at pH 10·4 did not expand upon solvation with ethylene glycol and exhibited practically no collapse after K-saturation and heating at 550°C. A small amount of interlayer was formed between pH 6·8 and 9·8 (10 days). In contrast, vermiculite exhibited no evidence of interlayer formation at pH values up to 9·7 (10 days). Chloritic intergrades formed at pH 10·7 did not collapse after K-saturation and heating at 300°C but did so at 550°C. Hydroxy-Mg interlayers were not formed in either mineral by using a drying method. This method apparently failed to provide the required alkaline conditions for interlayer formation.

The amount of magnesium interlayers present in the phyllosilicate systems decreased with time. The interlayers formed in vermiculite decreased more sharply than those in montmorillonite.

Sequential dissolution treatments included boiling 2 per cent Na2CO3, buffered sodium citrate-dithionite, a second citrate-dithionite treatment, and boiling NaOH. Hydroxy-Mg interlayers in montmorillonite exhibited a higher stability to sequential treatments than the interlayers formed in vermiculite. A stable 14 Å line was observed in interlayered montmorillonite after the dithionite-citrate and NaOH treatments.

The interlayers in montmorillonite showed a relatively high stability to HCl dissolution treatments. In contrast, most of the magnesium interlayer in vermiculite was removed by two HCl washings.

The reagents used in this study are sometimes used to remove coatings and cementing agents from soil surfaces prior to particle size and clay analysis. The present data show that these treatments also remove some hydroxy-Mg interlayers and produce changes in properties of clays. A proper interpretation of data for clay mineral identification and characterization must recognize these changes due to treatment.

Spherical halloysite aggregates have been identified for the first time in mineral matter isolated from bituminous coals. The spherules, found in Permian coals of the Sydney basin, New South Wales, range from 0.4 to 0.6 µm in diameter and have a delicate ring-like structure that helps to confirm the halloysite identification. They appear from their location to be related to influxes of pyroclastic debris, either directly or from nearby soils, into the original peat accumulation. Analytical electron microscopy indicates higher proportions of Si and Fe than coexisting particles of hexagonal platy kaolinite, and electron diffraction reveals a typical disordered halloysite structure. The aggregates are larger than those normally reported in soils, and comparison to growth rates in soils suggests development over a significantly longer time than that expected for accumulation of the host coal seams. The buckled structure in the ring-like pattern and the related crude polyhedral outlines probably reflect shrinkage with dehydration during the coalification process, but it may also be due to the different sample preparation techniques.

The stability of smectite separated from a Houston Black clay soil was studied by solubility methods in an acid environment. High Silicon levels (supersaturated with respect to amorphous Si) probably were due to dissolution of the smectite and slow precipitation of amorphous Silicon. Also, mica and vermiculite impurities may have contributed to high solution Si values. Solubility data from equilibrium solutions of various treatments and chemical structural analyses permitted the formulation of a solubility equation. The ΔG°f for the Houston Black smectite computed from pK values was —2433.9 ± 0.8 kcal/mole. The stability of this clay could then be determined by calculations for any desired solution environment. It was found that under some conditions this soil smectite could be more stable than Belle Fourche and Aberdeen montmorillonites. Therefore, it appears that this soil clay has the required stability area in which it can form in nature.

Investigation of mixed-layer illite/smectites with far-infrared (FIR) spectroscopy indicates the presence of torsional mode absorption bands associated with interlayer fixed-K sites. By contrast, hydrated montmorillonitic interlayer cation sites are transparent in the far IR. The presence or absence of bands for interlayer cation sites appears to be related to both the magnitude and site of negative layer charge within the 2:1 layer structure. The bimodal nature of illite/smectite spectra leads to the suggestion that two different fixed-K environments occur within illite/smectite structures. These two environments are controlled by the composition of the octahedral sheet. The torsional modes at 112 and 89 cm-1 represent fixed-K sites influenced, respectively, by an Al-rich, high-charge dioctahedral layer and a heterogeneous Al-Fe-Mg-bearing, low-charge layer. A general trend of increasing absorption of the 112 cm-1 band, relative to the 89 cm-1 band, is observed in a typical diagenetic illite/smectite sequence of Miocene shales from the Gulf of Mexico sedimentary basin. The absorbance strength of both torsional bands is also seen to increase with increasing degree of illitization and the amount of fixed potassium in the illite/smectite. These observations are consistent with the concept of shales undergoing illitization during burial diagenesis by both the collapse of high-charge smectite layers to form illite layers (i.e., transformation) and the formation of new high-charge (-0.9) illite layers at the expense of smectite layers (i.e., dissolution/ neoformation).

The structural formulas were determined of very pure specimens of montmorillonite, bei-dellite, saponite, nontronite, hectorite, wolchonskoite, vermiculite, and batavite. The lattice perfection of these minerals and the nature and variation of their crystalline swelling depend significantly on the surface density of the exchangeable cations between the silicate sheets.

In a neutral solution of NH4F, OH ions were replaced by F ions. The exchange capacity is smaller the greater the diameter of the silicate layers of the various montmorillonite minerals. Since the anion-exchange capacity of nontronite can be increased through oxidation of Fe2 it is probable that the exchangeable OH ions are located on the boundaries of the octahedral layers.

With kaolinite, the cation-exchange capacity decreases with increasing thickness of the crystal plates. The OH ions of the basal surface can be exchanged with F ions, which leads to the destruction of the lattice.

A one-dimensional Fourier synthesis of K-batavite, like the investigations of Walker on vermiculite and of Brown and also Méring on montmorillonite, provides confirmation of the structure and enables the lattice positions of the K ions to be determined.

On the basis of progressive acid dissolution the Thompson-Hower model states that mixed-layer glauconite/smectite and illite/smectite contain potassium in two structural sites: a mica-type K position (site 1) and a position of uncertain structural status more prone to dissolution (site 2). Site 2 was thought not to retain radiogenic argon (40Ar*). Using extensive progressive acid dissolution and K/Ar studies on a sized illite/smectite (I/S), determining the amount of K in site 2 is shown to be somewhat more complicated than previously thought because the dissolution pattern depends on acid normality. More important, site 2 fully retains 40Ar*, and no age correction is thus necessary as is required by the Thompson-Hower model, further affirming the geochronologic value of illite in mixed-layer clay. These data are also relevant to understanding the crystal and particle structure of I/S. Site 2 is probably a partly filled K interlayer that develops as an intermediate kinematic step on the way to being fully filled during the transformation of smectite to illite.

The cation exchange capacities (CEC) and extractable cations in smectite, corrensite and palagonitized glass from hydrothermally-altered pillow lavas and hyaloclastite breccias were measured by both bulk wet chemical and in situ microanalytical techniques. Smectite has CEC's between 60 and 120 meq/100 g, palagonitized glass between 30 and 60 meq/100 g, and corrensite approximately 35 meq/100 g as determined by the in situ CsCl-exchange method. These experiments generally verify that Cs exchanges for those cations that are presumed (from the stoichiometry implied by microprobe analyses) to occupy interlayer sites in sheet silicates. Results of conventional CEC determinations are consistent with those determined by the in situ experiments: the individual microanalytical values for smectite and palagonitized glass bracket the bulk CEC values. The in situ experiments imply that Mg is the major extractable cation in smectite, Ca in corrensite, and both Mg and Ca in the palagonitized glass. We speculate that discrepancies between the equivalents of extractable cations predicted from elemental analysis and the equivalents of Cs sorbed may be due to the presence of charge-balancing protons that are not detected by the microprobe analyses. The sum of equivalents of cations extracted by NH4-acetate is about the same as the CEC determined by both the in situ and the bulk methods. Cation proportions indicated by NH4-acetate extractions from bulk samples are also generally consistent with the in situ results for all elements except Mg, which is a minor leachate of the NH4-acetate extractions in all the samples. To explain this discrepancy, we propose that 1) Mg may occupy structural sites within palagonitized glass, which inhibit its extraction by NH4 or Cs, and/or 2) there is a significant quantity of smectite, unsampled by the electron microprobe analyses, which contains insignificant interlayer Mg.

Widespread lateritized ultramafic rocks in the southern part of the Muratdağı region of Turkey constitute a significant source of Ni-Cr-bearing ore with economic potential. However, no mineralogical or geochemical characterizations of these important materials have been performed previously. The aim of the present study was to describe the mineralogy, geochemistry, and genesis of Ni-Cr-bearing smectite in garnierite and ferruginous saprolite associated with the lateritized ophiolite-related ultramafic rocks. The lateritic zones are well developed over serpentinized harzburgitic mantle peridotites. The lateritized units and related bedrocks were examined using polarized-light microscopy, X-ray diffraction, scanning and transmission electron microscopies, and chemical and isotopic methods. The garnierite-containing saprolites are enriched in smectite, Fe-(oxyhydr)oxide phases, and opal-CT. Micromorphological images revealed that flaky smectite and, locally, Fe-rich particles, alunite, gypsum, gibbsite, and sulfur crystals developed along the fractures and dissolution voids. The development of saprolite demonstrates chemical weathering. The presence of silicified and Fe-(oxyhydr)oxide phases associated with gypsum, alunite, and local native sulfur in vertical and/or subvertical fractures and fault infillings are indicative of hydrothermal processes along the extensional, tectonically related fault systems. Chemical weathering and hydrothermal processes, which probably started during the Oligocene and Miocene, led to the formation of nontronite, Fe-bearing montmorillonite, and local Fe-rich kaolinite. Nickel and Cr are concentrated significantly in the saprolite zone and are positively correlated with Fe2O3 content, which is controlled by the formation of nontronite, montmorillonite, and Fe-(oxyhydr)oxide phases. Nickel-Cr-bearing nontronite and montmorillonite precipitated from alkaline water as a result of the increasing (Fe2O3+Al2O3+Cr2O5+Ni+Co)/(MgO+SiO2) ratio under the control of both chemical weathering and hydrothermal processes. The Fe and Mg (associated with Ni and Cr) required for the formation of smectite were supplied by solutions from both chemical weathering and hydrothermal alteration of Ni-Cr-bearing olivine and pyroxene in the harzburgitic bedrock; the Al was supplied by schists, granite, and volcanic units.

The crystal structure of biotite-1 M from Bancroft, Ontario, with the formula: $\left({\text{K}}_{1.96}{\text{Na}}_{0.13}{\text{Ca}}_{0.01}\right)\left({\text{Mg}}_{3.15}{\text{Fe}}_{2.59}^{2+}{\text{Ti}}_{0.17}{\text{Mn}}_{0.09}\right)\left({\text{Si}}_{5.98}{\text{Al}}_{1.92}{\text{Ti}}_{0.10}\right){\text{O}}_{20}\left[{\left(\text{OH}\right)}_{1.47}{\text{F}}_{1.98}\right]$, was determined by Rietveld refinement using high-resolution neutron powder diffraction at in situ temperatures ranging from 20 to 900°C. The room-temperature structure of the samples heated to between 400 and 900°C using an electric furnace in air was also refined. The crystal structures were refined to an RP of 2.98 — 5.06% and Rwp of 3.84–6.77%. For the in situ heating experiments in a vacuum, the unit-cell dimensions increased linearly to 600°C. The linear expansion coefficient for the c axis was 1.65 × 10−5°C−1, while those for the a and b dimensions were 4.44 × 10−6°C−1 and 5.21 × 10−6°C−1, respectively. Accordingly, the increase in the unit-cell volume up to 600 C occurred mainly along the c axis, resulting from the expansion in the K coordination sphere along that direction. Results for all K−O bonds were analyzed in terms of the lattice component and an inner component of the structural strain. The ditrigonal distortion decreased (3.76 at 20°C to 1.95 at 600°C) with temperature, because the shorter bonds expanded and the longer bonds contracted. The increase in the interlayer separation and the decrease in the interlayer octahedral flattening angle confirmed that the c-dominated expansion occurred in the interlayer region. In the case of the ex situ-heated samples, the cell dimensions decreased sharply at temperatures over 400 C. The octahedral sheet thickness and mean <M−O> distance decreased linearly due to oxidation of octahedral Fe. However, the interlayer separation and mean <K−O> distance decreased at temperatures over 400°C. At 400°C, dehydroxylation began to increase and interlayer regions became more constricted. The overall cell parameters decreased rapidly with increasing temperatures due to dehydroxylation. The large inner strain components in the K−O bonds also resulted in an increase in the considerable ditrigonal distortion (3.57° at 400°C to 6.15° at 900°C).

Fluoride is an essential component in the mineralization of bones and in the formation of dental enamel. Excessive intake may result, however, in teeth mottling and dental and skeletal fluorosis. With an average fluoride concentration of ~2.4 mg L−1 in Tunisian drinking water, the present study focused on promoting low-cost materials for removal of excess fluoride. Two Tunisian raw clays were used as adsorbents in a batch process to eliminate excess fluoride ions from drinking water and, thus, avoid fluorosis phenomena. Physicochemical characterization and chemical analysis of the raw clays were carried out using X-ray fluorescence, X-ray diffraction, and the BET method. For fluoride removal, the effects of contact time, adsorbent dose, and pH were evaluated. The optimum defluoridation capacity was at 30 min of contact time, 20 g/L of clay dose, and at pH = 3. The kaolinite tested removed more fluoride than smectite. The selected clay was used successfully to remove fluoride from contaminated water with high concentrations of foreign ions that exceeded the potability limits. Adsorption isotherms revealed that the data fitted well to both the Langmuir and Freundlich adsorption isotherms, thus confirming both monolayer and multilayer adsorption.

Electron energy-loss spectroscopy (EELS), energy-filtered transmission electron microscopy (EFTEM), and high-resolution transmission electron microscopy (HRTEM) have been applied in mineralogy and materials research to determine the oxidation states of various metals at high spatial resolution. Such information is critical in understanding the kinetics and mechanisms of mineral–microbe interactions. To date, the aforementioned techniques have not been applied widely in the study of such interactions. In the present study, the three techniques above were employed to investigate mineral transformations associated with microbial Fe(III) reduction in magnetite. Shewanella putrefaciens strain CN32, a dissimilatory metal-reducing bacterium, was incubated with magnetite as the sole electron acceptor and lactate as the electron donor for 14 days under anoxic conditions in bicarbonate buffer. The extent of bioreduction was determined by wet chemistry and mineral solids were investigated by HRTEM, EFTEM, and EELS. Magnetite was partially reduced and biogenic siderite formed. The elemental maps of Fe, O, and C and red-green-blue (RGB) composite map for residual magnetite and newly formed siderite were contrasted by the EFTEM technique. The HRTEM revealed nm-sized magnetite crystals coating bacterial cells. The Fe oxidation state in residual magnetite and biogenic siderite was determined using the EELS technique (the integral ratio of L3 to L2). The integral ratio of L3 to L2 for magnetite (6.29) and siderite (2.71) corresponded to 71% of Fe(III) in magnetite, and 24% of Fe(III) in siderite, respectively. A chemical shift (~1.9 eV) in the Fe-L3 edge of magnetite and siderite indicated a difference in the oxidation state of Fe between these two minerals. Furthermore, the EELS images of magnetite (709 eV) and siderite (707 eV) were extracted from the electron energy-loss spectra collected, ranging from 675 to 755 eV, displaying different oxidation states of Fe in the magnetite and siderite phases. The results demonstrate that EELS is a powerful technique for studying the Fe oxidation-state change as a result of microbial interaction with Fe-containing minerals.

The grey-green peloids from the Miocene period to Recent fine-grained deposits on the continental shelf close to Congo-Zaîire River mouth were studied by X-ray transmission diffractometry (XRD), SEM and by EDAX. The peloids have multiphase heterogenous mineral composition. Their most important constituents are detrital minerals like kaolinite, quartz, goethite, 7 Å phases with d(001) ≈ 7.3 Å, and in more maturated grains—nontronite. The d(060) values were used to estimate the general composition of phyllosilicate phases to compare with the composition determined by EDAX. It has been found that d(060) equal to 1.504 Å is common for Fe3+-bearing kaolinite, which is quite abundant for the Recent peloids. The d(060) equal to 1.535 Å and 1.55 Å is characteristic for the di-trioctahedral and trioctahedral 1:l phases, which are abundant within the more evolved Miocene peloids. Nontronite is characterized by d(060) equal to 1.524 Å within concordance with its highly ferrous composition, and partly by its potassic interlayer. It shows cabbage-like nannostructures proving neoformational origin of this mineral in the marine environment.

It has been shown that areas of the low sedimentation rate within the Congo Basin were favorable for the mineral changes and neoformation. For the Holocene vertical profile, we observed levels of slower sedimentation rates. The evolution is expressed by the disappearance of kaolinite at the expense of other 7 Å phases and nontronite. Although more advanced stages of maturation of the studied phases were observed in older peloids (104 to 107 y), one cannot detect a linear relationship of these processes with burial.