Temperature dependence of the magnetic rotation process of kaolinite microcrystals containing paramagnetic impurity ions dispersed in liquid medium

Temperature dependencies of magnetic rotation were measured in micron-sized silicates dispersed in ethanol for two different samples of kaolinite. Magnetic rotation proceeded by balance between thermal agitation energy and magnetic anisotropy energy. Measurements were performed between 195 and 343 K. The field intensity required to achieve magnetic alignment of microcrystals increases with temperature, because of the temperature dependence of paramagnetic anisotropy, and the temperature dependence of thermal agitation energy. The results indicate that the values of magnetic anisotropy of nonmagnetic materials might partially derive from the paramagnetic moments, which derive from paramagnetic impurity ions. The present experiment provides a technical basis for determining the precise values of diamagnetic anisotropy ()_DIA from minerals which have a concentration of paramagnetic ions and do not form a single crystal large enough to allow bulk measurements. The values of ()_DIA can be obtained by extrapolating the –T relations, which follow the Curie law, to the temperature limits.     

Radiation-induced paramagnetic defects in natural kaolinites: Alpha dosimetry with ion beam irradiation

Three types of paramagnetic radiation-induced defects (RID), namely A, A′ (Si-O^?-centers) and B (Al-O^?-Al center), had been identified in natural kaolinites by means of electron paramagnetic resonance spectroscopy. The A-center, stable at the scale of geological periods, was thought to be of particular relevance to quantify past transits of radionuclides in the geosphere. Alpha radiation being likely the main source of RID in kaolinite, the objective of this paper is to define the role of α-particles on the creation of RID and to test the use of A-centers for an α-dosimetry. Three kaolinites with different crystalline order and containing other clays as impurities were irradiated with He⁺ ion beams. The radiation dose range (0–750 MGy) was consistent with natural radioactivity in environments from the Earth's surface. Irradiation drastically enhanced the original signals due to RID. An important increase of concentration of the unstable B-center, partly due to unrealistic dose rate provided by accelerator ion beam, was observed from the lower doses. The most stable defects remained of the Atype all along experimental irradiations. The contribution of ancillary phyllosilicates to EPR spectra was negligible. The concentration of the three types of RID was related to radiation doses up to 30 MGy. Dosimetry growth curves for the A-centers exhibited variable efficiencies and saturation levels that were related to the structural order and the chemical purity of the kaolinites: the more ordered and purer the kaolinite, the higher the efficiency and the lower the saturation plateau. Moreover, these results are of geochemical significance: dosimetry based on A-centers could be directly used to quantify past migrations of radioelements in the geosphere, by determination of the naturally-cumulated doses (paleodose) of kaolinites.     

Evidence of native radiation-induced paramagnetic defects in natural illites from unconformity-type uranium deposits

This study presents the first unequivocal identification of natural radiation-induced defects in illites. Middle Proterozoic illites related to unconformity-type uranium deposits of Canada and Australia were studied using electron paramagnetic resonance (EPR) spectroscopy at X- and Q-band frequencies. The saturation behaviour of EPR spectra as a function of power demonstrates that native defects of illites are different from those known in other clays as kaolinite, dickite or smectite. Q-band spectra indicate the presence of several––at least two––native defects. The EPR signal is dominated by an axially distorted spectrum with apparent principal components as follows: g _∥ = 2.032 and g _⊥ = 1.993. The corresponding defect is named as A_i center. The study of oriented specimen confirms the strong anisotropy, and shows that the main defect has its g _∥ component perpendicular to the (ab) plane of illite. These defects in illite correspond to electron holes located on oxygen atoms of the structure and likely associated to Si, according to the lack of hyperfine structure. The A_i center in illite has similar EPR parameters to the A center in kaolinite and dickite. The isochronal annealing data suggest that illite can be used as a dosimeter in the geosphere. However, the determination of half-life and activation energy of the A_i center requires additional work.     

The ordered creation of paramagnetic defects at plastic deformation of natural diamonds

The EPR-study showed that natural purple diamonds from kimberlites of Eastern Siberia (Russia) contain well known P1, P2 (in some samples), W7, and N2 centers. The EPR spectra of these centers were typical of plastically deformed diamond single crystals. Besides, several intense additional spectra of di-nitrogen centers were observed in purple diamonds. The angular dependence analysis of these spectra showed that they can be attributed to known M2 centers. Comparison of principal axis directions observed for sites of the M2 center in purple diamond crystals with theoretically predicted directions in the twin crystal revealed that these centers are allocated exclusively to the twinned lamellae. Unusual phenomenon of the ordered distribution of paramagnetic centers in natural purple diamonds confirmed that the plastic deformation in natural diamonds can be induced not only by the slip of dislocations but also by the mechanical twinning.     

Formation and evolution of lateritic profiles in the middle Amazon basin: Insights from radiation-induced defects in kaolinite

The content of radiation-induced defects (RIDs) in kaolinite samples originating from lateritic soils and continental detritic sediments of the middle Amazon Basin (Brazil) is investigated using electron paramagnetic resonance. The paleodose registered by kaolinites ranges from 80 to 900 kGy. Present-day dose rates of radiation, determined from the whole-rock U and Th content, range between 4000 and 40,000 mGy/ka. In most samples, U and Th concentrations are correlated, suggesting that U has not been remobilized by lateritization. This observation is consistent with the fact that ∼80% of the total U content is incorporated in resistant minerals, such as zircon and Ti oxides. The heterogeneous distribution of U, observed by induced fission tracks mapping, makes it possible to neglect the α-radiation contribution of the U decay chains in the dose-rate calculation. The interpretation of the measured content of RIDs in kaolinite is then performed using the calculated present-day dose rate and assuming equilibrium in the radioactive decay chains. For the sedimentary samples, the amount of RIDs is broadly correlated to the dose rate and provides apparent absolute ages older than 20 Ma. The RID contents in kaolinites from the lateritic soils provide apparent ages ranging from 10 to 6 Ma. The high RID content of these lateritic kaolinites shows that their chemical, isotopic, and crystallographic properties are not representative of present-day weathering conditions. Models assuming the “dynamical equilibrium” of kaolinites with local physical-chemical conditions prevailing in lateritic soils are thus questionable. Alternatively, our findings bring strong support for the use of the isotopic composition of kaolinites to decipher continental paleo-climates.     

煤系高岭石的原始晶粒尺寸及结晶度对插层作用的影响

采用SEM、XRF、XRD和IR研究了大同、平朔和淮北煤系高岭石的结构、粒度及其特性,探讨了粒度大小对高岭石插层作用的影响及其机理。研究发现:粒度中等且结晶有序的平朔煤系高岭石插层率最高,其次为晶体粗大但结晶有序的大同高岭石,粒度最小且结晶无序的淮北高岭石插层率最低。研究认为高岭石原始晶粒粒度对高岭石插层作用有重要影响,中等粒度最有利于高岭石插层作用的进行,粒度过大或过小均不利于高岭石插层作用的进行,其原因是由于不同粒度的高岭石插层作用过程中导致的差异弹性变形引起的。结晶有序度对高岭石插层作用也有重要影响,结构无序不利于插层作用的进行。     

Structure of sediments of kaolinite

In an assembly of clay particles placed in a fluid, each particle is typically subjected to: (1) double-layer repulsive forces; (2) van der Waals attractive forces; and (3) contact mechanical forces. The study presented here concerns an approximate, quantitative analysis of clay suspensions, with considerations to the first two - the physico-chemical forces. Using recent theories to calculate the physico-chemical forces between two clay particles in an approximate model of an assembly, the equilibrium void ratio of a clay suspension is computed. The mechanical forces are ignored in the analysis. The results serve to verify the validity of physico-chemical theories employed and help interpret experimental data more fundamentally in terms of the system variables.     

Intercalation of kaolinite with acetamide

Upon intercalation of both ordered (low defect) and disordered (high defect) kaolinites with acetamide, two types of interaction are observed. Firstly, hydrogen bonding between the NH₂ groups of the acetamide with the siloxane oxygens is formed, as evidenced by the formation of two new bands at 3400 and 3509 cm^–1. Secondly, the appearance of additional bands at ∼3600 cm^–1 in both the infrared and Raman spectra of the acetamide intercalates is attributed to a second type of hydrogen bonding by the interaction of the C=O group and the inner surface hydroxyls. Changes in the intensity of the hydroxyl deformation modes in the 895 to 940 cm^–1 region are attributed to the changes in the hydrogen bonding of the kaolinite surfaces. It is proposed that the hydrogen bonding between the adjacent kaolinite layers is replaced with hydrogen bonding between both kaolinite surfaces and the acetamide molecule. Changes in the molecular structure of acetamide are observed upon intercalation. The amide 1 band is lost and replaced with a well-defined NH₂ deformation vibration. The loss of the amide 1 band is attributed the hydrogen bond formation between the amide hydrogens and the siloxane surface. The bands of the C=O group at 1680 and 1740 cm^–1 become a single band at 1680 cm^–1. The amide 2 band remains unchanged. The lack of intensity of the 1740 cm^–1 band is attributed to the formation of hydrogen bonding between the inner surface hydroxyl groups and the carbonyl group. Received: 4 February 1998/ Revised, accepted: 30 June 1998     

Electron paramagnetic resonance study of new paramagnetic centers in microcline-perthites from pegmatites

Four new types of paramagnetic centers — NH⁺ ₃, N^2?, Al-O^?, E ₁ — have been detected in microcline perthites from pegmatites in the Ukrainian Shield. Values are tabulated for their g and A tensors and limits of thermal stability determined. The NH⁺ ₃ center substitutes the K⁺ ion. It occurs naturally in potash feldspars but is intensified by gamma or X-ray irradiation. It is regarded as a radiational development of the more general NH⁺ ₄ ? K⁺ isomorphism. It disappears after heating to temperatures higher than 470 K. The N^2? center is an uncommon example of isomorphous substitution of a bridging oxygen, being located on a O _D(m) site between T ₂(o) and T ₁(m) silicon sites. It is stable to 820 K. The hole center, Al-O^?, has been detected on an O _A(l) oxygen shared by T ₁(o) and T ₁(m) tetrahedra. It is stable to 590 K. The E ₁ center in these alkali feldspars is similar to the E ₁ center in quartz, being an electron trapped in an oxygen vacancy in the O _B (o) position. It is stable to 420 K. The NH⁺ ₃, Al-O^? and E ₁ centers can be restored from temperatures above their stability limits by gamma radiation. Concentration of centers varies from sample to sample depending on conditions of formation and subsequent history of the minerals.     

Characterization of dehydration-induced luminescence of kaolinite

Dehydration-induced luminescence (DIL), the emission of light from a clay paste upon dehydration, was characterized experimentally for a colloidal kaolinite. The relationship between total photon count of the emitted light and film thickness is linear up to a thickness of 30 micrometers. The photon emission was obtained over a critical range of water contents (25-60%) of the oven-dry clay, and the kinetics of photon emission was presumed to be closely associated with the kinetics of film dehydration. Whether drying proceeded throughout the bulk or via a moving front was undetermined, but in either mode it was preceded by the formation of a thin dry film at the interface with the atmosphere. Grinding of the kaolinite for several minutes by mortar and pestle before paste preparations resulted in an overall increase of photon emission compared to unground kaolinite and in the formation of more than one emission peak, as well as a prolongation of the light emission. This effect on the kinetics of light emittance was observed for about two months after the application of the mechanical stress and suggests a means of detecting the mechanical stress history of a clay. An estimate was made of the spectral characteristics of the emitted light using optical filters and by incorporating tryptophan and salicylic acid into the kaolinite paste where they acted as fluorescent probes. The latter technique shifted the frequency of the light emitted by the kaolinite from the ultraviolet to the visible range where it was less effectively reabsorbed. The first method showed that the wavelengths of 97% of the emitted light was <460 nm and that 75% of the light had wavelengths < 410 nm. The second method showed that the total intensity of DIL increased in the presence of fluorescence molecules, suggesting that the emittance was in the ultraviolet range.     

Long-term behavior of lime-stabilized kaolinite clay

Clay soils create many problems for highway construction and they have to be replaced or improved by stabilization for satisfactory performance. Lime stabilization is a well-established technique to improve the performance of clays. Cementitious minerals form upon mixing of clay with lime causing an improvement in strength and durability. In the study, the changes in the microfabric of long-term cured lime-stabilized kaolinite clay using X-ray diffraction pattern, scanning electron microscope and unconfined compressive strength (UCS) is presented. Unconfined compression test samples at two different lime contents (4 and 12% by weight) were prepared and cured in a humidity room for long time curing. The UCS of pure kaolinite was originally 125 kPa, which increased to 1,015 kPa after 1 month and to 2,640 kPa (21 times the initial value) after 10 years for cured lime-stabilized kaolinite samples. Similar long-term strength increases were also observed for stabilized kaolinite with 12% lime. Calcium aluminate silicate hydrate minerals were detected in the structure of the kaolinite. This suggests pozzolanic reactions with lime stabilization may continue in the long-term for up to 10 years.     

铅在高岭石表面的吸附模式

采用表面络合模式,研究了高岭石表面的酸碱性质以及高岭石表面对铅的吸附行为。结果表明,高岭石表面在不同ｐＨ条件下对质子和铅离子的吸附反应可以用单一表面基团、无静电表面络合模式来描述。高岭石表面酸度常数拟合值分别为ｐＫａ１＝２．７５和ｐＫａ２＝５．５２,电荷零点ｐＨｚｐｃ＝４．１铅在高岭石表面的吸附量随ｐＨ值的升高而增加,吸附铅的两种表面化合态〉ＳＯＰｂ＾＋和〉ＳＯＰｂＯＨ的浓度也随ｐＨ值的变化而变化     

O-H stretching vibrations in kaolinite,and related minerals

The grades of ordering as represented by I.R. and X-ray spectra for platy kaolinite, fire clay mineral, ball clay, dickite, nacrite and tubular dehydrated halloysite samples of various origin, are related.Well ordered kaolinite, has four bands 3,693, 3,668, 3,652 and 3,620 cm^–1. In less ordered kaolinite, fire clay mineral and ball clay these bands have shifted a little or of the middle two, which are the weakest, only one band may be left (between brackets): less ordered kaolinite: 3,695, 3,667, 3,652 (3,653) and 3,620 cm^–1; fire clay mineral 3,696, 3,668, 3,653 (3,653) and 3,621 cm^–1; ball clay 3,697, 3,652 and 3,621 cm^–1.Dehydrated halloysite has 3,693–3,698, 3,668, 3,650–3,654 and 3,620–3,626 cm^–1 bands. In the most disordered dehydrated halloysite samples (Martinsberg and Baia Mare) only two bands are left at 3,696 and 3,624 cm^–1.Dickite has four bands: 3,708, 3,656, 3,627 and 3,622 cm^–1. For the lesser ordered Mexico sample it is 3,701, 3,652, 3,627 and 3,621 cm^–1. Nacrite also has four bands i.e.: 3,700, 3,650, 3,627 and 3,620 cm^–1.There is, apart from orientation effects, a wide variation in the absolute and especially in the relative intensities of the I.R. bands and X-ray reflections for each of the mineral groups investigated here, but of various origin. The need of a nomenclature adapted to the level of our knowledge about this matter to day and comprising all these variations included those in morphology is emphasized.Quantitative analyses, the adjective meant in the sense of an accuracy of minimal 5%, constitute a difficult problem which is perhaps even wholly impossible to solve with the conventional methods of today.     

Copper(II) humate mobility in kaolinite soil

An investigation of the influence of humate on the mobility of copper(II) ions in a kaolinite soil using leaching tests and electrokinetic experiments is reported. The data are interpreted in terms of humate–copper–clay interactions and humate electrical charge. Humate is mostly immobile below pH8 but is more mobile in alkaline conditions (sorption to kaolinite reduces its mobility in neutral conditions). Copper humate complexes are mobile in both acidic and alkaline conditions, but not in neutral conditions where they are sorbed. The dissolved copper humate complexes that form in acidic conditions are positively charged. The net effect of humate is to increase cupric ion mobility in kaolinite soil, especially in alkaline conditions.     

Strengthening mechanisms in cement-stabilized kaolinite revealed by cross-scale nanoindentation

Acta Geotechnica - The macroscale mechanical properties of cement-stabilized soil have been widely reported, but the microscale behavior remains largely unexplored. This paper presents the first...     

Sorption of lanthanides on smectite and kaolinite

Experiments were carried out to investigate the sorption of the complete lanthanide series (Ln or rare earth elements, REE) on a kaolinite and an a Na-montmorillonite at 22°C over a wide range of pH (3-9). Experiments were conducted at two ionic strengths, 0.025 and 0.5 M, using two different background electrolytes (NaNO₃ or NaClO₄) under atmospheric conditions or N₂ flow (glove box). The REE sorption does not depend on the background electrolyte or the presence of dissolved CO₂, but is controlled by the nature of the clay minerals, the pH and the ionic strength. At 0.5 M, both clay minerals exhibit the same pH dependence for the Ln sorption edge, with a large increase in the sorption coefficient (K_D) above pH 5.5. At 0.025 M, the measured K_D is influenced by the Cation Exchange Capacity (CEC) of the minerals. Two different behaviours are observed for smectite: between pH 3 and 6, the K_D is weakly pH-dependent, while above pH 6, there is a slight decrease in log K_D. This can be explained by a particular arrangement of the particles. For kaolinite, the sorption coefficient exhibits a linear increase with increasing pH over the studied pH range. A fractionation is observed that due to the selective sorption between the HREEs and the LREEs at high ionic strength, the heavy REE is being more sorbed than the light REE. These results can be interpreted in terms of the surface chemistry of clay minerals, where two types of surface charge are able to coexist: the permanent structural charge and the variable pH-dependent charge. The fractionation due to sorption observed at high ionic strength can be interpreted either because of a competition with sodium or because of the formation of inner-sphere complexes. Both processes could favour the sorption of HREEs according to the lanthanide contraction.