Article

Al-O - Al paramagnetic defects in kaolinite

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Abstract

Trapped holes located on Al-O-Al bonds in kaolinite were studied by electron paramagnetic resonance spectroscopy (EPR) at 9.3 and 35 GHz applied to well-crystallized, X-ray irradiated and oriented samples. The Q-band EPR spectrum is characterized by three clearly separated groups of 11 quasi-equidistant superhyperfine lines centered at gxx=2.040±0.0005, gyy=2.020±0.0005 and gzz=2.002±0.001. In each of these groups, the 11 superhyperfine lines exhibit intensities according to the ratios 1:2:3:4:5:6:5:4:3:2:1. An angular dependence of the Q-band EPR spectrum with respect to the magnetic field is demonstrated by measurements on oriented films of kaolinite. An appropriate numerical treatment of the EPR spectra is described, which allowed extraction of the SuperHyperfine Structures (SHFS). X-and Q-band spectra have also been simulated. It is concluded from these experiments that only one type of center is present. This center, labelled the B-center in the literature, is very probably a hole trapped on oxygen (O- center) atoms coupled to two octahedral aluminium.

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... Kaolinite, a ubiquitous clay mineral at the Earth's surface, is a crucial archive for deciphering the complex sequence of geological, soil, or paleoclimatic transformations over time (Murray, 1988(Murray, , 2007Balan et al., 2014;Mathian et al., 2019Mathian et al., , 2020Mathian et al., , 2022. Its structure exhibits a spectrum of various points or extended defects (Cases et al., 1982;Giese, 1988;Clozel et al., 1994), offering valuable insights into formation conditions and geological history. In particular, irradiation can induce radiation-induced defects, the influence of which on changes in material physicochemical properties, including specific surface area, cation exchange capacity, and mean layer charge, has been explored previously (Pushkareva et al., 2002;Plötze et al., 2003;Allard and Calas, 2009;Balan et al., 2011). ...
... Four RIDs were differentiated according to their EPR signal, their symmetry, and the presence of a superhyperfine structure or not, namely A-, A', B-, and H-centers (Allard et al., 2012). Some isochronal or isothermal annealing experiments demonstrated that these defects exhibit contrasting stabilities (Clozel et al., 1994). Furthermore, Goodman et al. (2016) proposed the existence of an additional radiationinduced defect known as the C-center, but its stability was not explored. ...
... The first investigation focussed on the relevance of the 250 • C, 2 h treatment to remove unstable defects after artificial irradiations to check if these conditions ensure a stable resulting concentration of A-centers. Among unstable defects are B and A' centers, as inferred from isochronal heating experiments (Clozel et al., 1994;Allard et al., 1994). Measurements were systematically carried out at specific time intervals: 2 h, 5 h, 27 h, 72 h, 144 h, and 408 h. ...
Article
Kaolinite naturally contains radiation-induced electron defects in its structure induced by ionizing radiations and can be analyzed with electron paramagnetic resonance spectroscopy (EPR). They include the so-called A-center, which is crucial for geochronological applications due to its stability at the scale of geological periods. Indeed, previous studies have substantiated the thermal stability of naturally occurring A-centers in kaolinite through annealing experiments. However, the stability of artificially created A-centers, which are used to determine paleodose through calibration via irradiation experiments and subsequent dating of kaolinite formation, still needs to be more adequately specified to confirm their relevance. Therefore, this research probed the thermal stability of A-centers produced by ionizing 1.5 MeV He + beams in two samples, i.e., the reference KGa-2 and a very disordered lateritic soil kaolinite. Heating experiments showed that the annealing of irradiated samples at 250 • C was relevant to determining A-center concentration. Second, the annealing of artificial A-centers at higher temperatures was found to be a second-order process, as for natural A-centers. Annealing parameters such as activation energy (2-2.6 eV) were determined with estimates of half-life at 300 K, which was found to be >10 12 years for natural and artificial A-centers. Thus, the parameters for artificial A-centers are consistent with the thermal annealing behavior of their natural counterparts. These new findings affirm that artificially generated A-centers mimic natural ones in terms of EPR spectra and thermal stability, enhancing the reliability and precision of the EPR dating methodology used in kaolinite dating.
... centers (Meyer et al. 1984 (Clozel et al. 1995) . This is also the case for the montebrasite (Bershov and Martirosyan 1970) which is investigated in this work. ...
... However, the classification scheme of Marfunin does not describe perfectly 328 this color center in topaz, because the deviation of the g-factor from that of free electron is 329 unusually large. Even though, the g values of the Ohole center in montebrasite are closely 330 comparable to those in jeremejevite(Li et al. 2012) and kaolinite(Clozel et al. 1995)). These O -331 centers, together with that in neutron-irradiated topaz can still be considered as variants of 332Marfunin's second type, i.e. those known as -type. ...
Article
Montebrasite is a lithium aluminum phosphate mineral with the chemical formula LiAlPO4(Fx,OH1–x) and considered a rare gemstone material when exhibiting good crystallinity. In general, montebrasite is colorless, sometimes pale yellow or pale blue. Many minerals that do not have colors contain hydroxyl ions in their crystal structures and can develop color centers after ionization or particle irradiation, examples of which are topaz, quartz, and tourmaline. The color centers in these minerals are often related to O− hole centers, where the color is produced by bound small polarons inducing absorption bands in the near UV to the visible spectral range. In this work, colorless montebrasite specimens from Minas Gerais state, Brazil, were investigated by electron paramagnetic resonance (EPR) for radiation-induced defects and color centers. Although γ irradiation (up to a total dose of 1 MGy) did not visibly modify color, a 10 MeV electron irradiation (80 MGy) induced a pale greenish-blue color. Using EPR, O− hole centers were identified in both γ- or electron-irradiated montebrasite samples showing superhyperfine interactions with two nearly equivalent 27Al nuclei. In addition, two different Ti3+ electron centers were also observed. From the γ irradiation dose dependency and thermal stability experiments, it is concluded that production of O− hole centers is limited by simultaneous creation of Ti3+ electron centers located between two equivalent hydroxyl groups. In contrast, the concentration of O− hole centers can be strongly increased by high-dose electron irradiation independent of the type of Ti3+ electron centers. From detailed analysis of the EPR angular rotation patterns, microscopic models for the O− hole and Ti3+ electron centers are presented, as well as their role in the formation of color centers discussed and compared to other minerals.
... Several samples were irradiated with He + ions and ~-rays in order to simulate the effects of 2 main ionizing radiations occurring in natural systems, a-particles and ",/-rays. Irradiation produces 3 types of electron-hole defects associated with oxygen atoms, referred to as B-(A1-O--A1), A-(Si-O) and A'-(Si-O-) centers, according to their nature and stability (Muller et al. 1992;Clozel et al. 1994Clozel et al. , 1995. ...
... In order to explain these results, it is necessary to specify the nature of the RIDs. Recent irradiation experiments with several natural kaolinites have shown that irradiations with He + beam and ~/ rays produce mainly B-centers, that is, AI-O -A1 centers Clozel et al. 1995). B-center concentration was at least 1 order of magnitude higher than the one of the other centers within the dose range investigated (<750 MGy). ...
Article
Full-text available
The Fe 3+ substituted for AP + at the 2 octahedral positions is one of the most common im-purities in the kaolinite structure detected by electron paramagnetic resonance (EPR). Evidence has been provided for a relationship between the shape of EPR spectra for structural Fe and the structural disorder in kaolinite. It is proposed that the structural Fe be used as a sensitive probe for the degree of disorder of natural kaolinites. With this aim in view, an EPR disorder index (E) is defined from the width of selected EPR lines. Using reference kaolinites, it is shown that this index can account as well for long-range disorder detected by means of X-ray diffraction (XRD) as for local perturbations such as radiation-induced defects (RID). It is shown that the disorder observed through EPR has some points in common with the XRD-measured one. The influence on E of the presence of RID is shown by the study of artificially and naturally irradiated kaolinites.
... Several samples were irradiated with He + ions and ~-rays in order to simulate the effects of 2 main ionizing radiations occurring in natural systems, a-particles and ",/-rays. Irradiation produces 3 types of electron-hole defects associated with oxygen atoms, referred to as B-(A1-O-A1), A-(Si-O) and A'-(SiO-) centers, according to their nature and stability (Muller et al. 1992;Clozel et al. 1994Clozel et al. , 1995. ...
... In order to explain these results, it is necessary to specify the nature of the RIDs. Recent irradiation experiments with several natural kaolinites have shown that irradiations with He + beam and ~/ rays produce mainly B-centers, that is, AI-O-A1 centers Clozel et al. 1995). B-center concentration was at least 1 order of magnitude higher than the one of the other centers within the dose range investigated (<750 MGy). ...
Article
The Fe 3+ substituted for AP + at the 2 octahedral positions is one of the most common im- purities in the kaolinite structure detected by electron paramagnetic resonance (EPR). Evidence has been provided for a relationship between the shape of EPR spectra for structural Fe and the structural disorder in kaolinite. It is proposed that the structural Fe be used as a sensitive probe for the degree of disorder of natural kaolinites. With this aim in view, an EPR disorder index (E) is defined from the width of selected EPR lines. Using reference kaolinites, it is shown that this index can account as well for long- range disorder detected by means of X-ray diffraction (XRD) as for local perturbations such as radiation- induced defects (RID). It is shown that the disorder observed through EPR has some points in common with the XRD-measured one. The influence on E of the presence of RID is shown by the study of artificially and naturally irradiated kaolinites.
... ions on well-ordered kaolinite. The studied sample is a hydrothermal kaolinite DCV with a very high cristallinity (Gaite et al. 1997; Clozel et al. 1995; Balan et al. 1999 Balan et al. , 2000). The ionizing irradiation has been performed in thin films, in which ballistic effects have been avoided. ...
... The EPR spectra of irradiated DCV kaolinite show an intense signal at g = 2 (i.e., 0.336 T), the intensity of which increases with the radiation dose (Fig. 3). Previous studies have shown that this signal corresponds to the superimposition of different defects (Clozel et al. 1995; Allard et al. 1994 ). The irradiated samples present a relative concentration of Al–O–Al defects (B-centers) higher than that of the pristine sample. ...
Article
Full-text available
Radiation effects on kaolinite were investigated using He+ ions of 1.5MeV at radiation doses up to 4.3×108Gy, which are comparable to the doses expected for clay barriers in high-level nuclear waste repositories. The concentration of paramagnetic radiation-induced defects in kaolinite reaches 2×1016 spins/mg (400 at. ppm), as determined by electron paramagnetic resonance spectroscopy. The broadening of X-ray diffraction patterns and transmission infrared (IR) absorption bands is mostly related to the structural strain induced by radiation-induced point defects. The broadening of IR absorption spectra is analyzed using an autocorrelation approach and is related to a change in the distribution of vibrational frequencies due to crystal heterogeneities. We theoretically analyze how the effective dielectric properties of kaolinite samples depend on macroscopic parameters and how irradiation can modify some of them. Irradiation leads to an increase in the electronic polarizability of kaolinite particles, related to the accumulation of radiation-induced electronic point defects.
... The hyperfine pattern was assigned to the interaction of an unpaired electron (S = ½) with two adjacent nuclei 27 Al (I = 5/2, natural abundance 100%) that are equidistant from the paramagnetic centre. Based on the work by Clozel et al. (1995) it was concluded that the B centre is a hole trapped on the oxygen bonding tow Al in adjacent octahedral positions (AlVI-O --AlVI) bridge). They showed that in the case of an AlIV-O-AlVI bond the AlVI-O bond is 17% longer than the AlIV-O bond. ...
Chapter
This chapter gives an overview of how spectroscopic methods such as Mid-Infrared spectroscopy, Raman spectroscopy and Magic-Angle-Spinning Nuclear Magnetic Resonance spectroscopy have lead over the decades to a better understanding of the layer structure and the effects of layer stacking i.e. order/disorder and differences between polymorphs. Though the interpretation of the different bands remains still some matter of debate, the advances in molecular modelling of the crystal structures of the different minerals has resulted in a much better understanding of the Infrared and Raman spectra of the kaolin minerals. Mössbauer and EPR spectroscopy show the presence of substitutions of transition metals such as Fe, Mn and V in the crystal lattice as well as the role of radiation induced defects. Finally, a possible link is shown between the defect structures and luminescence/cathodeluminescence.
... The B-centre is the most easily formed with ionizing radiations. Its nature was determined by Clozel et al. (1995). It consists of an electron hole located on an O atom, displaying an orthorhombic symmetry (Fig. 5.44a). ...
Chapter
This chapter presents an overview of the spectroscopic analyses of the effects of thermal, mechanical and chemical treatments on the kaolin group minerals. Thermal treatment and the formation of metakaolinite as an intermediate before the crystallization of mullite has been a matter of discussion for more than 70 years. Infrared, including Infrared Emission, spectroscopy, Raman spectroscopy and Magic-Angle-Spinning Nuclear Magnetic Resonance spectroscopy have played a major role in unravelling the reactions and the corresponding structural changes. The use of grinding and its effect on particle size and order/disorder has been extensively studied by in particular infrared spectroscopy. Chemical treatment, especially acid treatment and the leaching of metals has been a more limited subject of research. The effect of irradiation has been introduced in the previous chapter and is explored in more detail in this chapter.
... The A′-centre also has an anisotropic EPR signal without hfs, but with a smaller g / -value, of ∼2.040, than that of the A-centre; it is considered to be located on the surface of the tetrahedral sheet (Lombardi et al., 2002). In contrast, the B-centre contains extensive 27 Al hfs of ∼0.76 mT, and three 11peak features with g values of 2.040, 2.020 and 2.002 (Clozel et al., 1994(Clozel et al., , 1995. It is probably associated with (deprotonated) surface oxygen atoms on the octahedral sheet (Allard & Calas, 2009). ...
Article
The EPR spectrum of a previously unreported paramagnetic centre formed in kaolin minerals by exposure to γ radiation is described. This centre, which is referred to here as the 'C-centre', was seen initially during an investigation of the radiation dose response of the EPR signal in the natural Lampang kaolin from northern Thailand, and its EPR properties are now presented for a purified sample from this material. They suggest a paramagnetic centre with rhombic symmetry based on O- associated with a single ²⁷Al atom. Computer simulations suggest spin Hamiltonian parameters of g1 = 1.976, g3 = 2.0417 and A(²⁷Al) = 2.10 mT, with g² 2.01. This C-centre was also seen as a minor radiationinduced component in both crude and purified Ranong kaolin samples, along with a stronger signal from the B-centre radical. It seems to be associated with the kaolinite component, but was lost on annealing to 300°C after which only the signal from the A-centre was visible.
... Since this value is superior than the g factor of the free electron (e.g. 2.0023), it corresponds to a hole trapped on an oxygen atom of the structure [26,27]. This type of defect is often encountered in oxides [28] and has been well characterized in amorphous SiO 2 [29]. ...
Article
Hydrated zirconium molybdate is a precipitate formed during the process of spent nuclear fuel dissolution. In order to study the radiation stability of this material, we performed gamma and electron irradiation in a dose range of 10–100 kGy. XRD patterns showed that the crystalline structure is not affected by irradiation. However, the yellow original sample exhibits a blue–grey color after exposure. The resulting samples were analyzed by means of EPR and diffuse reflectance spectroscopy. Two sites for trapped electrons were evidenced leading to a d1 configuration responsible for the observed coloration. Moreover, a third defect corresponding to a hole trapped on oxygen was observed after electron irradiation at low temperature.
... The B-center: The B-center is the most easily formed with ionizing radiations. Its nature has been accurately determined by [19]. It consists of an electron hole located on an oxygen atom, displaying an orthorhombic symmetry (Fig. 2). ...
Article
Extensive information has been collected on radiation effects on clay minerals over the last 35 years, providing a wealth of information on environmental and geological processes. The fields of applications include the reconstruction of past radioelement migrations, the dating of clay minerals or the evolution of the physico-chemical properties under irradiation.The investigation of several clay minerals, namely kaolinite, dickite, montmorillonite, illite and sudoite, by Electron Paramagnetic Resonance Spectroscopy has shown the presence of defects produced by natural or artificial radiations. These defects consist mostly of electron holes located on oxygen atoms of the structure. The various radiation-induced defects are differentiated through their nature and their thermal stability. Most of them are associated with a π orbital on a Si–O bond. The most abundant defect in clay minerals is oriented perpendicular to the silicate layer. Thermal annealing indicates this defect in kaolinite (A-center) to be stable over geological periods at ambient temperature. Besides, electron or heavy ion irradiation easily leads to an amorphization in smectites, depending on the type of interlayer cation. The amorphization dose exhibits a bell-shaped variation as a function of temperature, with a decreasing part that indicates the influence of thermal dehydroxylation. Two main applications of the knowledge of radiation-induced defects in clay minerals are derived: (i) The use of defects as tracers of past radioactivity. In geological systems where the age of the clay can be constrained, ancient migrations of radioelements can be reconstructed in natural analogues of high level nuclear waste repositories. When the dose rate may be assumed constant over time, the paleodose is used to date clay populations, an approach applied to fault gouges or laterites of the Amazon basin. (ii) The influence of irradiation over physico-chemical properties of clay minerals. An environmental application concerns the performance assessment of the engineered barrier of nuclear waste disposals. In case of a leakage of transuranic elements from the radioactive waste form, alpha recoil nuclei can amorphize smectite after periods of the order of 1000 years according to a worst case scenario, whereas amorphization from ionizing radiation is unlikely. As amorphization greatly enhances the dissolution kinetics of smectite, the sensitivity of the smectites must be taken into account in the prediction of the long term behavior of engineered barriers.
... Two of these (center A of rhombic symmetry and center A' of axial symmetry) are located in the tetrahedral layer -centers of the -Si-Otype (Muller et al. 1992;Allard et al. 1994;Clozel et al. 1994). The third center (B) contains an Oion linked to two aluminum atoms of the octahedral layer (Clozel et al. 1995). EPR parameters for these centers differ in g-factor values and, in the case of centre B, in the presence of hyperfine structure (HFS) composed of 11 lines where the unpaired electron interacts with the nuclear spin of two neighbouring aluminum atoms. ...
Article
EPR Study of Paramagnetic Defects in Clay Minerals Radiation induced defects (RID-s) and transition metal ion impurities were revealed by EPR (Electron Paramagnetic Resonance) spectroscopy in kaolinites from a number of Polish deposits. Arelationship between the intensity of the EPR signals of the RID-s and quantity of radioactive elements was defined in these minerals. In one of the deposits of kaolinites (Wyszonowice) the EPR signal intensity depends on grain size. Other clay minerals studied (illites, montmorillonites) only show weak signals of the RID type.
... A signal due to the B-center is observed together with the A and A' centers. It is nearly isotropic at X-band, with a superhyperfine structure, due to the presence of two nearby 27 Al nuclear spins, which indicates a positive hole located on Al-O-Al bonds [8]. ...
Article
Full-text available
Natural short-lived radionuclides generate electronic defects in minerals, such as trapped electrons and positive holes, often associated with element impurities, which act as final traps over geological periods. Two main examples will be illustrated. The first example will concern the point defects, which are observed in clay minerals. The high specific surface area makes clay minerals sensitive to the geochemical radiation background and provides a record of the past occurrence of radionuclides in geological systems. In kaolinite, three types of hole-centers are trapped by oxygen atoms linked to Si- or Al-sites. An experimental dosimetry gives the paleodose, which can be used either to assess mean past U-concentration or for kaolinite dating, depending on the available geochemical parameters. The detection of past migrations of radioelements in natural analogues may be used in the safety assessment of radioactive waste disposals. The second example will concern the role of mineral impurities in defect formation and stabilization. Natural fluorites (CaF2) exhibit hole-and electron-centers trapped on several rare earths and oxygen impurities, often present at the ppm level, which are responsible for the wide range of coloration observed in natural fluorites. Ca colloids may form under severe irradiation and give rise to a characteristic absorption. The thermal stability of radiation-induced defects gives constraints on the evolution of fluorites as a function of temperature and time. Other minerals, such as apatite, confirm the importance of impurities in stabilizing radiation-induced defects over geological periods.
... X-and Qband EPR spectra of g-and He + -irradiated kaolinites (not shown) exhibited similar trends in terms of both the intensity and shape of the spectra. In particular, X-band spectra exhibited a signi®cant enhancement of the original signal and an increasing and major contribution of the superhyper®ne spectrum characteristic of B-centers (Clozel et al., 1995) as a function of dose. These observations were in full agreement with data previously obtained by Allard et al. (1994). ...
Article
The origin of 3 types of point defects (A-, A′- and B-centers) in kaolinite, due to natural irradiation and detected by electron paramagnetic resonance spectroscopy (EPR), has been demonstrated by artificial irradiation. The potential use of tracing the dynamics of the transfer of radionuclides through A-centers (i.e. the most stable centers) was qualitatively tested on different low-temperature alteration systems, some associated with U-concentrations. This paper proposes a quantitative approach to the reconstruction of the past migration of radionuclides by dosimetry of A-centers. With this aim in mind, the efficiency of α- and γ-radiations to produce A-centers was determined by experimental irradiation. Parameters extracted from A-center growth curves, together with their relationship with a parameter describing the degree of order of kaolinite, permitted (i) a definition to be made of the dose range in which a given kaolinite could be used as a dosimeter and (ii) the quantitative derivation of U-concentration from the cumulative dose (paleodose) of kaolinites. This was achieved by a formalism that accounted for the contribution of natural radiosources to the production of A-centers. The formalism was applied to the Nopal I U-deposit (Chihuhua, Mexico), considered as a natural analogue of a high level nuclear waste repository. Irrespective of the scenario considered, in terms of kaolinite age and of degree of isotopic disequilibrium in the system, A-center dosimetry permitted the determination of past occurrences of U which were several orders of magnitude higher than the present-day measured U-concentrations. Furthermore, this approach also provided evidence for several previous episodes of U-migration. EPR spectroscopy is thus a unique tool for the quantitative, indirect assessment of past radionuclide migration in the geosphere and kaolinite is a reliable in-situ dosimeter.
... Three kaolinites with increasing degree of disorder were used: respectively, DCV from Decazeville (France, Clozel et al., 1995), which has a hydrothermal origin, KGa2 from Georgia (USA, Chipera and Bish, 2001) and FBT2 from Fontbouillant (France, Cases et al., 1982). The reference gibbsite we used is AMAG-25, which results from a Bayer process (Morin, 1994). ...
Article
In the upper Amazon Basin, aluminum previously accumulated in lateritic formations is massively remobilised in soils by podzolization and exported in waters. We have investigated the speciation of aluminum in the clay-size fractions of eight horizons of waterlogged podzols lying in a depression of a plateau. The horizons illustrate the main steps involved in the podzolization of laterites. They belong to eluviated topsoil A horizons and illuviated subsoil Bhs, Bh and 2BCs horizons of weakly and better-expressed podzols located at the margin and centre of the depression. For the first time, aluminum speciation is quantitatively assessed in soils by spectroscopic methods, namely FTIR, 27Al magic angle spinning (MAS) and multiple-quantum magic angle spinning (MQMAS), nuclear magnetic resonance (NMR). The results thus obtained are compared to chemical extraction data.
Article
Full-text available
Kaolinite-rich Cretaceous clay sediment samples from Burgos (Spain) have been analyzed by elemental analylis, X-ray fluorescence, inductively coupled plasma mass spectrometry, X-ray diffraction and different spectroscopic techniques, as Fourier Transform Infrared, ultraviolet-visible and electron paramagnetic resonance. The clay sediment samples mainly contain quartz, muscovite and kaolinite. Different radicals, as A- and B-Centers in kaolinite and organic paramagnetic species, are detected. An illite/kaolinite FTIR band ratio parameter (IKB) is proposed to infer the illite/kaolinite proportion, which can be useful to graphically visualize the iron-substituted Al(III) sites. Studies of the activity as scavengers of DPPH and ABTS radicals show that samples with a larger amount of orthorhombic Fe(III) ions replacing Al(III) ions exhibit a higher antioxidant capacity.
Article
Electron paramagnetic resonance (EPR) spectroscopy, also known as electron spin resonance (ESR) spectroscopy, is a group of techniques used to study paramagnetic species that contain one or more unpaired electrons. The basic principles of EPR are analogous to those of nuclear magnetic resonance (NMR) spectroscopy, because they both deal with interactions between electromagnetic radiation and magnetic moments. However, the former is based on the excitation of electron spins, whereas nuclear spins are excited in the latter. EPR as a structural probe provides a wealth of information about the local structures and dynamic processes of the paramagnetic species studied, and is known for its unique sensitivity (~1012 spins/cm3 or parts per billion; Pan et al. 2002a; Weil and Bolton 2007), unmatched by any other structural techniques. In addition, quantitative EPR, provided that sufficient calibration and standardization are carried out, is possible and is useful for chemical analysis, dosimetry and geochronology, with applications to not only rocks and minerals but also other Earth and planetary materials such as coals, crude oils and meteorites (Ikeya 1993; Dyrek et al. 1996, 2003; Eaton et al. 2009). The basic principles of EPR spectroscopy can be found in various textbooks and monographs (Abragam and Bleaney 1970; Poole and Farah 1999; Schweiger and Jeschke 2001; Weil and Bolton 2007; Brustolon and Giamello 2009; Eaton et al. 2009; Misra 2011). Excellent reviews with emphasis on applications of EPR spectroscopy to minerals can be found in Marfunin (1979), Calas (1988), Vassilikou-Dova (1993), and Goodman and Hall (1994). However, most previous reviews on applications to minerals focused almost exclusively on conventional continuous-wave (CW) techniques, whereas more advanced techniques such as pulse electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) …
Article
Single-crystal electron paramagnetic resonance (EPR) spectra of a gem-quality jeremejevite, Al6B5O15(F, OH)3, from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by an 27Al hyperfine structure arising from interaction with two equivalent Al nuclei. Spin-Hamiltonian parameters obtained from single-crystal EPR spectra at 295 K are as follows: g 1 = 2.02899(1), g 2 = 2.02011(2), g 3 = 2.00595(1); A 1/g e βe = −0.881(1) mT, A 2/g e βe = −0.951(1) mT, and A 3/g e βe = −0.972(2) mT, with the orientations of the g 3- and A 3-axes almost coaxial and perpendicular to the Al–O–Al plane; and those of the g 1- and A 1-axes approximately along the Al–Al and Al–OH directions, respectively. These results suggest that this aluminum-associated hole center represents hole trapping on a hydroxyl oxygen atom linked to two equivalent octahedral Al3+ ions, after the removal of the proton (i.e., a VIAl–O−–VIAl center). Periodic ab initio UHF and DFT calculations confirmed the experimental 27Al hyperfine coupling constants and directions, supporting the proposed structural model. The VIAl–O−–VIAl center in jeremejevite undergoes the onset of thermal decay at 300 °C and is completely bleached at 525 °C. These data obtained from the VIAl–O−–VIAl center in jeremejevite provide new insights into analogous centers that have been documented in several other minerals.
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This document is part of Subvolume I5beta `Phyllosilicates - Part beta' of Volume 27 `Magnetic properties of non-metallic inorganic compounds based on transition elements` of Landolt-Börnstein - Group III `Condensed Matter'. It presents silicates belonging to the kaolin group and related silicates, presenting their crystal structure and lattice parameters, magnetic properties, nuclear gamma resonance (NGR), nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) data, dielectric properties, heat capacity, infrared and Raman data, and optical absorption spectra.
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This study deals with the radiation-induced defects in fracture filling clay-minerals from the El Berrocal U-deposit (Spain), which is considered as a natural analogue of a high-level nuclear waste repository in granitic rock. Mineralogical analyses shows the widespread occurrence of dickite as a secondary alteration phase, together with various amounts of associated smectite, illite, carbonates and primary minerals. Native radiation-induced defects accumulated in the dickite and smectite structure are identified by electron paramagnetic resonance spectroscopy (EPR). They are distinguished by their spectroscopic parameters and thermal stability, and can be reproduced by artificial irradiation. The dickite defects are similar to the well-characterized features in natural kaolinites, including defects stable at the scale of geological periods. Dickite can thus be used as a natural dosimeter to trace radioelement transfer in relevant geological systems. The concentration of stable defects in dickites is related to the present dose-rates, assuming dosimetry parameters from model kaolinites and several scenarii for dickite ages inferred from major tectonic events. This study demonstrates that the El Berrocal dickites correspond to different generations having recorded retention or migration events of radioelements within the whole system.
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Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated prehnite (Jeffrey mine, Quebec, Canada) measured at 298 and 160 K reveal an aluminum-associated oxygen hole center (Al-O-). Spin Hamiltonian parameters g and A(Al-27) fitted from the 298 K spectra suggest that this Al-O- center represents hole trapping on an apical hydroxyl oxygen atom (after removal of the proton) coordinated to an octahedral Al3+ ion (i.e., an [(OAlO4)-O-center dot(OH)] center from the [(OH)AlO4(OH)] precursor, where center dot denotes the unpaired spin). Pulsed electron nuclear double resonance (ENDOR) spectra measured at 25 K allow the identification and quantitative analysis of two sets of 27Al hyperfine structures and five proton hyperfine structures, which are all consistent with the proposed structural model. Isothermal and isochronal annealing experiments show that this center is bleached out completely at 375 degrees C, but can be readily restored by gamma-ray irradiation, and exhibits second-order decay kinetics. These results from the Al-O- center in prehnite provide support for and new insights into Clozel et al. (1995)'s Al-VI-O--Al-VI model for B-centers in kaolinite.
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ABSTRACT: Electron paramagnetic resonance (EPR) spectra of different particle size fractions of four kaolins from diverse sources in North America, Europe and Asia have been investigated in order to characterize their paramagnetic properties and heterogeneity. There were major differences in the sources of the EPR signals from transition metals; V and Mn were structural, Fe was both structural and as associated oxides, and Cu was in the form of an adsorbed ion. The radiation-induced free radical signals commonly known as the A- and B-centres were observed in three of the deposits; however, in addition to the previously reported 27Al hyperfine structure associated with the B-centre, we also observed much smaller 27Al hyperfine structure on the g\ feature of the A-centre. The other kaolin sample produced four free radical signals that have not previously been reported in kaolins. Each had substantial 1H hyperfine splitting; three are interpreted as corresponding to defect centres associated with Si-OH groups, and the other to a Si hole surrounded by protonated O atoms. The EPR spectra changed progressively with particle size, and measurements on the Asian specimens after grinding showed major differences in the Fe3+ signals from the same particle size fractions separated from the natural samples, thus supporting previous reports that grinding results in major structural changes in the minerals.
Article
Mineral components, which are characteristic of low temperature environments at the Earth's surface, are mostly finely divided, and owing to their high surface area constitute good tracers of solid/solution interactions. In order to understand the formation and evolution conditions of low temperature mineral materials, defects and trace elements in these solids can be studied. Spectroscopic tools, which are element specific and are sensitive to local order, together with quantitative tools for simulating X-ray powder diffraction patterns allow us to derive original information on the localization and quantification of these defects and impurities. These fingerprints are used for analyzing formation and evolution conditions of minerals, for distinguishing different generations of minerals, or for an indirect tracing of passed migrations of nuclides.
Article
Radiation-induced point defects have been discovered in several clay minerals such as kaolinite, dickite, montmorillonite, illite, and sudoite, using electron paramagnetic resonance spectroscopy. Besides, amorphization of smectites could be produced by electron or heavy ion irradiation with different cumulative doses. Two main applications are derived: (i) The tracing of past radioactivity using point defects. In geological systems where the age of the clay can be constrained, one can quantitatively reconstruct ancient migrations of radioelements. In closed systems where the dose rate can be assumed constant over time, the defects have been used to date clay populations from tropical soils. (ii) The impact of irradiation on physico-chemical properties. In a worst case scenario of leakage of transuranic elements in a high-level nuclear waste repository, alpha recoil nuclei may amorphize smectite after periods of the order of 1000 years. Moreover, radiation-induced amorphization of smectite significantly enhances its dissolution kinetics.
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A new appraisal of radiation-induced defects (RID) in natural kaolinite, i.e., positive trapped holes on oxygen atoms, has been undertaken using Q-band EPR spectra, recorded at 93 K, of irradiated annealed and oriented kaolinite samples originating from various environments. Three different centers were identified. Two of the centers, A- and A’-centers, are trapped holes on oxygen from Si-O bonds. They have a distinct signature and orthogonal orientation, i.e., perpendicular and parallel to the (ab) plane, respectively. The third center, the B-center, is a hole trapped on the oxygen bonding Al in adjacent octahedral positions (Al VI -O ⁻ -Al VI bridge). This confirmed some previous assignments from the literature, some others are no longer considered as valid. A least squares fitting procedure is proposed to assess the RID concentration in any kaolinite. It allows a quantitative approach of the thermal stability of RID. Isochronal annealing shows that the thermal stability of the centers decreases in the order A, A′, B over the temperature range 0–450°C: (1) B-center is completely annealed above 300°C; (2) A′-center can be annealed by heating at 400°C for more than two hours; (3) A-center is stable up to 450°C. The activation energy and the magnitude of the mean half-life for A-center is evaluated through isothermal annealing at 350, 375 and 400°C, with E a = 2.0 eV ± 0.2, and t ½ > 10 ¹² years at 300 K. The stability of A-center seems to decrease with increasing crystalline disorder. Nevertheless, it is high enough for radiation dosimetry using kaolinites from any environment on the Earth's surface.
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The structure of kaolinite (non-hydrogen atoms only) from Keokuk, Iowa, was refined in space group C1 using CuKα X-ray powder diffraction data and Rietveld refinement techniques (Rwp = 12.3%). A distance least-squares (DLS) model for the initial atomic coordinates avoided a false minimum and unrealistic results characteristic of previous Rietveld refinements of kaolinite. All Keokuk samples examined contain small amounts of dickite. The profile of the mixture was calculated using a fixed set of dickite atomic parameters and by constraining the dickite profile parameters to those refined for kaolinite. Results indicate that the individual kaolinite layer is very similar to the dickite layer; bond distances, which are similar to dickite and nacrite, are: Si-O, 1.60-1.63 Å; Al-O, 1.87-1.97 Å. The tetrahedral rotation angle is 6.9°, compared with 7.3° for dickite and 7.4° for nacrite. -from Authors
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Point defect centers in hydrothermal kaolinite have been investigated using electron paramagnetic resonance (EPR). Kaolinite was sampled in petrographically well-defined materials coming from uranium-rich hydrothermally altered volcanic tuffs (Nopal I uranium deposit, Chihuahua, Mexico), which show extensive kaolinization and an intense redistribution of uranium. Several kaolinite parageneses were defined according to their origin (fissure fillings and feldspar pseudomorphs); their location relative to the U ⁶⁺ mineralization at the scale of the deposit (mineralized breccia pipe vs. barren surrounding rhyolitic tuffs), and at the scale of mineral assemblages; and their crystal chemistry. Two types of centers of axial symmetry were identified (A- and A′-centers) and represent positive holes trapped on apical oxygens (Si-O ⁻ -centers). A-centers were stable to 400°C, whereas A′-centers annealed at 350°C. A relation between defect-center concentration and U content demonstrates that natural irradiation was responsible for these centers. On the other hand, defect-center concentration was not directly linked to the origin (fissural or feldspar pseudomorph) or the crystal chemistry (structural order and substitutional Fe ³⁺ content) of the kaolinite. According to petrographic data, and with respect to the relative thermal stability of A- and A′-centers, two successive irradiations of kaolinite were evidenced: (1) originally during crystallization of kaolinite from radioactive hydrothermal solutions, and (2) permanently when kaolinite was in contact with secondary U-silicates, which led to the formation of A′-centers. Because of the short half-life of U, these two radiation-induced centers were created by short-lived elements of the U-decay series. As a consequence, variations of defect-center concentration possibly reflect variations in radioactive disequilibrium during the history of the alteration system. This provides a unique tool for tracing the dynamics of the transfer of radionuclides in the geosphere: kaolinite may be used as a sensitive in situ dosimeter, which may be useful in the fields of weathering petrology and nuclear waste management.
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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.
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Defect centres are demonstrated in natural kaolinites by means of electron paramagnetic resonance. The irradiation origin of these paramagnetic defect centres (PDC) is demonstrated by artificial irradiation (four types have been used: X-rays, γ-rays, He+ and Pb2+ ion beam implantation) of kaolinites representing different formation conditions (hydrothermal, weathering, synthesis). Several types of PDC (designated A, A′ and B-centres) are clearly identified and can be separated with respect to their relative stability. One (A-centre) is stable over geological periods. Moreover, it is shown that defect centres in kaolinites reflect effectively the cumulative radiation dose.The potential use of tracing radionuclide migration through PDC in kaolinite has been tested on three low-temperature alteration systems, some associated with U concentrations: (1) a hydrothermal alteration system (Nopal I, Chihuahua, Mexico), and (2) two weathering systems in tropical areas (latentes from Brazil and Cameroon). The kaolinites investigated show a significant concentration of PDC which corresponds with background natural irradiation. The different types of PDC detected may form a record of successive irradiations of kaolinite linked directly to geochemical conditions prevailing, during and after kaolinite growth. Because of their different thermal stability, these different centres can be considered as being formed at different periods. This makes a unique tool for tracing the dynamics of the transfer of radionuclides and allows the use of kaolinite as a very sensitive in situ dosimeter. Because some of the geological settings chosen for nuclear wastes are located in environments containing kaolinites, PDC dosimetry could be directly useful to a safety assessment of these wastes.
Article
A rubefied formation within the carboniferous coal beds of Decazeville is identified as a ferrallitic alteration up to the stage of argillaceous bauxite. It is the oldest bauxite of this kind in W Europe. An hypothesis on its formation mode is proposed.-English summary
Article
Metastable electron-hole centers in single crystals of Amelia albite were studied using electron paramagnetic resonance at 9.2 GHz between 5 and 260 K. Six distinct O1- centers were identified as four O1-/227Al centers, one O1-/27Al × 223Na center, and one O1-/[Si,M2+] center. Values of their g and A tensors and limits of thermal stabilities were determined. The four O1-/227Al centers could be assigned to AlT1(O)-OC(O)-AlT2(m), AlT1(O)-OD(O)-AlT2(m), AlT2(O)-OC(m)-AlT1(m), and AlT2(O)-OD(m)-AlT1(m) bridges (violations of the rule of Loewenstein) and are designated as co, do, cm, and dm, respectively. The O1-/27Al × 223Na center could be assigned to a SiT1(O)-OAl-AlT1(m) bridge and is designated as a′1. The O1-/[Si,M2+] center could be assigned to O1- at the D(m) position that links T2(O) with T1(m) and is designated as hm. After destruction by heating, all centers could be reactivated by X-rays. -from Authors
Chapter
In recent years, electron spin resonance (esr) spectroscopy has proven to be a powerful tool in studies of clay mineral chemistry. The orientations, dynamics, and reactions of a variety of intercalated paramagnetic species have been elucidated by esr spectroscopy. The esr spectra of certain transition metal ions, which substitute for aluminum or silicon in the oxygen framework, can provide useful information on the nature of thermal processes and the state of order or disorder of the clay structure being probed. This chapter provides recent examples of the kind of information that can be obtained through the application of esr to the study of clay minerals. Esr can be an exceptionally powerful tool for observing the formation of metal complexes on the interlamellar surfaces of clays. The esr parameters of the complex should be different from those of the simple solvated ion, providing that rapid tumbling of the complex does not average these parameters. Studies of the orientation dependence of film samples can provide information on the orientation of the complex.
Article
Trente échantillons d'argiles kaoliniques provenant de Cornouailles anglaise et française, de Géorgie, des Charentes, ont été étudiés par analyses chimique, cristallographique, spectroscopique (I.R.), granulométrique, thermique différentielle et mesures des surfaces spécifiques. Le fer, sous forme d'oxydes ou hydroxydes dans l'espace interfoliaire ou en couche octaédrique, semble principalement responsable des variations nombreuses des propriétés physicochimiques, en particulier du nombre de défauts aléatoires ou non dans le plan (a , b). Les défauts cristallins selon l'axe c*, compte tenu de leur relation avec le fer total, traduisent vraisemblablement la présence des oxydes de fer ou de feuillets micacés interstratifiés. Il est alors possible de classer les échantillons par ordre de cristallinité décroissante. On constate que la hauteur moyenne et le diamètre moyen des particules élémentaires décroissent au fur et à mesure que les kaolinites sont de plus en plus désordonnées. Cet effet est mis en relation avec l'existence d'un milieu de croissance riche en impuretés.
Article
A number of natural kaolinites from a variety of world sources have been investigated using electron spin resonance at both X-band and Q-band. The results show systematic differences some of which are related to the crystallinity of the material, some to the presence of transition metal ions other than Fe ³⁺ and hole-trapping defects. The results provide evidence for at least three sites for substitution of Fe ³⁺ having large crystal fields : (i) a site with near maximum rhombic character having crystal field parameters D > 1·2, λ = ⅓ the occurrence of which is correlated with lack of crystalline perfection due to stacking defects or to the proximity of surfaces, (ii) a site with less rhombic character with λ = 0·234, D = 0·585 as likely parameters, and (iii) a site with parameters near the values λ = 0·207, D = 0·322. Mössbauer and ESR evidence suggests that the principal sites of Fe ³⁺ substitution are octahedral. The presence of Fe ³⁺ in adjacent cation sites leads to a very broad resonance centred near g eff = 2. Other resonances in the spectra are attributed to the effect of trapped holes, some at least of which are situated at oxygen sites adjacent to Al ³⁺ ions substituting in sites normally occupied by Si ⁴⁺ . Hyperfine effects due to the presence of the transition ions Mn ²⁺ and (VO) ²⁺ are also observed.
Article
Synthetic kaolinites of varying crystallinity, and in some instances with ideal morphology, have been produced by hydrothermal reaction of aluminiosilicate gels. Synthetic kaolinites doped with Mg ²⁺ and Fe ³⁺ were also obtained. Synthetic kaolinite doped with Fe ³⁺ produced an ESR spectrum at g = 4 identical to spectra observed in natural kaolinites. Following X-irradiation and annealing at ~200°C, synthetic kaolinite doped with Mg ²⁺ exhibited an ESR signal at g = 2·0 identical to a resonance observed in natural kaolinites. It is concluded that the g = 2·0 signal in kaolinite is due to a defect centre stabilized by Mg ²⁺ substitution. All synthetic and natural kaolinites exhibit an additional resonance at g = 2·0 following X-irradiation which can be repeatedly created and destroyed by irradiation and annealing. This resonance is attributed to defects possibly associated either with the substitution of Al ³⁺ for Si ⁴⁺ in kaolinite or with lattice vacancies.
Article
A program to compute powder EPR spectra for noncoincident tensors has been written. The spin Hamiltonian used includes anisotropic g, A, D, and Q tensors, isotropic nuclear Zeeman term, fourth- and sixth-order terms in S and first-order anisotropic ligand hyperfine terms. All terms except the ligand hyperfine terms are treated exactly. Data can be saved and reused if the program runs out of time. An efficient method is used for the variation of the orientation of the magnetic field, and for locating resonance fields.
Article
Aqueous Al passes from octahedral to tetrahedral coordination over a narrow pH interval, or threshold. This interval is 5.5–6.5 at 25°C and shifts to lower pH as temperature increases. The concentration of aqueous tetrahedrally coordinated Al is a quasi-step function of the solution pH, and, by the mass-action law, so should be the amount of tetrahedral Al incorporated by a silicate that crystallizes from the aqueous solution. Qualitative support for this prediction (which applies to quartz, opal-CT, kaolin-group minerals, pyrophyllite, micas, chlorites, and other low-temperature silicates) comes from the very topology of equilibrium activity diagrams and from several pairs of associated waters and authigenic silicates from weathering, hydrothermal, and diagenetic environments. The uptake of tetrahedral Al also depends on the aqueous concentrations of monovalent cations and silica, and on the mineral's structural constraints. Solid solution of tetrahedral Al in halloysite in turn produces the characteristic bent or tubular crystals of this mineral. This genetic link between aqueous chemistry (mainly pH), tetrahedral-Al uptake by a low-temperature silicate, and the mineral's crystal morphology may operate also in other silicates.
Article
Electron spin resonance (ESR) spectroscopy has contributed significantly to the identification and characterization of paramagnetic impurities associated with clays. Following a brief discussion of the general principles of the technique, a review is given of the application of ESR to the study of those paramagnetic species (chiefly iron and radiation-induced lattice defects) located either within the aluminosilicate structure or present as an external impurity phase.
Article
A B STRACT: Calibration of the internal and external signals appearing at geff ~ 4 in the electron paramagnetic resonance (EPR) spectra of twenty-four kaolinites, has led to the recognition of two types of kaolinite-iron. The amount of iron associated with the internal signal (I iron) shows good inverse correlations with parameters used to describe the degree of crystalline perfection in kaolinite. Iron associated with the external signal (E iron) is insensitive to crystalline order. By considering both the amount and nature of the iron, the kaolinites investigated are placed in three groups. The first contains well-crystallized varieties characterized by tow total iron and EPR spectra consisting mainly of an E signal. Poorly-crystallized kaolinites contain more iron and are characterized by a fifty-fifty partition of this iron into E and I sites. The third group contains samples which are intermediate with respect to both the amount and nature of the iron.
Article
The analysis of the intensities of 3Si, 2Si + 1Al, 1Si + 2Al, and 3Al lines in the 29Si NMR spectra of a series of phyllosilicates with a wide range of compositions has permitted the determination of the degree of dispersion of Al in the tetrahedral sheet. This dispersion is higher than that required by the Loewenstein's rule (Al-O-Al avoidance), but clearly lower than that corresponding to the criterion of maximum dispersion of charges (MDC model). The most probable Si,Al distribution is one in which the number of Al per hexagonal ring is close to that given by the chemical composition which assures the homogeneous distribution of the layer charge (HDC model).
Article
27Al and 29Si MAS NMR results are presented for the mixed-layer 2:1 phyllosilicate rectorite. Separate 29Si spectra, obtained by partially relaxed inversion-recovery experiments, for the two types of tetrahedral sheets present are consistent with one sheet being smectite-like, i.e. low aluminum occupancy of tetrahedral sites, and the other sheet being mica-like, i.e. high aluminum occupancy of tetrahedral sites. Comparison of the relative areas of the Si(0-3Al) resonances from the mica-like sheets with those predicted by computer modeling indicates a high degree of ordering of substituted aluminum. The observed areas indicate that minimization of both Al-O-Al (Loewenstein's rule) and Al-O-Si-O-Al (Dempsey's rule) interactions is important. For the tetrahedral sheets Si/Al ratios derived from 27Al MAS spectra indicate a much higher aluminum occupancy than determined by XRF or 29Si NMR.
Article
The electron paramagnetic resonance (EPR) spectra of Fe3+ in a well cristallized kaolinite from Decazeville in France are well resolved. It is shown that in this sample there are mainly two slightly different spectra, well separated at low temperature and characterized at -150° C by the constants B 20= 0.112 cm−1, B 22= 0.0688 cm−1 for one and B 20= 0.116 cm−1, B 22= 0.0766 cm−1 for the second. These two spectra arise from Fe3+ substituted for Al3+ at the two octahedral positions in equal amounts. The temperature dependence of EPR spectra was studied and was explained by a modification of the octahedral sites.
Article
Various radiation defects were characterized and analyzed by electron paramagnetic resonance (EPR) in 15 feldspars of different compositions after X-ray irradiation. A hole center on oxygen adjacent to two aluminum ions is formed in most feldspars, except those with very high An content. Since the hole is not localized at room temperature, clusters of more than two Al must be present in all feldspars in amounts of at least 100 ppm. Less frequent radiation defects are trivalent titanium and holes on oxygen ions adjacent to a small divalent ion of a yet unidentified nature on a T site with Si and in some cases also Pb as further neighbors. The directions of the magnetic axes for these centers allowed their assignment to specific sites in the feldspar structure. Characteristic absorption and thermolu-minescence emission bands could also be assigned to these centers. Their properties are remarkably independent of composition and Al, Si disorder of the feldspars.
Article
Thesis (doctoral)--Institut national polytechnique de Lorraine, 1977. Includes bibliographical references (leaves [329-336]). Photocopy (positive).
Kaolinite from Warsaw geodes, Keokuk region, Iowa
  • Hayes
Les d6fauts paramagn6tiques induits par irradia-tion dans les kaolinites Approche exp&imentale et implications g6ochimiques Experimental and modelised EPR spectra of Cr 3 + in kaolinite
  • France Gaite
  • J-M Mosser
Clozel B (1991) Les d6fauts paramagn6tiques induits par irradia-tion dans les kaolinites. Approche exp&imentale et implications g6ochimiques. PhD Dissertation, Paris 7 University, France Gaite J-M, Mosser C (1993) Experimental and modelised EPR spectra of Cr 3 + in kaolinite. J Phys C 5:49294934
Electron Spin Resonance studies of doped synthetic kaolinite. I Ordering of aluminium in tetrahedral sites in mixed-layer 21 phyllosilicates solid-state high resolution NMR Rietveld refinement of non-hydro-gen atomic positions in kaolinite
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Angel BR, Jones JPE, Hall PL (1974) Electron Spin Resonance studies of doped synthetic kaolinite. I. Clay Minerals 10:247 255 Barron PF, Slade P, Frost RL (1985) Ordering of aluminium in tetrahedral sites in mixed-layer 21 phyllosilicates solid-state high resolution NMR. J Phys Chem 89:3880-3885 Bellenguez G, Revel J-C (1986) Etude pr61iminaire d'une bauxite lat6ritique st6phanienne du bassin de Decazeville (d6partment de l'Aveyron, France). C.R. Acad. Sci Paris 302:131-134 Bish DL, von Dreele RB (1989) Rietveld refinement of non-hydro-gen atomic positions in kaolinite. Clays Clay Minerals 37 : 289
Paramagnetic centers in hydrothermal kaolinite from an altered tuff in the Nopal Urani-um deposit. Chihuahua, Mexico Radiation-in-duced defects in kaolinites: indirect assessment of radionuclide migration in the geosphere A program for EPR powder spec-trum simulation
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Muller J-P, Ildefonse P, Calas G (1990) Paramagnetic centers in hydrothermal kaolinite from an altered tuff in the Nopal Urani-um deposit. Chihuahua, Mexico. Clays Clay Minerals 38: 600-608 Muller J-P, Clozel B, Ildefonse P, Calas G (1992) Radiation-in-duced defects in kaolinites: indirect assessment of radionuclide migration in the geosphere. Appl Geochem, Suppl Issue n~ Nettar D, Villafranca J (1985) A program for EPR powder spec-trum simulation. J Mag Res 64:61
The dispersion of charge deficits in the tetrahedral sheet of phyllosilicates Analysis from 29Si NMR spectra Contribution fi l'6tude des propri6t6s physicochi-miques cristallographiques et morphologiques des kaolins The distribution of aluminium in the tetrahe-dra of silicates and aluminates
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Herrero CP, Sanz J, Serratosa JM (1989) The dispersion of charge deficits in the tetrahedral sheet of phyllosilicates. Analysis from 29Si NMR spectra. J Phys Chem 93:4311~4315 Lietard O (1977) Contribution fi l'6tude des propri6t6s physicochi-miques cristallographiques et morphologiques des kaolins. PhD Dissertation, Institut National Polytechnique de Lorraine, France Lowenstein W (1954) The distribution of aluminium in the tetrahe-dra of silicates and aluminates. Am Mineral 39:92-96
Kaolinite from Warsaw geodes, Keokuk region
  • Jb Hayes
Electron spin resonance studies of clay minerals Advanced Techniques for Clay Minerals Analysis, Developments in Sedimentology 34
  • Tj Pinnavaia
Experimental and modelised EPR spectra of Cr3+ in kaolinite
  • J-M Gaite
  • C Mosser