Iron distribution in the octahedral sheet of dioctahedral smectites. An Fe K-edge X-ray absorption spectroscopy study

The distribution of iron atoms in the octahedral sheet of a series of dioctahedral smectites with varying unit-cell composition and iron content was investigated by Fe K-edge XAS spectroscopy. First-step analysis reveals that the patterns corresponding to backscattering by atoms located between 3 and 4 Å from the absorbing atom are very sensitive to the relative amount of light (Si, Al, Mg) and heavy (Fe) atoms. Detailed modelling of this domain then provides valuable information on the number of iron atoms surrounding octahedral iron. By comparing the number of iron neighbours deduced from EXAFS with that determined from unit-cell composition assuming a statistical distribution, three groups of montmorillonites can be distinguished: (1) clay samples from Wyoming display an ordered distribution of iron atoms; (2) clay samples from Georgia, Milos, China and Washington exhibit a close to random distribution of iron atoms; (3) clay samples from North Africa, Germany, Texas and Arizona display extensive iron clustering. These results complement previously obtained IR results and show that the combination of these two spectroscopic techniques could provide an additional crystal-chemistry-based framework for typological analysis of montmorillonite deposits.     

Thermomechanical model of hydration swelling in smectitic clays: I two-scale mixture-theory approach

A thermomechanical theory of hydration swelling in smectitic clays is proposed. The clay is treated as a three-scale swelling system wherein macroscopic governing equations are derived by upscaling the microstructure. At the microscale the model has two phases, the disjoint clay platelets and adsorbed water (water between the platelets). At the intermediate (meso) scale (the homogenized microscale) the model consists of clay particles (adsorbed water plus clay platelets) and bulk water. At the macroscale the medium is treated as an homogenized swelling mixture of clay particles and bulk-phase water with thermodynamic properties defined everywhere within the macroscopic body. In Part I, the mesoscopic model governing the swelling of the clay particles is derived using a mixture-theoretic approach and the Coleman and Noll method of exploitation of the entropy inequality. Application of this procedure leads to two-scale governing equations which generalize the classical thermoelastic consolidation model of non-swelling media, as they exhibit additional physico-chemical and viscous-type terms accounting for hydration stresses between the adsorbed fluid and the clay minerals. In Part II the two-scale model is applied to a bentonitic clay used for engineered barrier of nuclear waste repository. The clay buffer is assumed to have monomodal character with most of the water essentially adsorbed. Further, partial results toward a three-scale thermomechanical macroscopic model including the bulk phase next to the swelling particles are derived by homogenizing the two-scale model with the bulk water. A notable consequence of this three-scale approach is that it provides a rational basis for the appearance of a generalized inter-phase mass transfer between adsorbed and bulk water. Copyright © 1999 John Wiley & Sons, Ltd.     

Stabilization of swelling clays by Mg(OH)2. Factors affecting hydroxy-Mg-interlayering in swelling clays

The damage caused to structures and other human endeavours, on or in the ground, by swelling clay soils is considered as a natural hazard. In order to mitigate this hazard an attempt was made to stabilize the swelling clay mineral structure by the addition of Mg(OH)₂. This will turn the swelling minerals, like montmorillonite, into non-swelling ones, e.g., chloride. Accordingly, the various factors affecting the precipitation-adsorption of the Mg-hydroxide by clays, was investigated, and the conditions giving the best results were established. The factors examined were: the base used as precipitant (NH₄OH and NaOH); the preparation of the precipitate, inside or outside the clay suspension; the sequence and the duration of the reagents addition; the OH/Mg ratio; the time of clay-chemical contact; and the drying time and temperature. The method, was applied to different clay minerals (swelling and non-swelling). The material produced after the Mg-hydroxide precipitation was examined by the methylene blue dye adsorption test, XRD, DTA and DTGA methods; the results were treated statistically by factorial analysis. From the results obtained, it is concluded that the factors affecting the Mg-hydroxide adsorption by clays are: the base used as a precipitant, the preparation of the hydroxide directly inside or outside the clay suspension, the drying temperature, and the rate of titration of the reagents. Therefore, the best conditions for the laboratory preparation of hydroxy-Mg-interlayers are a dilute (1–2%) and well-dispersed clay suspension, pH between 10–12; some 12 meq Mg²⁺, as Mg-salt (e.g., MgCl₂) per gram of clay, added before the base and followed by the dropwise titration of 1–2 N NaOH solution into the clay suspension, to give a molar ratio OH/Mg of about 1.5; vigorous agitation of the suspension during titration of the reagents; centrifugation and decantation of the supernatant liquid, and drying the product at about 250°C. Satisfactory results are also obtained with the fast titration of the base and drying at 105°C to complete dryness.     

Water retention characteristics of swelling clays in different compaction states

The soil–water retention (SWR) characteristics of the clays play an important role in controlling their engineering behaviour, particularly, in the unsaturated state. Although, researchers have attempted to understand the water retention characteristics of the clays in their reconstituted or remoulded state, such studies are rare for the clays in their intact state. In this context, it becomes important to understand the influence of initial state of compaction, which would create different pore and fabric structure (viz., microstructure), on the water retention characteristics of the clays. With this in view, SWR behaviour was experimentally determined for the swelling clays (dried from different compaction states, viz., intact, reconstituted and remoulded) by employing Dewpoint PotentiaMeter (WP4C®). The changes in the pore size distribution of the clays at different stages of drying cycle were also studied by employing the Mercury Intrusion Porosimetry. The study reveals that the SWR curves for the intact and reconstituted specimens of the clays converge beyond a certain stage of drying. Also, a critical analysis of changes in the pore structure of the swelling clay specimens, during drying, indicates that the progressively deforming pore structure plays an important role in controlling its water retention characteristics to a great extent.     

The structure and dynamics of 2-dimensional fluids in swelling clays

The interlayer pores of swelling 2:1 clays provide an ideal 2-dimensional environment in which to study confined fluids. In this paper we discuss our understanding of the structure and dynamics of interlayer fluid species in expanded clays, based primarily on the outcome of recent molecular modelling and neutron scattering studies. Counterion solvation is compared with that measured in bulk solutions, and at a local level the cation-oxygen coordination is found to be remarkably similar in these two environments. However, for the monovalent ions the contribution to the first coordination shell from the clay surfaces increases with counterion radius. This gives rise to inner-sphere (surface) complexes in the case of potassium and caesium. In this context, the location of the negative clay surface charge (i.e. arising from octahedral or tetrahedral substitution) is also found to be of major importance. Divalent cations, such as calcium, eagerly solvate to form outer-sphere complexes. These complexes are able to pin adjacent clay layers together, and thereby prevent colloidal swelling. Confined water molecules form hydrogen bonds to each other and to the clays' surfaces. In this way their local environment relaxes to close to the bulk water structure within two molecular layers of the clay surface. Finally, we discuss the way in which the simple organic molecules methane, methanol and ethylene glycol behave in the interlayer region of hydrated clays. Quasi-elastic neutron scattering of isotopically labelled interlayer CH₃OD and (CH₂OD)₂ in deuterated clay allows us to measure the diffusion of the CH₃- and CH₂-groups in both clay and liquid environments. We find that in both the one-layer methanol solvates and the two-layer glycol solvates the diffusion of the most mobile organic molecules is close to that in the bulk solution.     

An infrared study of clay minerals, 2. The identification of kaolinite-group clays in deep-sea sediments

The kaolinite-group mineral in deep-sea sediments is usually identified as kaolinite, and occasionally as halloysite. A study has been made of the infrared spectra of these two minerals; their absorption bands have been described, and compared to those of the other principal clay groups, i.e., illites, montmorillonites and chlorites, found in deep-sea sediments. It is shown that kaolinite and halloysite have diagnostic absorption bands at ca. 915 cm^?1 and ca. 3700 cm^?1. These bands are made the basis of a rapid infrared technique for the identification of kaolinite and or halloysite in deep-sea sediments.     

A GIS model for preliminary hazard assessment of swelling clays,a case study in Harran plain (SE Turkey)

Swelling behavior of clayey soils regarded as a hidden disaster, causes a great deal of damage in light hydraulic structures such as drinking water network, irrigation pipes or open canal linings through which water can easily leak and penetrate into soil during loading and unloading stages. Early identification during site investigation and laboratory testing is extremely important to ensure that the appropriate design strategy is adopted. The clay soils having swelling potential are generally found in arid and semi-arid regions, such as in the Şanlıurfa-Harran plain located in the southeast of Turkey. The problems associated with swelling clays occurred for the lightweight hydraulic structures constructed in Turkey have been met during the construction of irrigation structures in the scope of Southeast Anatolian Project as the Turkey’s greatest water resources project. Therefore, the identification of such soils and the assessment of their swelling potential parameters are necessary for hazard mitigation planning and land-use management. In this scope, extensive geotechnical study is executed for an investigation area. The samples were obtained from the Harran plain where many irrigation canal structures are constructed, and geotechnical study was performed on these samples. Atterberg limit tests often provide the basic field information to substantiate the soil’s swelling nature. The plasticity index (PI) and liquid limit (LL) values are also used extensively for classifying swelling soil and should always be determined during preliminary investigations. Because of this, LL and PI are assessed by using geographical information system (GIS)-based computer software, and LL and PI contour maps are created. Swelling percentages of the soil samples were carried out on both undisturbed and compacted soil samples by using direct methods. As a result of this extensive study, the values of swelling percentages determined for each location are used to obtain the swelling potential hazard map of the area by means of a GIS program. The results of GIS analyses for this area indicate that the analyses based on a lot of data introduce meaningful results for this study. It is expected that these maps will be a useful tool for planners and engineers in their efforts to achieve better land-use planning and to decide necessary remedial measures.     

Engineering characteristics of british over-consolidated clays and mudrocks I. Tertiary deposits

There is much information in the literature relating to the classification, strength, deformability, permeability and other geotechnical properties displayed by British overconsolidated clays and mudrocks. The opportunity has been taken in this paper to collate published values of the more important geotechnical parameters for a number of Tertiary formations and to present ranges and averages in a convenient form.

The data are intended to serve as a reference of typical values of parameters, but it must be recognised that the results were derived from tests on material from particular locations and will not necessarily be typical of formations as a whole. Meaningful interpretation is possible only when there is complete awareness as to why values may vary. Since the degree of weathering is particularly important, values for both weathered and unweathered material are presented, where possible. However, discussion of the data indicates that mineralogical composition, geological loading, present burial depth and method of testing are also significant causes of variation.

Some of the implications regarding the choice of design parameters are discussed; of particular relevance is the degree and orientation of fissuring present in the material.     

Experimental study of strain localization in sensitive clays

For evaluation of slope stability in materials displaying strain-softening behavior, knowledge concerning the failed state material response is of importance. Here, soft sensitive clay is studied. Such clays behave contractant at failure, which for undrained conditions yields a strain-softening behavior governed by the generation of excess pore water pressure. Strain softening is further linked with material instability and the phenomenon of strain localization. In the case of shear band formation, internal pore pressure gradients are then expected to be present for globally undrained conditions in the sensitive clay due to its low permeability. In the present study, this hypothesis and its implications on the global response and shear band properties are investigated. Utilizing an experimental setup with a modified triaxial cell allowing for shear band formation, the effect of varying the displacement rate is studied. Onset of strain localization is interpreted to occur just before or at the peak shear strength. A strong rate dependency of the softening response is observed. Increasing displacement rates give raised brittleness in terms of the slope of the global softening curve due to accumulating pore pressure. Also, reduced shear band thickness and a shear band inclination approaching 45° are obtained for increasing rates. In the context of slope failure in such materials, the rate dependency in the post-peak state opens up for a large variation in behavior, all depending on time as an important factor.     

合肥重塑膨胀土三向膨胀力试验研究

以合肥重塑膨胀土为研究对象,采用改进的三向胀缩仪开展了16组不同初始含水率与干密度的三向膨胀力试验。研究结果表明,(1)在所研究的含水率、干密度范围内竖向膨胀力总是大于横向膨胀力,快速膨胀阶段大致在0~2 h以内,该阶段竖向膨胀力可达到极限膨胀力的80%以上;(2)同一干密度下竖向膨胀力随初始含水率增大而减小,竖向膨胀力与初始含水率之间具有良好的线性关系,且干密度越大,竖向膨胀力随着初始含水率的变化速率越大;(3)在竖向膨胀力与干密度的关系图中,每条曲线均以干密度1.6 g/cm~3为分界点呈双线性关系;(4)由膨胀力对数与初始干密度的关系,不同初始含水率下的ln(P_z)-ρ_d关系为一系列近似平行的递增直线,直线斜率大致相同,说明膨胀力随初始干密度的变化速度不随含水率的变化而变化。     

Geochemical study of clays used as barriers in landfills

This is a hydraulic and geochemical study on 2 materials: natural clay (AN) and a regenerated material made up of a sand-bentonite (SB) mixture. The hydraulic part allowed us to conclude that a 10% industrial bentonite content mixed with sand offers hydraulic properties that are similar to those of AN material and are lower than the required standards. The geochemical properties of both AN and SB matrixes are comparable with those of some of the synthetic leachates studied. Furthermore, the Langmuir model helped us to identify the adsorption capacities of both matrixes with the following selectivities: Pb > Cu > Cd > Zn.     

Stabilization of swelling clays by Mg(OH)2. Changes in clay properties after addition of Mg-hydroxide

Stabilization of the swelling clay structure is attempted by intercalation of Mg(OH)₂ and the development of a brucite interlayer between the clay layers. The properties of the product obtained by applying the technique, formulated as described in a previous work, are considered here. The materials used were Wyoming bentonite (USA), Fuller's Earth (UK), kaolinite, illite, lignite, and silica gels. The Mg(OH)₂-clay products were examined by the methylene blue dye test, X-ray diffraction analysis (XRD), differential thermal analysis (DTA), and derivative thermogravimetry analysis (DTGA). From the results obtained it is concluded that: the Mg-hydroxide is adsorbed by swelling clays both on their external and internal surface, whereas it is adsorbed on the external surface by non-swelling clays. The internally adsorbed phase of Mg-hydroxide forms an ill-defined interlayer of brucite, retarding swelling, whereas the external phase covers the particles modifying drastically their surface properties, like the adsorption of the MB dye. The material produced after precipitation of Mg-hydroxide on swelling clays (smectites) did not re-expand on wetting or after glycolation. The adsorption of MB dye was also reduced by some 80–90%, due to coating effect, preventing the measurements of the external surface area of the clay by polar molecules. The principal forces involved in the process are believed to be physical adsorption on the external surface, along with chemisorption and some chemical bonding, mostly in the internal surface. Cementation due to crystallization and, in the long term, some pozzolanic reactions take also place. Internal adsorption of the Mg-hydroxide is postulated to be in the form of positively charged mono- and/or small polymers and it is, chiefly, diffusion controlled. Since Mg-hydroxide is internally adsorbed by swelling clays, whereas Ca-hydroxide(lime) is not, and the (Mg, Ca)-clay aggregates are more stable than the Ca-clay or the Mg-one, the combination of the two hydroxides could give better results in soil stabilization than each hydroxide alone.     

Rietveld refinement study of the cation distribution in (Co, Mg)-olivine solid solution

The Co,?Mg-cation ordering in the Mg₂SiO₄–Co₂SiO₄ solid solution series as a function of the chemical composition has been studied by X-ray powder diffraction methods. The structures of nine polycrystalline samples prepared at 1200?°C and equally cooled to room temperature within 2 minutes have been refined by the Rietveld technique. The results corroborate earlier studies showing a strong preference of Co²⁺ for the M1 site. Considering the agreement of the powder diffraction results with those of single crystal studies allows the conclusion that the powder method is well suited for investigating the cation distribution in compounds exhibiting significant ordering effects. According to the cation distributions derived from first rapid quench experiments, the cation order of the slowly cooled samples corresponds to an estimated equilibrium temperature of 800?°C.     

On the driving force of cation exchange in clays: Insights from combined microcalorimetry experiments and molecular simulation

We study the origin of the ionic exchange enthalpy in montmorillonite clays using microcalorimetry measurements and molecular simulation. We first determine the standard reaction enthalpy for well-defined interlayer water contents. We then show by a detailed analysis based on thermodynamic cycles that replacing Na⁺ ions by Cs⁺ in the interlayer of montmorillonite clays is an endothermic process, and that the overall exchange is exothermic only because it is dominated by the exothermic replacement of Cs⁺ by Na⁺ in the aqueous phase. This conclusion from ionic exchange enthalpies supports the one of a recent study of the ionic exchange free energy by Teppen and Miller [Teppen B. J. and Miller D. M. (2006) Hydration energy determines isovalent cation exchange selectivity by clay minerals. Soil Sci. Soc. Am. J.70(1), 31-40] and contradicts long-held views on the role of ion-clay interactions in determining the ionic exchange thermodynamics. This calls for a paradigm shift for the origin of this exchange: The driving force is the “hydrophobicity” of Cs⁺ compared to Na⁺ and not its affinity for clay surfaces.     

Pressure and temperature dependence of cation distribution in Mg-Mn olivine

Synthetic (Mg_0.51, Mn_0.49)₂SiO₄ olivine samples are heat-treated at three different pressures; 0, 8 and 12 GPa, all at the same temperature (～500° C). X-ray structure analyses on these single crystals are made in order to see the pressure effect on cation distribution. The intersite distribution coefficient of Mg and Mn in M1 and M2 sites, K _D = (Mn/Mg) _M1/(Mn/Mg) _M2, of these samples are 0.192 (0 GPa), 0.246 (8 GPa) and 0.281 (12 GPa), indicating cationic disordering with pressure. The small differences of cell dimensions between these samples are determined by powder X-ray diffraction. Cell dimensions b and c decrease, whereas a increases with pressure of equilibration. Cell volume decreases with pressure as a result of a large contraction of the b cell dimension. The effect of pressure on the free energy of the cation exchange reaction is evaluated by the observed relation between the cell volume and the site occupancy numbers. The magnitude of the pressure effect on cation distribution is only a fifth of that predicted from the observed change in volume combined with thermodynamic theory. This phenomenon is attributed to nonideality in this solid solution, and nonideal parameters are required to describe cation distribution determined in the present and previous experiments. We use a five-parameter equation to specify the cationic equilibrium on the basic of thermodynamic theory. It includes one energy parameter of ideal mixing, two parameters for nonideal effects, one volume parameter, and one thermal parameter originated from the lattice vibrational energy. The present data combined with some of the existing data are used to determine the five parameters, and the cation distribution in Mg-Mn olivine is described as a function of temperature, pressure, and composition. The basic framework of describing the cationic behavior in olivine-type mineral is worked out, although the result is preliminary: each of the determined parameters is not accurate enough to enable us to make a reliable prediction.     

Synthetic coprecipitates of exopolysaccharides and ferrihydrite. Part I: Characterization

Iron(III) (hydr)oxides formed at extracellular biosurfaces or in the presence of exopolymeric substances of microbes and plants may significantly differ in their structural and physical properties from their inorganic counterparts. We synthesized ferrihydrite (Fh) in solutions containing acid polysaccharides [polygalacturonic acid (PGA), alginate, xanthan] and compared its properties with that of an abiotic reference by means of X-ray diffraction, transmission electron microscopy, gas adsorption (N₂, CO₂), X-ray absorption spectroscopy, ⁵⁷Fe Mössbauer spectroscopy, and electrophoretic mobility measurements. The coprecipitates formed contained up to 37 wt% polymer. Two-line Fh was the dominant mineral phase in all precipitates. The efficacy of polymers to precipitate Fh at neutral pH was higher for polymers with more carboxyl C (PGA ∼ alginate > xanthan). Pure Fh had a specific surface area of 300 m²/g; coprecipitation of Fh with polymers reduced the detectable mineral surface area by up to 87%. Likewise, mineral micro- (<2 nm) and mesoporosity (2-10 nm) decreased by up to 85% with respect to pure Fh, indicative of a strong aggregation of Fh particles by polymers in freeze-dried state. C-1s STXM images showed the embedding of Fh particles in polymer matrices on the micrometer scale. Iron EXAFS spectroscopy revealed no significant changes in the local coordination of Fe(III) between pure Fh and Fh contained in PGA coprecipitates. ⁵⁷Fe Mössbauer spectra of coprecipitates confirmed Fh as dominant mineral phase with a slightly reduced particle size and crystallinity of coprecipitate-Fh compared to pure Fh and/or a limited magnetic super-exchange between Fh particles in the coprecipitates due to magnetic dilution by the polysaccharides. The pH_iep of pure Fh in 0.01 M NaClO₄ was 7.1. In contrast, coprecipitates of PGA and alginate had a pH_iep < 2. Considering the differences in specific surface area, porosity, and net charge between the coprecipitates and pure Fh, composites of exopolysaccharides and Fe(III) (hydr)oxides are expected to differ in their geochemical reactivity from pure Fe(III) (hydr)oxides, even if the minerals have a similar crystallinity.     

An elastoplastic description of frictional destructuration in natural clays and shales

The existence of structuration in natural clays and shales is believed to change their stiffness, yielding, dilatancy and strength characteristics. These constitutive features are widely known to ultimately reunite with those of the reconstituted parent soil upon large straining. However, some experimental results show that such reunification may not occur in isotropic/one-dimensional compression, especially with regard to the critical state friction angle. This peculiar phenomenon has been barely addressed in constitutive models for natural geomaterials. Hence, the present study aims at introducing a structure-dependent critical state friction angle within the subloading yield framework. A new internal variable is introduced in the model of Nakai et al. (Soils Found 51(6):1149–1168, 2011) to capture subtle irreversible degradation of the structured critical state line which also serves as the threshold between contractive and dilatant volume changes. Additionally, a new evolution rule for the proposed destructuration factor is developed by considering important microstructural information revealed by discrete element method simulations. The proposed new modifications not only enhance the model capabilities in predicting bonding effects, but also enrich the classical stress-dilatancy equation by rendering it a function of void ratio, mean stress and the microstructural state. Model simulations of laboratory experimental tests on the Colorado shale as well as Bacinetto clay are presented in order to illustrate the improved predictive capabilities of the new model.     

Combined inelastic neutron scattering and solid-state DFT study of dynamics of hydrogen atoms in trioctahedral 1M phlogopite

Inelastic neutron scattering (INS) was used to study the vibrational dynamics of the hydrogen atoms in natural trioctahedral phlogopite, K_0.93Na_0.03(Mg_2.47Fe_0.22Al_0.16Fe_0.04Tl_0.06)[Si_2.84Al_1.16]O₁₀OH_1.71F_0.28Cl_0.01, within the 50–1,000?cm^?1 energy range. The INS spectra collected using direct geometry spectrometer SEQUOIA (ORNL) were interpreted by means of the solid-state DFT calculations covering both normal mode analysis and molecular dynamics. To optimize the structure and to calculate the vibrational modes under harmonic approximation, both a hybrid PBE0 and the AM05 functional were used, while the molecular dynamics calculations (60?ps/1?fs) were performed only with the computationally less-demanding AM05 functional. The main contributions to the dominant band within ~750–550?cm^?1 are symmetric and antisymmetric Mg–O–H bending modes, overlapping with the skeletal stretching and bending modes causing weaker secondary movements of H atoms of inner hydroxyl groups. Signatures of the Mg–O–H bending modes appear down to ~400?cm^?1, where a region of octahedra deformation modes starts. These deformations cause just shallow movements of the hydrogen atoms and are mirrored by the modes with close vibrational energies. The region from ~330?cm^?1 down to the low-energy end of the spectrum portrays induced vibrations of the H atoms caused by deformation of individual polyhedra, translational vibrations of the parts of the 2:1 layer relative one to another, and librational and translational vibrations of the layer. The main difference between the INS spectrum of dioctahedral Al-muscovite and trioctahedral Mg-phlogopite is that the Mg–O–H modes are all assigned to in-plane vibrations of the respective hydrogen atoms.     

Determination of quantitative cation distribution in orthopyroxenes from electronic absorption spectra

Electronic and Mössbauer absorption spectra and electron microprobe data are correlated for iron-bearing orthopyroxenes. The correlation provides a means of quantitatively determining the distribution of Fe²⁺ between the M(1) and M(2) sites of orthopyroxene crystals from electronic spectra and electron microprobe analysis. The electronic spectra are used to analyze the changes in the Fe²⁺ distribution produced during heating experiments and confirm earlier results from Mössbauer spectra. Two components of the spin-allowed transition of Fe²⁺ in the M(1) site are identified at about 13,000 cm^?1 and 8,500 cm^?1 in γ. Molar absorptivity (?) values for all spin-allowed Fe²⁺ absorption bands in the near-infrared region are determined. The M(2) Fe²⁺ band at ～5,000 cm^?1 in β is the analytically most useful for site occupancy determinations. It remains linear with concentration (?=9.65) over the entire compositional range. The band at ～10,500 cm^?1 in α is the most sensitive to M(2) Fe²⁺ concentration (?=40.8), but deviates from linearity at high iron concentrations. The origins of spin-forbidden transitions in the visible region are examined.