Applied Clay Science 102 (2014) 51–59

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Applied Clay Science

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Research Paper

Stability of lanthanum-saturated montmorillonite under high pressureand high temperature conditions

Vicente Fiorini Stefani a,⁎, Rommulo Vieira Conceição a,b, Larissa Colombo Carniel b, Naira Maria Balzaretti a,c

a Programa de pós-Graduação em Ciências dos Materiais, UFRGS, Av. Bento Gonçalves, 9500, P.O. Box 15051, CEP: 91501-970 Porto Alegre, Brazilb Instituto de Geociências, UFRGS, Av. Bento Gonçalves, 9500, prédio 43126, P.O. Box 15001, CEP: 91501-970 Porto Alegre, Brazilc Instituto de Física, UFRGS, Av. Bento Gonçalves, 9500, prédio 43133, CEP: 91501-970 Porto Alegre, Brazil

⁎ Corresponding author. Tel.: +1 647 713 3358.E-mail address: vicente.stefani@ufrgs.br (V.F. Stefani).

http://dx.doi.org/10.1016/j.clay.2014.10.0120169-1317/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 December 2013Received in revised form 16 October 2014Accepted 18 October 2014Available online xxxx

Keywords:SmectiteInterlayer waterIon exchangeRadioactive waste disposalHigh pressureSubduction zone

Smectite has been used to capture radioactive cations through adsorption in deep radioactive waste repositoriesin various parts of the world. Smectite is also important in the transport of water and some trace element cationssuch as rare earth elements (REE), which are captured in its structure, back to themantle in subduction environ-ments. Such captures are based on the ionic strength of the surrounding solution and the adsorption coefficient ofsmectite. However, captured cations can be released from the smectite structure once the ionic strength of thesolution changes. In this work, the stability of a particular smectite (montmorillonite) structure saturated withlanthanum was verified at high pressures (up to 12 GPa) and room temperature and at high pressure and hightemperature (HPHT) concomitantly. La3+-montmorillonite remains stable up to 12 GPa at room temperaturewith a small variance in its vibrational mode. At HPHT, however, the structure becomes muscovite-like andrich in La3+. When in contact with a Ca2+-enriched solution, La3+ is partially replaced by Ca2+ in the newphase, returning to its original Ca2+-montmorillonite phase, whereas another part remains La-muscovite-like.These results were confirmed by X-ray diffraction and scanning electron microscopy with energy disper-sive X-ray spectroscopy.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Smectites are clay minerals with a tetrahedral:octahedral structuralratio of 2:1. Due to isomorphic substitutions in the octahedral andtetrahedral sheets, a net negative charge is established in the structure;this charge is balanced through cation adsorption, usually in the inter-layer spaces, which results in a high cation exchange capacity (CEC)(Bergaya et al., 2006). In addition to the adsorption of mono- anddivalent cations such as Na+, K+, and Ca2+, smectite can also adsorbtrivalent elements (e.g., trivalent rare earth elements (REE3+) and triva-lent actinides), as has been shown in several studies (Takahashi et al.,1998; Stumpf et al., 2001; Bradbury and Baeysns, 2002; Coppin et al.,2002; Stumpf et al., 2002; Coppin et al., 2003; Stumpf et al., 2004;Rabung et al., 2005; Brandt et al., 2007; Tan et al., 2010). Due to theseand other features, smectites are used as geochemical barriers in differ-ent contexts including secondary barriers for deep nuclear wastedisposal (Pusch, 1998), where smectites are employed to adsorbactinide trivalent cations. In the geologic environment, smectites canplay an important role in oceanic subduction-related zones becausethey transport water in higher amounts compared to micas or kaolin-ites; thus, some elements such as K+ and Na+ along with very minor

quantities of incompatible elements such as REE3+ are transportedback to the mantle, causing mantle refertilization.

The different reaction mechanisms between clay minerals andcations are as follows: outer-sphere interaction, inner-sphere interac-tion, cation exchange within the interlayer spaces, and structure incor-poration in the octahedral layer (Takahashi et al., 1998; Strawn andSparks, 1999; Stumpf et al., 2001; Bradbury and Baeysns, 2002;Coppin et al., 2002; Stumpf et al., 2002; Coppin et al., 2003; Stumpfet al., 2004; Rabung et al., 2005; Brandt et al., 2007; Tan et al., 2010).The different modes of clay mineral–cation interaction are highlydependent on the pH of the surrounding environment. At pH valuesequal to or lower than 5, the adsorption is outer-sphere; while at pHvalues higher than 5, adsorption becomes inner-sphere (Strawn andSparks, 1999; Stumpf et al., 2001, 2002).

Several studies have shown that smectite stability is strongly depen-dent on temperature. Temperatures between 105 °C and 240 °C desta-bilize 40% of smectite, transforming it into illite in 3.4 million years(Kamel et al., 1990). At temperatures lower than 100 °C, the rate oftransformation is 0.3% per million years (Chapman et al., 1984), whilesmectite remains unchanged for over 106 years at temperatures lowerthan 90 °C (Pusch and Karnland, 1988; Pusch et al., 1989).

The effect of temperature on the structure of La3+-montmorillonite(La3+-Mt) was studied by Mozas et al. (1980), who showed thatLa3+-Mt loses all its interlayer water at 320 °C. In this condition,

52 V.F. Stefani et al. / Applied Clay Science 102 (2014) 51–59

La3+-Mt can be re-hydrated once temperature decreases. However,at temperatures higher than 500 °C, the re-hydration of La3+-Mtno longer occurs (Alba et al., 1997).

Although some studies have investigated the temperature de-pendence of montmorillonite stability, few works have exploredmontmorillonite stability under high pressure conditions. When Ca2+-montmorillonite (Ca2+-Mt) is exposed to pressures up to 13 GPa atroom temperature, the vibrational mode of the tetrahedral Si\O bondis affectedwithout compromising the Ca2+-Mt structure. Once pressureis released, Ca2+-Mt returns to its original structural condition(Alabarse et al., 2011). Under high pressure and high temperature(HPHT) conditions such as 7.7 GPa and 250 °C, Ca2+-Mt remains stable(Alabarse, 2009).

The goal of this work is to study the effect of pressure and tempera-ture on the stability of La3+-Mt. The results of this study will be used tounderstand the transportation of elements in subduction environments.In addition, we intend to transform La3+-Mt into a new in La3+-richstructure under HPHT and verify whether this new structure releasesLa3+ when in contact with an aqueous solution enriched in otherelements. Lanthanum is analogous to actinide radionuclides due totheir similar chemical properties (Krauskopf, 1986), and this experi-ment focuses on radioactive waste disposal. Future works will examinesmectites saturatedwith different lanthanides (samarium, neodymium,gadolinium and lutetium).

2. Experimental methods

2.1. Starting material

Ca2+-Mt was extracted from bentonite collected on the border ofBrazil andUruguay in the regions of Aceguá (Brazil) andMelo (Uruguay).

The structural formula of the montmorillonite used in this workwas determined by Calarge et al. (2003) from a bulk bentonite andis given by:

Si3:87Al0:13½ �O10 Al1:43 Fe3þ0:08Mg0:53Ti0:01

� �OHð Þ2Kþ

0:01Ca2þ0:2: ð1Þ

The material is montmorillonite with calcium as themain interlayercation, and the calculated layer charge was 0.47 per O10(OH)2.

Ca2+-Mt was separated from bentonite by particle decantation inorder to concentrate particle sizes smaller than 2 μm (Day, 1965).First, the bentonite was gently ground, and 200 g of the sample wasplaced into 500-ml bottles (50 g of bentonite and 300 ml of distillatedwater per bottle) followed by shaking for 24 h. Subsequently, thesolution was left in a beaker for 24 h and 30 min to allow particlessmaller than 2 μm to separate according to Stoke's law.

2.2. Cation exchange process

Cation exchange was performed in order to produce La3+-Mt fromCa2+-Mt via the substitution of Ca2+ with La3+ using the saturationmethod. Ca2+-Mt (0.1 g) was added to 25 ml of 1 mol/l LaCl3 solutionover 1 h. Subsequently, the solution was replaced with another 25 mlof 1 mol/l LaCl3 solution and left for 24 h. The sample was then centri-fuged and washed with ethyl alcohol until the AgCl test was negativefor the presence of Cl in the final material. The pH was buffered at 5during the entire procedure. The sample was dried at room tempera-ture. Finally, the process was repeated in order to obtain a greateramount of sample.

2.3. High pressure and high temperature (HPHT) experiments

2.3.1. Toroidal chamber experimentsHPHT experiments were performed on a hydraulic press with a

toroidal chamber. A detailed description of the equipment used is

provided elsewhere (Khvostantsev, 1974; Sherman and Stadtmuller,1987; Stefani, 2012). This apparatus can reach pressures of up to7.7 GPa and temperatures up to 2000 °C. The pressure cell consists ofa graphite heater (height of 12.0 mm, diameter of 8.0 mm, and wallthickness of 2.0 mm) and two small disks of pyrophyllite calcinated at1000 °C (diameter of 4.0 mm and height of 1.5 mm). To apply isostaticpressure, the La3+-Mt sample was placed inside a hexagonal boron ni-tride (hBN) capsule (2.0-mm internal diameter and height). Finally,the hBN capsule containing the sample was placed in between thetwo pyrophyllite disks. In all experiments, pressurewas initially appliedat room temperature and kept at the desired value for 15 min forpressure stabilization. The pressure calibration was performed using bis-muth (Bi), which has phase transitions at 2.5 and 7.7 GPa (Sherman andStadtmuller, 1987).

Heating was applied simultaneously to pressure by passing anelectric current through the reaction cell, which heats according toJoule's first law. Temperature calibration was carried out with a plat-inum and platinum rhodium thermocouple (13% Type-R). The ratioof applied voltage and temperature is known from the literature(Bundy, 1988).

After the experimental run, the samples were ground in an agatemortar just before XRD analyses. The experiments performed in thisapparatus have pressure and temperature accuracies of ±0.5 GPa and±25 °C, respectively (Alabarse, 2009).

2.3.2. Diamond anvil cell experimentsA Piermari-Block diamond anvil cell (DAC) (Piermarini and Block,

1975)was used to reach pressures of up to 12 GPa at room temperature.A mixture of 1 mass% La3+-Mt powder in KBr and a small ruby wereplaced in a 250-μm diameter hole drilled in a Waspaloy gasket pre-indented to a thickness of 80 μm (Piermarini et al., 1975). Pressurewas determined using the ruby technique (Piermarini et al., 1975)with an accuracy of ±0.2 GPa.

2.4. Analytical techniques

The samples were analyzed by scanning electron microscopy withenergy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction(XRD) and Fourier transform infrared spectroscopy (FTIR).

SEM-EDS was performed on a sample of natural montmorillonite(Ca2+-Mt), La3+-Mt and La3+-Mt processed at 2.5 GPa and 700 °C for8 h. The equipment usedwas a JEOL JSM 5800with an acceleration volt-age of 10 kV, which allows the detection of any element heavier thanboron.

XRD analyses were conducted using a Siemens D500 XRD powderdiffractometer equipped with a CuKα source and a graphite monochro-matic in the secondary beam. The spectra of all samples were obtainedfrom 3° to 65° with a step of 0.05° at 2 s/step and from 58° and 65°with a step of 0.02° at 4 s/step in order to investigate the b-parameter.

FTIR spectroscopywas performed on Ca2+-Mt and La3+-Mt samples.An in situ FTIR analysiswasperformed on La3+-Mt under pressure in theDAC using a Bomem FTIR model MB100 equipped with a DTGS detectorand a KBr beam splitter. The spectral rangewas 350 to 4000 cm−1, and atotal of 512 scans was performed with 4 cm−1 resolution.

3. Results and discussion

3.1. Ca2+–La3+ exchange in montmorillonite

The compositions of the natural Ca2+-Mt and La3+-Mt obtained bySEM-EDS are shown in Table 1. The amount of calcium in Ca2+-Mt isin agreement with the literature (Alabarse et al., 2011). The XRD analy-sis (Fig. 1) of Ca2+-Mt suggested the presence of a minor amount ofquartz, and Ca2+-Mt is characterized by the presence of the (001)basal plane at a distance of 15.21 Å. The 001 basal plane changes to

Table 1Average composition in mass% determined by SEM-EDS in a selected area of the naturalmontmorillonite (Ca2+-Mt) and montmorillonite saturated with La (La3+-Mt) samples.The total amount sums to 100%.

Na2O MgO Al2O3 SiO2 K2O CaO Fe2O3 LaO3 Cl

Ca2+-Mt 0.77 5.46 22.84 66.3 0.00 3.10 1.52 0.00 0.00La3+-Mt 0.48 5.61 21.52 54.26 0.14 0.10 3.35 14.54 0.48

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16.83 Å when the sample is saturated with ethylene glycol and 9.47 Åwhen it is calcined at 550 °C.

After La3+ ion exchange, the amount of Ca2+ in La3+-Mt is highlyreduced, while the amount of La3+ is greatly increased. Furthermore,the minor presence of Cl− indicated by the analyses suggests that Lano longer exists in the chloride form. Therefore, these results indicatethat the Ca2+ in the interlayer spaces of montmorillonite wasexchanged by La3+. Indeed, La3+ is the only cation in our system ableto replace the Ca2+ in the interlayer spaces.

The structural formula calculated for La3+-Mt based on the EDSresults (Table 1) is given by:

Si3:46Al0:54½ �O10 Al1:08 Fe3þ0:16Mg0:53

� �OHð Þ2Na0:06La3þ0:29: ð2Þ

EDS only gives a qualitative estimate of the composition; therefore,the formula is approximate.

Fig. 1 shows the XRD patterns of Ca2+-Mt and La3+-Mt, with thedetail of the b-parameter (reflection 060). The two patterns have verysimilar reflections, with a slight variance in the 001 reflection (15.23 Åfor Ca2+-Mt vs. 15.80 Å for La3+-Mt). The b-parameter (in detail) indi-cates that both the Ca2+-Mt and La3+-Mt structures are dioctahedral.

The difference in the basal distance might be associated with thepresence of each cation (lanthanumand calcium) and the arrangementsof their hydration shells.

Fig. 1. XRD patterns of La3+-Mt and natural Ca3+-Mt with the respective planes and distanceparameters, indicating dioctahedral structures for both samples.

Previous studies on Ca2+-Mt have shown that the presence of twowater layers leads to a basal distance of approximately 15.1 Å (Ferrageet al., 2005), while La3+-Mt with two water layers is expected to havea basal distance of 15.8 Å (Mozas et al. 1980).

The presence of water is evidenced by the peak at 1645 cm−1 in theFTIR spectrum (Fig. 2), which was measured by diffuse reflectance.

The vibrationalmodeof La3+-Mt is very similar to that of the originalCa2+-Mt (Fig. 2). The band between 3000 and 3600 cm−1 correspondsto the hydroxyl groups in the octahedral and interlayer water of bothCa2+- and La3+-montmorillonite (Mookherjee and Redfern, 2002;Aranha, 2007). Peaks at 848 and 916 cm−1 due to Al\OH\Al bonding,characteristic of montmorillonite with a high amount of Al (Wagneret al., 1994), are present in both the Ca2+- and La3+-Mt spectra. Thepeak at 1114 cm−1 corresponds to the Si\O bond, and the peak at670 cm−1 corresponds to the Si\O\Mg bond (Calvert and Prost,1971). The band at approximately 2350 cm−1 is due to carbon dioxideadsorbed in the sample.

3.2. La3+-montmorillonite under high pressure

In situ FTIR analysis was performed on the La3+-Mt sample underhigh pressure (up to 12 GPa) using the DAC system. The sample wasdispersed in KBr (1 mass% of La3+-Mt, 99 mass% of KBr) and mea-sured by transmittance at different pressures. The spectra obtainedat all pressures were very similar, with the exception of the peak at1040 cm−1 (Fig. 3), which corresponds to the tetrahedral Si\Obond (Mookherjee and Redfern, 2002); this peak shifted from1039 cm−1 at atmospheric pressure to 1063 cm−1 at 12 GPa. Thechange in this bond frequency from lower to higher frequency withincreasing pressure was also observed by Alabarse et al. (2011) forCa2+-Mt and is related to the approach of the Si\O apical bondfrom the tetrahedron, which increases its vibrational modes. Thisprocess is reversible; as the pressure is released, the peak returnsto its original position (Fig. 3).

s. Both patterns indicate minor amounts of quartz (Q). The figure in detail shows the b-

Fig. 2. FTIR spectrum of La3+-Mt and natural Ca2+-Mt.

54 V.F. Stefani et al. / Applied Clay Science 102 (2014) 51–59

3.3. La3+-montmorillonite under high pressure and high temperature(HPHT)

La3+-Mtwas processed at a pressure of 2.5 GPa and temperatures of200 °C, 250 °C, 300 °C, 400 °C, 500 °C, 650 °C and 700 °C as well as at apressure of 7.7 GPa and temperatures of 250 °C, 300 °C, 350 °C and900 °C.

EDS analysis was performed for the experiment carried out at2.5 GPa and 700 °C for 8 h on the La3+-Mt sample. The La3+ is still

Fig. 3. In situ FTIR spectra for La3+-Mt under high pressures using the DAC system. The uparrows indicate that the pressure is being applied,while the downarrows indicate that thepressure is being released.

present in the structure at HPHT, as indicated by the high amount mea-sured by EDS (13.20 mass%); the approximate structural formula isgiven by:

Si3:13Al0:87½ �O10 Al1:07 Fe3þ0:13Mg0:83

� �OHð Þ2La3þ0:28: ð3Þ

XRD analyses were performed on all samples. Fig. 4 shows the XRDresults for experiments performed at 2.5 GPa, and Fig. 5 shows thepatterns for experiments performed at 7.7 GPa. All experiments werecarried out for 8 h.

At 2.5 GPa, the La3+-Mt structure remains stable up to 200 °C. At250 °C, however, the (001) basal reflection shifts to a higher angle,corresponding to a basal distance of 10.5 Å; together with the otherreflections, this infers that the structure has collapsed and changedto a La3+-muscovite-like structure. Although an illite-like structurewas expected, the XRD reflections are in better agreement with amuscovite-like structure. For this reason, we refer to this new HPHTphase as La3+-muscovite-like from now on. Based on the structuralformula calculated from the EDS results (Eq. (3)), the structure has ahigh layer charge; however, the nature of this phenomenon cannotbe determined.

As temperature increases, the La3+-muscovite-like diffractionreflections becomenarrower andmore intense due to themore orderedstructure.

At 7.7 GPa, the structure of La3+-Mt is stable up to 300 °C and turnsto La3+-muscovite-like at 350 °C. At 900 °C, the quartz present in thestarting material becomes coesite, and the La3+-muscovite-like struc-ture remains stable. In all experiments at both 2.5 and 7.7 GPa, theLa3+-muscovite-like structure remains dioctahedral, and La3+ is partof the muscovite-like structure.

When the La3+-Mt structure becomes La3+-muscovite-like, thestructure loses its interlayer water, and the distance between layersdrops to approximately 10.5 Å. Fig. 6 shows the detailed changesobserved in the 001 reflection of the La-muscovite-like structure alongwith the corresponding basal distances for each HPHT condition. As

Fig. 4.XRD patterns of La3+-Mt at 2.5 GPa and different temperatures. The 001 peak of muscovite, indicated by an asterisk (*), is better specified in Fig. 7. At 200 °C, the structureremains as montmorillonite, while the patterns show muscovite-like structures at temperatures equal to and higher than 250 °C. The patterns also indicate the presence ofquartz and contamination due to hBN.

Fig. 5. XRD patterns of La3+-Mt at 7.7 GPa and different temperatures. The 001 peak of La-muscovite, denoted by an asterisk (*), is better specified in Fig. 7. The structure is La3+-Mt up toa temperature of 300 °C, while it becomes La3+-muscovite-like at 350 °C and higher. The patterns also show reflections of quartz and coesite as well as low intensity reflections of hBNfor the high pressure assemblage.

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Fig. 6. The detailed 001 reflection of La3+-muscovite-like samples at different pressureand temperature conditions.

56 V.F. Stefani et al. / Applied Clay Science 102 (2014) 51–59

temperature increases, the structure becomes better ordered and the001 reflections shift to higher angles.

Ethylene glycol (EG) saturation tests were performed on the La3+-muscovite-like samples obtained at 2.5 GPa/250 °C and 2.5 GPa/650 °C (Fig. 7) in order to analyze the behavior of their structuresunder these conditions. These samples were selected for EG testingbecause the 2.5 GPa/250 °C sample is one that is most expected to re-saturate due to the lower temperature condition and the 001 reflectionshape (see Fig. 6); the sample obtained at 2.5 GPa/650 °C was selectedbecause its conditions are closest to those of the sample used in theCa2+ adsorption experiment of the La3+-muscovite-like structure (seeSection 4). None of these samples showed total or partial expansion of

Fig 7. Comparison of La3+-Mt obtained at 2.5 GPa/250 °C

the 001 distance, suggesting that there is no illite–smectite structurein these samples. However, the simple EG treatment is not sufficientto re-expand all smectite layers (Ferrage et al., 2011). EG saturationwas performed as described in Bradley (1945).

Fig. 8 summarizes the pressure and temperature conditions atwhichLa3+-Mt collapses irreversibly (became a La3+-muscovite-like struc-ture) and reversibly (remains as La3+-Mt) after the experiments. Inthe figure, black squares represent the experimental conditions atwhich the La3+-Mt structure returns to its original phase. In contrast,the black triangles represent the experimental conditions at which theLa3+-Mt structure collapses to become La3+-muscovite-like and doesnot rehydrate afterwards.

When the pressure increases, the interlayer water does leave thestructure due to osmotic pressure; therefore, the structure returns toits original phase after the release of pressure. On the other hand, astemperature increases, the interlayer water has more mobility and candehydrate, leading to an irreversible phase transition. Yet, as pressureincreases, higher temperatures are necessary to generate an irreversiblephase transition (Fig. 8). At these conditions, we assume that the inter-layer spaces collapse and trap the lanthanum ions; we call this phaseLa3+-muscovite like.

The phase diagram shown in Fig. 8 is similar to that of water(Wagner et al., 1994; Saitta and Datchi, 2003; Lin et al., 2004; Vegaet al., 2009).

To verify the stability of La inside the La3+-muscovite-like structure,we performed an ion exchange experiment inwhichwe tried to replaceLa with Ca2+ in the La3+-muscovite-like structure. The experimentalsample was the one produced at 2.5 GPa/700 °C/8 h because its struc-ture exhibits good crystallinity.

An XRD scan between 3° and 10° with a step of 0.02° at 4 s/stepwas carried out after the ion exchange experiment in order to verifythe position of the first reflection (001). The diffraction pattern hastwo reflections: 5.73° (d(001) = 15.25 Å) and 8.62° (d(001) = 10.24 Å;Fig. 9). Therefore, two different structures are present: one thatremains unchanged (La3+-muscovite-like) and another that chang-es into Ca2+-Mt (15.25 Å). This result implies the presence of amixed layer structure due to the partial readsorption of Ca2+ bythe La3+-muscovite-like structure, turning it into the original

and 2.5 GPa/650 °C saturated with ethylene glycol.

Fig. 8.Diagram based on the obtained results. Squares represent the conditions at which the structure returns to the original phase, while triangles represent the conditions at which thestructure transforms.

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Ca2+-Mt phase and releasing La3+ to the solution. As the cationexchange was performed at a pH of approximately 5, we presumethat this pH is responsible for the partial cation leaching of theHPHT La3+-muscovite-like structure. Fig. 11 shows the backscatter-ing image of this sample (Fig. 10a) along with the distributions ofCa2+ and La3+ (Fig. 10b and c, respectively). Table 2 confirms the

Fig. 9.XRDpattern of the La3+-muscovite-like structure processed at 2.5 GPa/700 °C/8 h and suthe reflection at 10.24 Å corresponds to the La3+-muscovite-like structure.

presence of areas enriched in Ca2+ and depleted in La3+ along withother areas enriched in La3+ and depleted in Ca2+.

The approximate structural formulas calculated from the data inTable 2 are given in Eq. (4) for area 1 and Eq. (5) for area 2.

Si4O10 Al0:88 Fe3þ0:03Mg0:05

� �OHð Þ2Na0:65Ca2þ0:95La3þ0:08 ð4Þ

bsequently saturatedwith calcium. The reflection at 15.25Å corresponds to La3+-Mt, while

Fig. 10. a) Backscattering image of the La3+-muscovite-like structure saturatedwith Ca2+ from the areas (1 and 2)where EDSwasperformed, b) Ca2+distribution and c) La3+ distribution.

58 V.F. Stefani et al. / Applied Clay Science 102 (2014) 51–59

Si3:15Al0:85½ �O10 Al0:87 Fe3þ0:249Mg0:41

� �OHð Þ2Ca2þ0:02La3þ0:43: ð5Þ

4. Significance to geologic and radioactive waste deposits

Our experiments are applicable to two scenarios: subduction envi-ronments and radioactive waste disposal. Some amount of REE canbe found in authigenic illite–smectite, as was shown by Uysal andGolding (2003); greater enrichment of heavy REE3+ is associated witha high degree of illitization, whereas less illitic clays show a higher con-centration of light REE3+. During the deep burial of sediments in thesubduction zone, hydrous minerals such as micas are considered to bethe major carriers of near-surface H2O that is able to enter in thelower crust and mantle (Huang and Wyllie, 1973). However, smectitescontain a much larger amount of water (±15 mass%) than micas(±4.7 mass%) or even kaolinites (±13.8 mass%) and are widespreadin pelagic sediments. La3+-montmorillonite (or La3+-muscovite orLa3+-illite) does not occur in nature, although minor amounts of La3+

can be trapped in the interlayer spaces of montmorillonite, as describedelsewhere, to share the interlayer space with other major cations suchas K+, Ca2+ or Na+ (Huang and Wyllie, 1973; Uysal and Golding,2003). When the mineral reaches pressure and temperature conditionsthat lead to dehydration according to the diagram above (Fig. 8), thewater lost from the mineral can decrease the melting point of thesurrounding rocks and promote their partial melting. Our experimentsof Ca2+–La3+ replacement in the La3+-muscovite-like structure alsosuggest that in the presence of fluids, La3+ can be partially releasedfrom the La3+-muscovite-like structure to the surroundings, enrichingthe mantle in water and incompatible elements (e.g., La3+), possiblycausing mantle metasomatism. However, part of the La3+ may stillremain in the La3+-muscovite-like structure in deeper and hotterregions; thus, mantle incompatible element enrichment can be extend-ed to higher pressure conditions.

For radioactive waste disposal, our experiments show that La3+

(or its actinide counterpart) can be easily placed inside of La3+-Mt viacation exchange. If this mineral is exposed to high pressures andtemperatures (beyond 2.5 GPa and 250 °C), it transforms into a La3+-muscovite-like structure by trapping La3+. These results suggest adifferent approach to radioactive waste disposal where the hazardouselements are trapped in a stable crystalline structure. However, such

Table 2EDS in mass% from different areas (1 and 2) of La-muscovite saturated with Ca. The totalamount sums to 100%.

Na2O MgO Al2O3 SiO2 Cl CaO Fe2O3 LaO3

Area 1 5.30 0.57 11.75 63.56 0.00 13.96 0.68 4.18Area 2 0.00 4.17 22.16 47.74 0.04 0.33 5.02 20.54

trapping has been shown to be inefficient or partially inefficient asLa3+ can be totally or partially released from La3+-Mt and La3+-musco-vite-like structures, respectively, if the surroundingwater leaches thesemineral structures. Therefore, further studies are needed to identify astructure that completely traps lanthanum cations regardless of thesurrounding environment. One of these high pressure structures couldbe garnet, which has a greater ability to contain REE, particularlyheavy-REE3+, which better mimic the actinides. Our group is currentlydeveloping studies on this subject.

5. Conclusion

Ion exchange with La3+ was achieved in Ca2+-Mt, and the structureremained very similar with the exception of the cations in the interlayerspaces. High pressures applied to the La3+-Mt structure at roomtemperature resulted in minor variations in the vibrational mode ofthe tetrahedral Si\O bond; however, this effect was reversible. Whenpressure and temperature were simultaneously applied to La3+-Mt, itsstructure remained stable up to around 2.5 GPa/250 °C and 7.7 GPa/350 °C; in these cases, the interlayer water had enough mobility toleave the structure, transforming La3+-Mt into a La3+-muscovite-likestructure. At this point, no evidence for an illite–smectite structurewas observed in our experiments. With increasing temperature, thestructure became more ordered, and the HLW radionuclides could betrapped into the structure before being deposited in a multi-barriersystem. Nevertheless, the La3+-muscovite-like structure in contactwith an aqueous solution rich in other cations (Ca2+, for instance)would cause partial La3+ leaching; in this case, part of the structurewould transform to Mt, while part of the structure would remain withthe cation trapped in a La3+-muscovite-like structure.

Acknowledgments

We would like to thank the financial support provided by ComissãoNacional de Energia Nuclear (CNEN) from Brazil.

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