DSpace at VNU: Characterization of Fe-smectites and their alteration potential in relation to engineered barriers for HLW repositories: The Nui Nua clay, Thanh Hoa Province, Vietnam

Research paperCharacterization of Fe-smectites and their alteration potential in relation to engineered barriers for HLW repositories: The Nui Nua clay, Thanh Hoa Province, Vietnam a Ins

Research paper

Characterization of Fe-smectites and their alteration potential in relation

to engineered barriers for HLW repositories: The Nui Nua clay, Thanh

Hoa Province, Vietnam

a

Institut für Angewandte Geowissenschaften, Technische Universität Darmstadt, Germany

b

Institut für Geographie und Geologie, Ernst-Moritz-Arndt-Universität, Greifswald, Germany

c

Hanoi University of Science, Vietnam National University, Hanoi, Viet Nam

d Gesellschaft für Anlagen- und Reaktorsicherheit mbH, Braunschweig, Germany

e

Institute of Geological Sciences, Vietnamese Academy of Science and Technology, Viet Nam

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 24 December 2012

Received in revised form 17 June 2014

Accepted 28 July 2014

Available online 15 August 2014

Keywords:

Fe-smectite

Fe-montmorillonite

Kinetic experiment

Alteration

HLW repository

The stability of smectite-rich clay minerals is of interest because they could be candidates for engineered barriers for high-level radioactive waste repositories This research characterized the chemical and mineralogical proper-ties of the Nui Nua clay which forms from the weathering of the Nui Nua serpentinized ultramafic–mafic massif (Thanh Hoa Province, Viet Nam) using several methods (including Fourier transform infrared spectrometry and transmission electron microscopy) The Nui Nua clay, taken from Co Dinh and My Cai valleys, is composed of Fe-smectites as the main phase with minor phases of normal smectite, quartz, talc, chlorite, kaolinite, amphibole, antigorite, feldspars and magnetite The Fe-smectites were characterized as mixed-layer minerals (including Fe-montmorillonitic as an end-member) composed of illitic or dioctahedral vermiculitic layers and Fe-rich smec-titic layers The proportion of the smecsmec-titic layer is approximately 80%; the interlayer sheet is dominated by Ca and Mg, while the octahedral sheet is dominated by Fe3+(not Al) The stability of the Nui Nua smectites was also investigated by a simulation in saturation of 1 M NaCl and deionized water under kinetic impaction The average tetrahedral-Si content of the smectites increased or decreased depending on the“dynamic solution” or hydraulic regime By chemical identification, the alteration of Fe-smectites is mainly increased by the smectitization process This research suggests that the Nui Nua clay is a potential candidate for an engineered barrier because during the alteration process, neo-formation of a montmorillonitic layer occurs

© 2014 Elsevier B.V All rights reserved

1 Introduction

Montmorillonite is well known as an octahedral Al-dominating

smectite, whereas nontronite is known as an octahedral Fe-dominating

smectite (Brindley, 1980; Güven, 1988; Moore and Reynolds, 1997;

Newman, 1987) Smectites containing more than 0.3 Fe per [O10(OH)2]

are considered as Fe-rich smectites or Fe-rich montmorillonite (Brigatti,

1983; Güven, 1988) These minerals are formed mostly from serpentinized

ultramafic rock (Aleta et al., 2002; Caillaud et al., 2004, 2006; Ducloux

et al., 1976; Köster et al., 1999; Lee et al., 2003; Seki and Yurdakoç,

2007; Wildman et al., 1968, 1971)

In a weathering profile of serpentinite rock, it has been observed that

dioctahedral smectite is more stable than trioctahedral smectite and

Fe-montmorillonite, which was replaced with saponite (Wildman

et al., 1971) The formation of low-charge Fe-montmorillonite in a weathering profile of serpentine rock is very rapid under tropical mon-soon climate conditions and results in several centimeters of formation depth above the local bedrock surface as described bySchnellman (1964) A substitution of trioctahedral silicates (e.g saponite) by dio-ctahedral silicates (e.g low-charge illite/smectite mixed-layer mineral) and an Fe-pathway of a weathering profile of serpentine rock was also determined byNguyen-Thanh (2012) Thesefindings were based on research on clay in the Nui Nua area, Thanh Hoa Province, Viet Nam There has been interest in the use of the clay as engineered barriers for

a high-level radioactive waste (HLW) repository because the government

of Viet Nam has decided to erect nuclear power plants in the next ten years to guarantee electricity supply for the future

Smectitic clay is well known as a material for engineered barrier sys-tems (EBS) for HLW repositories because of its engineering properties that can meet many of the required functions (Pusch, 1992; Pusch and Yong, 2006) However, in considering candidate materials for an EBS,

⁎ Corresponding author at: 334 Nguyen Trai Road, Thanh Xuan District, Hanoi, Viet Nam.

Tel.: +84 4 38585097; fax: +84 4 38583061.

E-mail address: hoangminhthao@vnu.edu.vn (T Hoang-Minh).

http://dx.doi.org/10.1016/j.clay.2014.07.032

Contents lists available atScienceDirect Applied Clay Science

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c l a y

it is necessary to assess the stability of all the mineral components,

in-cluding smectite which forms a substantial proportion of the Nui Nua

clay It is primarily the shearing processes associated with movements

of the host rock and tension-related expansion of the clay embedding

canisters (made of pure iron or iron lined with 50 mm copper on the

contact surface) which can fracture the waste canisters exposing the

clay to iron (Fe) Iron-related corrosion can accelerate the alteration

processes in the smectite by replacing the octahedral sheet of the

smec-tite (Herbert et al., 2011; Nguyen-Thanh, 2012) Increased octahedral Fe

has been demonstrated in laboratory experiments (Charpentier et al.,

2006; Herbert et al., 2011; Ishidera et al., 2008; Müller-Vonmoss et al.,

1991; Nguyen-Thanh, 2012), modeling studies and natural analogs of

clay weathering (Herbert et al., 2011; Nguyen-Thanh, 2012; Savage

et al., 2010; Wilson et al., 2006a,2006b)

Groundwater in the local environment of a HLW repository is likely

to have large concentrations of dissolved Ca2+and Na+(or even K+and

Mg2+) because of upwelling of saline ground water or extraction of salt

from cements (Pearson et al., 2003; Stober and Bucher, 2002) The

Al-rich smectites may not be stable in this environment (Adamcova et al.,

2008; Bauer et al., 2001; Castellanos et al., 2008; Eberl et al., 1978;

Hoffman et al., 2004; Herbert et al., 2004, 2008; Karnland et al., 2007;

Kaufhold and Dohrmann, 2009, 2010; Suzuki et al., 2008; Zysset

and Schindler,1996) Alternatively, this could lead to Si-precipitation

and cementation of the smectite forming collapsed quasi-crystals and

wider pores (Pusch et al., 1991, 2007) Some researches detected the

illitization, kaolinitization and pyrophylitization of the smectite (Herbert

et al., 2004; Kasbohm et al., 2004) and several studies have identified

changes in the geotechnical properties of smectite including cation

exchange capacity, swelling and adsorption capacities (Adamcova

et al., 2008; Hofmann et al., 2004; Suzuki et al., 2008; Weiss and

Koch., 1961) However, the stability of Fe-rich smectite has not been

considered

Therefore, this research on the Nui Nua clay, which is a product of

natural weathering processes of the serpentinized ultramafic–mafic

massif, was performed in order to determine the chemical and

mineral-ogical properties of the smectite The research also investigated the

sta-bility of this smectite phase, specifically in relation to simulation of

reactions relating to the use of this material as a HLW repository

through saturation in 1 M NaCl and deionized water under kinetic

impaction

2 Geological setting and materials

2.1 Geological setting

The Nui Nua clay deposits are located in the Thanh Hoa Province, in

the northern Central region of Viet Nam (Fig 1) The clay is related to

the Nui Nua massif The biggest ultramafic–mafic massif in Viet Nam

is well known as part of the Song Ma ophiolite zone (Bach et al., 1982;

Chuong et al., 2001; Findlay and Trinh, 1997; Hung, 1999; Hutchison,

1975) and is associated with a series of economically important

chromite and serpentine mines in this area (Tri, 2005; UNP, 1990)

The massif is composed mainly of dunite, harzburgite, lerzolite, gabbro

and diabase of Cambrian and Ordovician age (Chien, 1964; Khuc,

2000; Lien, 1980; Thang et al., 1999; Thanh et al., 2005; Thuc and

Trung, 1995; Tri, 1979; Tri et al., 1986; Vuong et al., 2006) and was

altered to serpentinite and schist of actinolite and talc-actinolite by

serpentinization during the Cambrian period (Chien, 1964; Son et al.,

1975; Tong-Dzuy and Vu, 2011) More recently (less than 2 million

years before present), the Nui Nua rocks have undergone

pre-Quaternary erosion and pre-Quaternary weathering (Tong-Dzuy and Vu,

2011) The weathering continued under tropical climatic conditions

cre-ating a large amount of Fe-rich minerals including Fe-rich smectite The

Nui Nua clay was concentrated by re-sedimentation in two big valleys,

My Cai and Co Dinh (Fig 1)

2.2 Materials The physical samples used in this research were taken in the My Cai and Co Dinh valleys, which are situated in the northeastern part of the Nui Nua massif (Fig 1) The clay body in My Cai is thicker than that in

Co Dinh, but both the clays are homogenous and brownish-yellow in color Based on application of the Atterberg sedimentation method, the two clay samples are composed of 65.8–67.3 mass% of the b2 μm particle size fraction, 19.5–20.0 mass% of 6.3–2 μm fraction, 12.3–12.8 mass% of 6.3–20 μm fraction and only 0.9–1.2 mass% of 20–63 μm fraction; parti-cles with diametersN63 μm could not be detected

3 Experiments and methods 3.1 Kinetic experiments The kinetics of Fe- and Mg-driven alteration processes of the Nui Nua clay as well as the potential of alteration of Fe-smectite were investigated The aim of these experiments was to focus on alteration processes in buffer materials of HLW repositories containing canisters made of iron

The hypothesis this research wishes to test concerns the alteration rate of Fe-smectite based on the proposal byČičel and Novak (1976)

that large amounts of Fe and Mg in the octahedral sheet of smectite can accelerate the alteration process The larger ionic diameters of Fe and Mg in comparison with Al may well be responsible for larger sheet stresses, which facilitate dissolution of the smectites The other mechanism proposed byLaird (2006)andKaufhold and Dohrmann (2008)concerning this process, is the space stabilization of Ca2+and

Mg2+cations as quasi-crystals in the interlayer

The Co Dinh clay samples, ground tob40 μm, were saturated in

1 M NaCl solution and deionized water with a“liquid:solid” ratio of 4:1 and 10:1, respectively for 30 days, followed by the application of soft gels for mechanical agitation by overhead rotating at room temper-ature at 20 revolutions per minute (rpm) and 60 rpm The higher the speed of overhead rotating, the greater the energy, which leads to the removal of dissolved elements from the particles It is expected that at

60 rpm, the number of dissolved elements removed was higher than that at 20 rpm The reaction products of experiments of saturation in

1 M NaCl solution were dialyzed using the QuickStep-system for 1 mg material in approximately 2 h Below, these products are referred

to as Co Dinh clay + 1 M NaCl + 20 rpm and Co Dinh clay +

1 M NaCl + 60 rpm as well as Co Dinh clay + H2O + 20 rpm and Co Dinh clay + H2O + 60 rpm

3.2 X-rayfluorescence (XRF) spectroscopy Using the material collected from thefield sites, the bulk materials (milled to b40 μm) were analyzed by XRF spectroscopy using a wavelength-dispersive X-ray Philips PW 2404 spectrometer equipped with a 4 kW Rh X-ray source (10 mA, 20 kV) The analyses used a non-wetting agent and/or oxidizer Loss on ignition (LOI) was deter-mined at 1100 °C as an approximate measure of volatile H2O 3.3 X-ray diffraction (XRD)

The mineralogical composition of randomly oriented powder sam-ples withb40 μm size fraction of the Nui Nua Fe-rich clay was investi-gated using a Siemens D5000 X-ray diffractometer (Cu tube, Kα1,2

radiation, 40 kV, 30 mA) Oriented mounts withb2 μm size fraction in-cluding air-dried, ethylene glycolated, and heated (to 550 °C for 4 h) specimens were investigated with a Freiberg Präzitronic diffractometer HZG 4A-2 equipped with a Seifert C3000 control unit (Co tube, Kα1,2 ra-diation, 30 kV, 30 mA) The XRD data were processed using BGMN-Rietveld software (Bergmann et al., 1998; Kleeberg et al., 2005; Ufer

et al., 2004, 2008) in cross-checking with the XRF results

3.4 Fourier transform infrared spectrometry (FT-IR)

Infrared spectra of bulk samples milled tob40 μm were recorded in

the mid-infrared range, which extends from 400 cm−1to 4000 cm−1,

using a Nicolet 6700 FT-IR spectrometer (64 scans, 4 cm−1resolution)

The FT-IR spectra were deconvoluted by an Origin Pro Peak Fitting

(ver-sion 8.5) technique A Gaussian distribution function was applied to

smooth the spectra and to obtain exact values of peak positions, full

width at half maximum (FWHM), intensity and area The baseline

cor-rection of two wavenumber regions surrounding 970 cm−1 and

720 cm−1was subtracted by the same technique The accuracy of the

deconvolution was assessed by the adjustment of peak position to

achieve an R2N 0.98 The stable adjustment was documented by the

positions of a band of quartz at ~795 cm−1and of a significant doublet

of quartz at 780 and 800 cm−1(Farmer, 1974; Craciun, 1984)

3.5 Transmission electron microscopy (TEM)

Selected individual clay particles were characterized by their

morphology, crystal habit, chemical composition, electron diffraction

properties, and element distribution using a JEOL JEM-1210 microscope

(120 kV, LaB6 cathode) coupled to an ISIS LINK-OXFORD

energy-dispersive X-ray (EDX) system Suspension of clay samples was

pre-pared on carbon-coated Cu-grids by air-drying The particle

morphol-ogies were described according toHenning and Störr (1986) Mineral

formulae were calculated using the semi-quantitative data of

approxi-mately 150 particles per sample based on the theory ofKöster (1977)

and the software toolkit ofKasbohm et al (2002)

In this study, any illite identified during TEM-EDX analyses is

referred to as illite in the sense ofŚrodoń et al (1992); specifically

conforming to the following structural formula:

FIX0.89(Al1.85Fe0.05Mg0.10)(Si3.20Al0.80)O10(OH)2

where: FIX representsfixed K + Na cations in the interlayer space Furthermore, K- and/or charge-deficient dioctahedral micas, with tetrahedral Sib 3.3 per O10(OH)2, are referred to as dioctahedral vermic-ulite The acronyms“IS-ml” and “diVS-ml” refer to an illite/smectite mixed-layer and a dioctahedral vermiculite/smectite mixed-layer, respectively

4 Minerological characterization of Nui Nua clay Bulk chemical compositions of the two clays, as determined by XRF spectroscopy, are presented inTable 1 The My Cai clay contains the larger concentrations of SiO2and MgO and smaller Al2O3and Fe2O3 con-centrations in comparison with the Co Dinh clay The Fe2O3 concentra-tions (23.3 mass% for the My Cai clay and 25.5 mass% for the Co Dinh clay) are comparable to the former publications about the Nui Nua clay byBinh and Duc (1998),Lam et al (1998), andKhai and Tau (2003) Such large Fe2O3concentrations are comparable with that of the Fe-rich Saint-Laurent, Limousin, France (24.4%, lower saprolite zone) (Caillaud et al., 2004), and higher than the Fe2O3concentrations

of the Fe-rich Çamlıca clay, Turkey (9.97% and 12.12%) (Seki and Yurdakoç, 2007)

The XRD patterns of the Co Dinh and My Cai powder samples (Fig 2) verified the presence of smectite-phases with peaks at 15.4 Å, 4.51 Å, 2.58 Å, 2.26 Å, 1.71 Å and 1.51 Å The occurrence of the following min-eral phases was demonstrated by specific XRD positions (as Å units): anitogorite (7.32 Å), quartz (4.26 Å, 3.34 Å, 1.82 Å, 1.54 Å and 1.46 Å), amphibole (3.14 Å, 2.71 Å) and magnetite (2.53 Å) The 15.4 Å basal spacing at room temperature indicated that interlayer space of the smectite is dominated by divalent cations The XRD patterns showed that mineral compositions of these two clay samples from the Co Dinh and My Cai valleys are similar Semi-quantitative mineral compositions

of the bulk samples and of theb2 μm fraction samples of these two clays

Fig 1 Geological map of the Nui Nua massif and the two sampling locations.

are presented inTable 2 Smectite is the dominant mineral phase and its

proportion increased significantly from the bulk samples (65–70 mass%)

to theb2 μm fraction samples (85–87 mass%)

Based on the XRD patterns of the oriented specimens (

Nguyen-Thanh, 2012) and calculation of the proportion of illitic layers in the

illite/smectite mixed-layer mineral (according toMoore and Reynolds,

1997), the Nui Nua smectite was characterized as an illite/smectite

mixed-layer series with around 10% illitic layers The positions of the

d06–33peaks at 1.51 Å indicated that the clay samples contain Fe-rich

dioctahedral smectite (Caillaud et al., 2004; Köster et al., 1999; Petit

et al., 2002; Seki and Yurdakoç, 2007)

Mineral characteristics of the two clays were investigated in greater

detail by TEM-EDX and shown inFig 3a andTable 3

Most often, the clay particles of the Nui Nua clay samples presented

xenomorphic form, with less distinct edges and aggregates Some

parti-cles were present as separate laths Based on the electron diffraction

analyses, the Nui Nua clay showed mainly ring-like polytype structure

(Fig 3a), which is typical for smectite phases

Based on the results from TEM-EDX analyses, the particles can be

di-vided into two groups based on their structural chemical composition

Thefirst group was identified as the illite/smectite mixed-layer series

including Fe-montmorillonite as the end-member containing the

ma-jority of Fe in the octahedral sheet The second group was characterized

as the dioctahedral vermiculite/smectite mixed-layer phase including

the Fe-rich dioctahedral vermiculite/smectite mixed-layer series

(Fe-diVS-ml) and the Al-rich dioctahedral vermiculite/smectite

mixed-layer series (Al-diVS-ml) The Al-diVS-ml particles with both

tet-rahedral and octahedral charges can be assumed composed of a

smec-titic layer, which is intermediate between an Fe-montmorillonite

end-member and beidellite (or nontronite) (Christidis, 2006; Petit et al.,

2002) The ratio of the number of the Al-diVS-ml particles and the

number of the Fe-diVS-ml was approximately 1:9 for both clays

According toŚrodoń et al (1992), the Fe-rich smectite from the Nui

Nua clay belongs to the low-charge mixed-layer phases containing large

proportions of montmorillonitic layers; these were mostly 40–80% for

the diVS-ml phases and ~100% for the IS-ml series (Table 3) The

octahe-dral sheets of most particles are dominated by Fe, but also contain traces

of Cr3+

A comparison of the Co Dinh and My Cai clays shows that the former is

characterized by a higher K index, corresponding to a lower interlayer

charge and a diVS-ml series with a higher Fe index in the octahedral sheet

Further information on the mineral composition was obtained by the FT-IR method The FT-IR spectra (Fig 4) of bulk samples from the

Co Dinh and My Cai clays verified the presence of the Fe-smectite based on the strong absorption bands of the OH-stretching region at

3548 cm−1and 3554 cm−1, respectively (Bishop et al., 2002; Caillaud

et al., 2004; Farmer and Russell, 1964; Fialips et al., 2002; Goodman

et al., 1976; Madejová and Komadel, 2001; Petit et al., 2002; Vantelon

et al., 2001) These publications related these absorption bands to

Fe3+

\OH\Fe3+ The weak spectral bands observed near 3625 cm−1 were due to stretching vibrations of the Al\OH\Al stretching of octa-hedral sheet of smectite (Andrejkovičová et al., 2006; Farmer, 1974) Moreover, the other weak bands at 3683 cm−1(from My Cai clay) and 3675 cm−1 (from Co Dinh clay), can be assigned to either

Al\OH\Mg stretching of the octahedral smectite sheet (Farmer,

1974) or νMg3OH stretching of trioctahedral occupancy (Caillaud

et al., 2004; Decarreau et al., 1992; Petit et al., 2002) In some publica-tions, such asFarmer (1974)andMadejová and Komadel (2001), these bands were interpreted to OH-stretching of inner hydroxyl groups of kaolinite, but the peaks must be sharper

Based on the bending region, the Fe3+\OH\Fe3+bending vibra-tion at 818 cm−1represents the large content of Fe3+in the octahedral sheet of smectite (Fialips et al., 2002; Goodman et al., 1976) was ob-served for both clays The bands obob-served at 870 cm−1and 910 cm−1 (My Cai clay) as well as 870 cm−1and 913 cm−1(Co Dinh clay) were assigned to the Al\OH\Al and Al\OH\Fe bending bands, respectively

In regard to other regions, the spectral feature observed at 1632 cm−1 from both clays demonstrated the presence of the H\O\H bending vi-brations at the smectite phase (Seki & Yurdakoç, 2007) The tetrahedral

Si\O\Al bending and the tetrahedral Si\O\Si bending bands ap-peared at 519 cm−1and 465 cm−1(from My Cai clay) and 515 cm−1 and 467 cm−1(Co Dinh clay) Additionally, the strong spectral bands

at 1023 cm−1(from My Cai clay) and 1021 cm−1(from Co Dinh clay) were attributed to typical Si\O stretching vibrations of smectites (Bishop et al., 2002; Madejová and Komadel, 2001) or Fe-rich smectites (Fialips et al., 2002)

The bands at 778 cm−1 and 798 cm−1 were also typically interpreted to Si\O stretching of quartz (Madejová and Komadel, 2001; van der Marel and Beutelspacher, 1976; Vantelon et al., 2001) This evidence indicated that in the Nui Nua clay, the characteristics of the smectite phase was dominantly characterized by the quantity of

Fe3 +and also the minor amount of Al in the octahedral sheet The

Table 1

Chemical composition (mass%) of Nui Nua clay.

Sample SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2 O P 2 O 5 LOI Sum

My Cai clay 49.71 0.37 5.99 23.3 0.36 9.34 1.01 0.11 0.36 0.03 8.28 98.86

Co Dinh clay 47.67 0.35 6.24 25.5 0.30 9.01 0.66 0.08 0.31 0.03 8.99 99.14

Fig 2 XRD patterns of a) Co Dinh clay (above), and b) My Cai clay (below) in samples from randomly oriented powder, °2Θ CuK position.

Al\OH\Mg (bending) bond was not observed in both clays; this could

be explained by the relatively low Mg content in the octahedral sheet

Therefore, the identified spectral bands of Mg were due to the presence

of local trioctahedral magnesian clusters within the dioctahedral Fe3+

and Mg-rich matrix (Petit et al., 2002)

In conclusion, based on the good agreement among the reported

re-sults by XRD, TEM-EDX and FT-IR studies, the Nui Nua clay (including

clays from the Co Dinh and My Cai valleys) formed as the weathering

products of the serpentinized ultramafic–mafic Nui Nua massif (Thanh

Hoa Province, Vietnam) was characterized by the dominant of

low-charge Fe-smectite The impurities detected in both clay samples

in-cluded quartz, chlorite, talc, kaolinite, amphibole, antigorite, feldspars

and magnetite There is only a small difference between the Co Dinh

and the My Cai clay in regard to structural formulae of the smectite as

well as chemical and mineralogical compositions From both types of

bulk sample andb2 μm sample fraction, the Co Dinh clay was

character-ized by a larger amount of smectite and a smaller quantity of quartz in

comparison with the My Cai clay (Table 2) By contrast with the My

Cai clay, measurements on samples of the Co Dinh clay showed that

there were slightly smaller concentrations of MgO (Table 1), lower Mg-index values from the structural formulae (Table 2) and a smaller quantity of Mg associated with the less intensive band ofνMg3OH stretching vibration (Fig 4) In contrast, the Co Dinh clay was character-ized by a higher Fe2O3mass% (Table 1), a higher Fe3+amount in the oc-tahedral sheet yielded from TEM-EDX analyses (Table 3) and more intensive bands of Fe3+\OH\Fe3+stretching and bending vibrations

5 Kinetic experiments: minerological characterization of the Nui Nua clay

The Co Dinh clay was selected for kinetic experiments because of its verified larger content of Fe-smectite, larger Fe2O3mass% and larger

Fe3 +quantity in the octahedral sheet of the smectite by comparison

to the My Cai clay The XRD curves of the randomly oriented specimens from the reaction products indicated that Fe-smectite is still the main mineral phase after the experiments (Nguyen-Thanh, 2012) The XRD quantitative study of mineralogical composition determined that the

Table 2

Mineralogical composition (in mass%) of bulk and b2 μm fraction samples of untreated Co Dinh and My Cai clays as well as Co Dinh clay after kinetic experiments, determined from XRD data with the BGMN-Rietveld software.

Phases My Cai clay (untreated) Co Dinh clay (untreated) Co Dinh clay after kinetic experiments (b2 μm fraction)

Bulk sample b2 μm fraction Bulk sample b2 μm fraction Co Dinh clay + 1M NaCl + 20 rpm Co Dinh clay + 1M NaCl + 60 rpm

Note: Smectite comprises mainly Fe-montmorillonite, illite/smectite mixed-layer mineral and dioctahedral vermiculite/smectite mixed-layer mineral, which were verified by TEM measurement.

Fig 3 TEM bright-field (above) and electron diffraction (below) images of a) Co Dinh clay (untreated ), b) Co Dinh clay + 1 M NaCl + 20 rpm, and c) Co Dinh clay + 1 M NaCl + 60 rpm.

b2 μm fraction of the reaction products contain more than 85 mass% of

Fe-smectite (Table 2)

The TEM analyses revealed that the initial Fe-smectite particles

were changed in morphology and polytype The products presented

thefiner aggregates and rearranged as mainly 1 M-polytype (Co Dinh

clay + 1 M NaCl + 20 rpm,Fig 3b) and ring-like polytype (Co Dinh

clay + 1 M NaCl + 60 rpm,Fig 3c) The structural formulae of the

smec-tites are presented inTable 3 The obvious changes of Si indexes of the

tetrahedral sheet or proportion of the smectitic layer (S%) (calculated

according toŚrodoń et al., 1992) as well as the accompanied interlayer charge were identified

The changes were also observed using the FT-IR method, wherein the higher Si contents in the tetrahedral sheet and the higher wavenum-ber values of the Si\O stretching-vibration band (around ~1000 cm−1

and ~1100 cm−1;Farmer, 1974; Lerot and Low, 1976; Stubičan and Roy,

1961) were observed

Fig 5shows a linear relationship between the Si indexes of the tet-rahedral sheet calculated from TEM-EDX data and the wavenumber

Table 3

Mineral formulae (cations per [O 10 (OH) 2 ]) of clay mineral phases from untreated My Cai and Co Dinh clays as well as Co Dinh clay after kinetic experiments, determined by TEM-EDX analyses.

Phases Interlayer space Octahedral sheet Tetra sheet

Ca Mg Na K Cr 3+

Al Fe 3+

Mg Ti Al Si XII n VI

S%

My Cai clay (untreated)

Fe-IS-ml 0.03 0.08 0.00 0.15 0.06 0.50 1.17 0.22 0.01 0.06 3.94 0.37 1.96 97 Fe-diVS-ml 0.02 0.08 0.00 0.13 0.05 0.57 1.20 0.16 0.02 0.16 3.84 0.31 2.00 78 Al-diVS-ml 0.01 0.12 0.00 0.16 0.04 1.39 0.49 0.07 0.01 0.37 3.63 0.43 2.00 44 average 0.02 0.08 0.00 0.15 0.05 0.61 1.11 0.19 0.01 0.12 3.88 0.35 1.97 85

Co Dinh clay (untreated)

Fe-IS-ml 0.07 0.07 0.00 0.02 0.04 0.48 1.19 0.16 0.01 0.03 3.97 0.30 1.88 100 Fe-diVS-ml 0.03 0.11 0.00 0.03 0.09 0.46 1.25 0.16 0.04 0.19 3.81 0.32 2.00 73 Al-diVS-ml 0.08 0.14 0.00 0.05 0.03 1.48 0.39 0.09 0.01 0.40 3.60 0.50 2.00 40 average 0.04 0.10 0.00 0.04 0.08 0.56 1.16 0.17 0.03 0.19 3.81 0.32 2.00 73

Co Dinh clay after kinetic experiments

Co Dinh clay + 1 M NaCl + 20 rpm 0.02 0.13 0.00 0.04 0.03 0.59 1.21 0.15 0.02 0.21 3.79 0.35 2.00 68

Co Dinh clay + 1 M NaCl + 60 rpm 0.04 0.07 0.02 0.06 0.10 0.48 1.20 0.18 0.02 0.06 3.94 0.30 1.98 97

Co Dinh clay + H 2 O + 20 rpm 0.02 0.11 0.00 0.05 0.03 0.58 1.18 0.19 0.01 0.10 3.90 0.31 1.99 89

Co Dinh clay + H 2 O + 60 rpm 0.03 0.13 0.00 0.02 0.05 0.55 1.13 0.23 0.02 0.11 3.89 0.34 1.98 87 Note: the values are average of measured particles; Fe-IS-ml included high-charge and medium-charge Fe-montmorillonite as end-member; Fe-diVS-ml and Al-diVS-ml included low-charge Fe-montmorillonite and low-low-charge Al-montmorillonite as end-members, respectively; average was calculated for Fe-IS-ml, Fe-diVS-ml and Al-diVS-ml; XII: interlayer low-charge,

n VI

: number of octahedral occupation, S%: proportion of montmorillonitic layer

values of the Si\O stretching-vibration band fitted using the FT-IR data.

This verified that the TEM-EDX and FT-IR data are in agreement and the

changes of smectites after the similar kinetic experiments are consistent

with this trend

Two processes are postulated to occur during the kinetic

ex-periment Thefirst, which occurred in only two experiments using

1 M NaCl solution, is an alteration process affecting smectite in the

NaCl solution with the“open” dynamic system described byPusch

and Kasbohm (2002) This process leads to a reduction of Si

concentra-tions The second process is the dissolution of smectite, followed by the

removal of Si, andfinally representing a new montmorillonitic layer

This process, called“dynamic solution”, depends on the flow rate of

overhead rotating or the energy of the hydraulic regime; the lower

theflow rate, the larger the amount of dissolved Si resulting in

neo-formed montmorillonite The alteration is mentioned byPusch et al

(1991, 2007)and can be proven by the occurrence offiner aggregates

of smectite under TEM observations (Fig 3b, c)

The observation of the lower tetrahedral Si of smectite in the Co

Dinh clay + 1 M NaCl + 20 rpm (Table 3,Fig 5) demonstrates that

thefirst alteration process was dominant With the Co Dinh clay +

H2O + 20 rpm, there was only“dynamic solution” at the low energy,

so that the tetrahedral Si content was slightly increasing in comparison

with the starting material When the speed of overhead rotating was

increased to 60 rpm, the influence of “dynamic solution” was more

significant in comparison to the alteration by salt solution Therefore,

the Co Dinh clay + 1 M NaCl + 60 rpm and the Co Dinh clay +

H2O + 60 rpm resulted in a larger tetrahedral Si concentration (or S%)

in comparison with the starting material (Table 3,Fig 5)

6 Conclusions

Under tropical monsoon climate conditions, low-charge

Fe-montmorillonite is formed very rapidly by weathering of serpentine

rock This study of the Nui Nua clay (formed from the weathering of

the Nui Nua serpentinite, Viet Nam) based on chemical, TEM-EDX,

XRD, FT-IR analyses indicated that the clay contains mainly smectite,

in-cluding Fe-smectite and normal smectite (N65 mass% of bulk samples

andN85 mass% of b2 μm fraction samples) and impurities of quartz,

chlorite, talc, amphibole, kaolinite, antigorite, feldspars and magnetite

(Table 2;Figs 2, 3a,4) The Fe-smectite phases were specifically

charac-terized as mixed-layer minerals (including Fe-montmorillonitic as

end-member) composed of illitic or dioctahedral vermiculitic layers and a

Fe-rich smectitic layer In regard to the Fe-smectite, the proportion of

the smectitic layer (%S) is 80%, while the remainder is formed from an

interlayer sheet dominated by Ca and Mg, and an octahedral sheet

dom-inated by Fe3+but not Al (Table 3;Fig 4) The normal smectite phase

made up approximately 10% of all the smectite phases They were

char-acterized as mixed-layer minerals with large proportions of smectitic

layer phases, which is intermediate between Fe-montmorillonite end-member and beidellite (or nontronite), and a smaller quantity of Fe in comparison to the Fe-smectite phases

By investigating the alteration processes of the Nui Nua clay using simulation by saturation in 1 M NaCl and deionized water under kinetic impaction, this study showed that the smectite phase of the Co Dinh clay (Nui Nua clay) could be altered by two processes which have impli-cations for its potential use as an engineering barrier Thefirst is an al-teration process due to the NaCl solution which influences the smectite phase, and the second one is“dynamic solution” The “dynamic solution” process, which leads to the dissolution of smectite then removal of Si, andfinally leading to neo-formed montmorillonite, de-pends strongly onflow rate of the overhead rotating activity Each smectite may have a specific alteration potential, which was observed with some other clays such as the MX-80 and the Friedland clay (Herbert et al., 2011; Nguyen-Thanh, 2012) With the Fe and Mg-rich octahedral sheet and bivalent cation-dominated interlayer sheet, the

Co Dinh Fe-smectite has a high alteration or dissolution potential be-cause the larger ionic radius of Fe and Mg compared to Al increases the mechanical stresses in the octahedral sheet (Čičel and Novak,

1976) and the larger proportion of divalent cations (e.g Ca, Mg) in the interlayer sheet promotes unmixing of monovalent and divalent inter-layer cations or leads to the stabilization of quasi-crystals (Laird, 2006; Kaufhold and Dohrmann, 2008) The alteration processes may lead to the formation of a richer smectitic phase; the alteration process is smectitization This hypothesis is consistent with natural formation processes, which under ideal conditions leads to the complete alteration

of serpentinite to montmorillonite

This study also indicates that a clay dominated by Fe-smectite (illite/ smectite layer and dioctahedral vermiculite/smectite mixed-layer series) like the Nui Nua clay can be a good candidate for a buffer

or backfill material of a HLW repository because clay-mineral alteration

of this kind leads to neo-formation of montmorillonite which has effective barrier properties Otherwise, if we use Fe-smectite with a pro-portion of the smectitic layer (S%) of approximately 100% or pure Fe-montmorillonite as original material end-members, reaction processes will lead to Si-cementation or Si-clogging as described byPusch and Touret (1988) This process would cement collapsed quasicrystals to-gether and broaden the pores, which can allow other external agents

to penetrate channel-like pathways into the clay and increase the hydraulic conductivity and the shear strength

Acknowledgements

We gratefully acknowledge the support from the National Foundation for Science and Technology Development, Vietnam (project code 105.02.54.09) We also thank the support from the Gesellschaft für Anlagenund Reaktorsicherheit (GRS) mbH and the Mineralogical Laboratories— University of Greifswald, Germany We are very grateful

to Prof Reto Gieré (University of Freiburg, Germany) for the XRF data,

Dr Le Thi Thu Huong for the FT-IR measurement, and Dr Barry Rawlins (British Geological Survey) for upgrading the use of English in the man-uscript We also thank two anonymous reviewers and the associate editor for their evaluation

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