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June 2004
Work ing Repor t 2004 -24
Results of the Studies onBentonite Samples from
Serrata de Nijar, Almería, Spain
Nur ia Marcos
June 2004
Nur ia Marcos
He l s i nk i Un i ve r s i t y o f Techno logy
Work ing Repor t 2004 -24
Working Reports contain information on work in progress
or pending completion.
The conclusions and viewpoints presented in the report
are those of author(s) and do not necessarily
coincide with those of Posiva.
Results of the Studies onBentonite Samples from
Serrata de Nijar, Almería, Spain
Marcos, N. 2004. Results of the Studies on Bentonite Samples from Serrata de Nijar, Almería, Spain. Posiva Oy, Eurajoki. Working Report 2004-24. 19 p.
ABSTRACT
Bentonite samples collected from an outcrop in Serrata de Nijar (Almería, Spain) were mineralogical and chemically characterized. The main objectives were to study the mode of occurrence of iron in bentonite and to find out whether there is as positive cor-relation between the iron content in the samples and the position of the sample with respect to a possible iron source. The use of the bentonite outcrop as a possible natural analogue study site was also evaluated.
Typical reddish hematite pigment is observed in almost half of the samples. These sam-ples show also the highest content in iron, but no iron oxide minerals were identified in any of the collected samples, nor in the reddish or in the greyish ones. Structural iron in the greyish samples was most as Fe(III) (75%) as revealed by Mössbauer spectroscopy. The average content of smectite in the samples stays well below the average content of smectite of any of the bentonites currently considered for use in nuclear waste disposal.
The use of the outcrop as a natural analogue to study iron-bentonite interactions and the influence of iron in the properties of bentonite can be questioned because of the dissimi-larity between the mineral content of these samples and the bentonite to be used in nu-clear waste disposal. Also it is difficult to explain why the possible release of iron from smectite has happened in samples at such small interval of occurrence (about 15 cm). In the field no physical contact (fissure, fault) was observed between reddish and greyish samples. Another line of reasoning is the influence of the pore structure of the original bentonite in the outcrop: the more connected internal porosity, the more possibility of drainage and weathering. In spite of the disadvantages mentioned, useful studies could still be done with respect to the influence of iron in the properties of bentonite. The already collected samples could be mixed to form two groups: iron rich (all reddish samples) and iron poor (all greyish samples). From each group, a smectite rich fraction could be separated to per-form detailed investigations about the mode of iron occurrence and a comparison be-tween the physical properties of the selected smectite fractions.
Keywords: Almería, bentonite, smectite, iron, structural iron, Mössbauer, natural ana-logue
Marcos, N. 2004. Tutkimustuloksia Serrata de Nijarin bentoniittinäytteistä (Almería, Espanja). Posiva Oy, Eurajoki. Työraportti 2004-24. 19 s.
TIIVISTELMÄ
Espanjassa Serrata de Nijarin alueella (Almeríassa) on useita bentoniittiesiintymiä. Yksi niistä valittiin raudan esiintymisen perusteella, ja näytteitä kerättiin huomioiden asteit-tainen värimuutos punertavasta harmahtavaan. Tavoitteena oli selvittää 1) raudan esiin-tymismuotoja bentoniitissa, 2) onko korrelaatiota bentoniitin rautapitoisuuden ja näyt-teiden etäisyyden (raudan lähteeltä) välillä ja 3) arvioida esiintymän soveltuvuutta luon-nonanalogiakohteena, josta voitaisiin tutkia raudan ja bentoniitin kontaktissa tapahtunei-ta ilmiöitä.
Puolet näytteistä (punertavat) sisälsivät hematiittia pigmenttiä, mutta rautamineraaleja ei havaittu missään näytteissä. Harmahtavista näytteistä tutkittiin raudan esiintymismuo-toa. Mössbauerin analyysin perusteella tuli ilmi, että lähes 75 % raudasta esiintyy Fe(III) muodossa. Keskimäärin näytteiden smektiittipitoisuus oli selvästi pienempi kuin käytetyn polttoaineen loppusijoituksessa käytettävän bentoniittipuskurin smektiittipitoi-suus. Näytteiden mineralogia ei ole täysin verrattavissa muihin bentoniitteihin.
Esitetyistä syistä ei ole suositeltavaa, että ko. bentoniitiesiintymää käytettäisiin luon-nonanalogiana. Korrelaatiota bentoniitin rautapitoisuuden ja näytteiden etäisyyden (rau-dan lähteeltä) välillä ei voitu havaita, koska rautapitoisuus muuttuu yhtäkkisesti 15 cm:n etäisyydellä. Raudan lähde punertavissa näytteissä voisi olla itse smektiitissa, joka muuntuessaan esim. kaoliniitiksi vapauttaa rakenteellista rautaa ympäristöön. Myös itse bentoniitin rakenteella (esim. huokoisuus) voi olla vaikutusta raudan vapautumiselle.
Vaikka ko. bentoniittiesiintymää ei voida käyttää luonnonanalogiana ja raudan lähde on yksi epävarmuustekijä, voisi olemassa olevat näytteet vielä hyödyntää. Olisi mahdollista jakaa olemassa olevia näytteitä kahteen ryhmään (punertavat ja harmahtavat). Silloin voitaisiin erottaa tarpeeksi suuri smektiittifraktio kummastakin ryhmästä. Näin erote-tuista smektiittifraktioista voitaisiin tutkia sekä raudan esiintymismuotoa että raudan vaikutusta smektiitin fysikaalisiin ominaisuuksiin.
Avainsanat: Almeria, bentoniitti, smektiitti, rauta, rakenteellinen rauta, Mössbauer, luonnonanalogia,
PREFACE This study has been performed within the Basic Design phase of SKB and Posiva's development programme for the KBS-3H (i.e. horizontal) disposal concept.
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TABLE OF CONTENTS:
PREFACE
ABSTRACT TIIVISTELMÄ
TABLE OF CONTENTS ………………….………………………………… 1 1 INTRODUCTION ……………………………………….…………………… 2
1.1 Sampling site selection …………………..……………………………….. 2
1.2 Sampling method …………………….…………………………………… 5
2 HANDLING OF THE SAMPLES AND ANALYTICAL METHODS ……… 6
2.1 X-ray diffractometry (XRD) …………………….………………………… 6
2.2 Fourier Transform Infrared Analyses (FTIR) …………….……………..… 6
2.3 Mössbauer spectroscopy ……………………………………..………. 7 3 RESULTS ………………………………………………………………..……. 8
3.1 XRD-Analyses ……………………………………………………..……… 8
3.2 FTIR-Analyses ………………………………………………….……...…. 8
3.3 XRF-Analyses …………………………………………………………….. 8
3.4 Mössbauer Analyses ………………………………………………………. 10 4 DISCUSSION …………………………………………………………………. 13
4.1 Comparison of the samples in this study to other bentonite samples from Serrata de Nijar ……………………...................… 13
4.2 Estimation of the site Collado del Aire as a natural analogue
on the influence of iron on the properties of bentonite …….…………..….. 14 5 CONCLUSIONS …………………………………………………………..…... 15
5.1 Further studies with the samples used in this study …………………….…. 15 6 REFERENCES ………………………………………………………………... 16 APPENDIX A …………………………………………………………………….. 17
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1 INTRODUCTION 1.1 Sampling site selection
The so-called “Pecho de los Cristos” bentonite outcrop (Fig. 1) in Serrata de Nijar (Almería, Spain) was in principle the site selected to collect bentonite samples, where bentonite-iron interaction can be observed. The exact position of the outcrop was uncertain and mostly based on a geological map (scale 1:50.000), on a photograph of the outcrop (Fig. 1), and on the notes and observations made by the author in an excursion to the area during spring 1999. Fig. 1. The “Pecho de los Cristos” outcrop After two days tracking the areas of bentonite deposits in Serrata de Nijar called: Cortijo de Archidona, Collado del Aire and Pecho de los Cristos, an outcrop similar to the one in Fig. 1 and close to the site in the old map was found (Fig. 2), but in case this were exactly the same outcrop, it was totally changed. Small-scale landslides are observable in the area and especially in this outcrop all the bentonite mass to the right of the photograph had sledded. The iron oxide front (see arrow in Fig. 1) was not observable anymore (Fig. 2).
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Fig. 2. The “Pecho de los Cristos” outcrop or the most similar site to it. Because the selected outcrop was not available anymore, other similar sites in Serrata de Nijar area were searched for the same purpose. The aim was to find bentonite affected by iron-rich solutions coming from faults or other discontinuities. These kinds of bentonite outcrops were seen in the area, and one of them was selected for sampling (Fig. 3). The advantages of the selected outcrop with respect to others were the situation far from even small roads or paths to avoid the invasion of wastes, the fact that the coordinates of the place are known (thanks to the use of a GPS device), which will help to locate it by anybody, and confidence that samples were taken from a real outcrop and not from whatever abandoned pile of bentonite. A probable disadvantage is the uncertainty of where the iron-rich solutions come from and if they came from outwards. The UTM coordinates of the sampling site are X = 0575203, Y = 4081393. A general view of the site is shown in Fig. 4.
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Fig. 3. Sampling in an outcrop of the “Cortijo del Aire” zone.
Fig. 4. General view of the sampling site. Site is encircled. Eight samples were collected from six holes in an outcrop from Cortijo del Aire (Serrata de Nijar, Almería) as shown in Figure 5. There was interest in the study of these samples because of the colour gradation, which was expected to imply a gradation also in the content of Fe-oxides. The outcrop is an almost vertical wall at the site (Fig. 4).
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1.2 Sampling method Sampling was done perpendicular to the wall. The distance between samples ALM-2 and ALM-6 is about 120 cm. ALM-2 and ALM-6 consist of two sub-samples (Table 1). Each sample was taken with a tube of fibreglass to avoid contamination from one sample to another and from possible iron contamination from standard steel tubes.
ALM-6 ALM-1 ALM-3 ALM-5
2 10 cm
Figure 5. Sample positions and codes. Table 1. Code, length and colour of the samples Sample code Sub-samples Length (mm) Colour ALM-2 ALM-2A 0 - 350 Greyish ALM-2B 350 - 440 Greyish ALM-1 0 - 190 Greyish ALM-3 0 - 180 Greyish ALM-4 0 - 160 Reddish ALM-5 0 - 120 Reddish ALM-6 ALM-6A 0 - 120 Reddish ALM-6B 120 - 350 Reddish to grey
ALM-
ALM-4
6
2 HANDLING OF THE SAMPLES AND ANALYTICAL METHODS The most distant samples (ALM-2 and ALM-6) were fractionated first by sieving and then by selecting the pieces of the fractions richest in smectite by testing the stickiness of the fraction on a damp surface. These fractions of the most distant samples (ALM-2 and ALM-6) were then analysed by X-ray diffractometry (XRD)1. Fourier Transform Infrared spectrometry (FTIR) was used to determine the location and abundance of the structural AlFeOH group in the clay (Appendix A). All samples were analysed by X-ray fluorescence spectroscopy (XRF) to get the chemical composition. The greyish samples (ALM-2A, ALM-2B, ALM-1 and ALM-3) were analyzed by Mössbauer spectrometry2 to get an insight in the iron oxidation state of the structural iron in the smectite. The reddish samples were not analysed with Mössbauer because of the possible “external iron oxy-hydroxides” would have interfered the spectra of structural iron in smectite.
2.1 X-ray diffractometry (XRD) The samples were ground under acetone and the thick slurry spread on glass slide to avoid preferred orientation. X-ray diffraction patterns were recorded from 2 to 70°2θ CuKα using a Philips X-Pert X-ray diffractometer at the Research laboratory of GTK. Minerals were identified using software provided by Philips. Semiquantitative mineral composition was evaluated using intensities of selected peaks and factors determined experimentally at the Research laboratory of GTK. To identify clay minerals the fraction of about < 20 µm was separated by suspending the samples in deionized water and letting them settle. Oriented mounts were prepared using the filter-membrane peel-off technique. The clay suspension was vacuum-filtered on membrane filter and laid face down on a glass slide. The filter was removed carefully. After air-drying the preparations the XRD patterns were recorded from 2 to 20°2θ. The preparations were then kept for 16-20 hours in ethylene glycol atmosphere at 60°C and the XRD patterns recorded again from 2 to 20°2θ. Ethylene glycol solvation is necessary for the identification of smectite. The material left after fractionation was further washed free of finess, observed with light microscope and its XRD patterns were recorded.
2.2 Fourier Transform Infrared Analyses (FTIR) Infrared spectroscopy gives information e.g. about the composition of octahedral sheets of clay minerals. In the OH-bending frequencies between 800 and 1000 cm-1 the Al2OH (920-930 cm-1), the AlFe3+OH (880-890 cm-1) and the AlMgOH (840 cm-1) bands are well resolved. The samples selected for FTIR were ground to fine powder because it was known that they contain several minerals in addition to smectite. Pellets were pressed of a mixture of 0.3 – 1.4 mg of sample and 200 mg of KBr and the spectra were
1 Descriptions of XRD and FTIR methods are given by Liisa Carlson (LC) at the Geological Survey of Finland (GTK). LC interpreted the XRD patterns and the FTIR spectra. XRD analyses were done at GTK and FTIR-analyses at the Department of Geology in the University of Helsinki. 2 Professor François Martin at the University of Limoges (France) did and interpreted the Mössbauer analyses.
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recorded with a Perkin Elmer Spectrum One FT-IR spectrometer at the Department of Geology, University of Helsinki. Each spectrum consists of 10 scans at a resolution of 4 cm-1. All pellets were dried at 150°C for 18-20 hours before recording the spectra in order to remove excess water. 2.3 Mössbauer spectroscopy Mössbauer spectroscopy relies on the phenomenon of recoil-free nuclear resonance fluorescence or the Mössbauer effect, which was discovered by Rudolph Mössbauer in 1957. A Mössbauer spectrum is simply a plot of the amount of nuclear absorption of gamma-rays (γ-rays) as a function of γ-ray energy. The required differences in γ-ray energy are so small, that it is possible to change the energy of the γ-rays by simply moving the source at very low velocities (mm/s). This gives rise to plots of absorption (or transmission) versus source velocity. Three effects contribute to the shape of a Mössbauer spectra; isomer shift, electric quadrupole interaction and magnetic hyperfine interaction. Further background can be found in textbooks, e.g., Greenwood & Gibb (1971) or Bhide (1973). In this study a 57Fe Mössbauer absorption spectrum over the range ± 14 mm/s in first time to observe possible presence of Fe-oxyhydroxydes and over the range ± 4 mm/s in second time (no presence of Fe-oxyhydroxydes) in 512 channels was recorded in the “Laboratoire de Chimie de Coordination” (Toulouse, France). The Mössbauer spectrometer is composed of a compact detector γ-system for high counting rates and a “Wissel” conventional constant acceleration Mössbauer device. A 57Co (in Rh) source with nominal activity of 50 mCi was used. The spectra were obtained at 80 °K to benefit the 2nd order Doppler effect and recorded on a multichannel analyzer Cambera, coupled to a computer. The isomer shift was recorded with respect to α-Fe metal. According to Rancourt et al. (1993), the absorber sample thickness was approximated for phyllosilicates (kaolinite, illite, smectites, etc.) to those of the phlogopite-annite series. The values are around 200 mg of mineral by cm2 for talc and 150 mg of mineral by cm2 for chlorites. Powders were finely ground under acetone (to minimize possible oxidation of Fe) and placed in the plexiglass sample holder. Lorentzian line shapes were assumed for deconvolutions, based on least squares fitting procedures. The χ2 and misfit values were used to measure the goodness of the computer fit. For phyllosilicate mineral spectra (Rancourt et al., 1992, 1993; Rancourt, 1994 a, b; Rancourt et al., 1994), no orientation effects are observed: the electric field gradients of [6]Fe3+ (6-coordinate, octahedral), [4]Fe3+ (4-coordinate, tetrahedral) and [6]Fe2+ (6-coordinate, octahedral) have different orientations. Parameters of the fit are δ (isomer shift in mm/s), ∆ (Quadrupole splitting in mm/s), Γ (Full Width at Half Maximum in mm/s) and % (relative area of the resolved components or peak intensity).
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3 RESULTS 3.1 XRD Analyses The average composition of samples 6A, 6B, 2A and 2B are presented in Table 2. Of each sample two sub-samples were examined. The sub-samples were separated with sieving and presented grain/aggregate sizes 56-500 µm and 500-1000 µm, respectively. Table 2. Average mineral compositions (%) of samples 6A, 6B, 2A and 2B. Smectite Kaolinite Quartz Plagioclase Calcite Dolomite Illite K-feldspar 6A <5 5-10 25-30 10-15 10-15 30-35 <5 - 6B 10-15 5-10 25-30 15-20 5-10 15-20 <5 - 2A 10-15 20-25 10-15 20-25 25-30 <5 - 5-10 2B <5 20-25 15-20 15-20 30-35 <5 - -
In the coarse fraction washed free of fines, following minerals were observed with light microscope: 2A: biotite, muscovite, hornblende, possibly couple of grains of a sulphide mineral; 2B: biotite, hornblende; 6A: biotite, hornblende, hematite pigment in most light-coloured mineral grains (probably carbonate and feldspar); 6B: less hematite pigment than in 6A. In the coarse fraction of samples 2A and 2B amphibole (hornblende) was identified by XRD, in sample 6B gypsum. The smectite content of sample 6A was 15-20%, 6B 45-50%, 2A 5-10% and 2B 5-10%. Thus, especially in sample 6B smectite occurs mainly as sand-sized aggregates. 3.2 FTIR-Analyses The Al2OH-, AlFe3+OH- and AlMgOH-bands in the FTIR-spectra (Appendix A) are weak because the samples contain only a small amount of smectite and other clay minerals. In samples 2A and 2B intensive calcite bands at 880 and 720 cm-1 can be observed. Unfortunately, the AlFe3+OH-band of smectite coincides with the intensive calcite band at 880 cm-1 so that in case of samples 2A and 2B no information can be obtained on the iron content in octahedral sheets of smectite. Also in the spectrum of sample 6A, the calcite 720 cm-1 band is present, so that the intensive band at 880 cm-1 cannot be assigned to AlFe3+OH alone. More work should be done with FTIR. Preferably, pure smectite should be separated by fractionation in deionized water; fine enough fraction usually contains only smectite. Another possibility is to destroy carbonate minerals with chemical methods. This, however, does not solve the problem of dilution. 3.3 XRF-Analyses The chemical composition of all sample series was determined with XRF. The results for main components are presented in Table 3 in weight % (wt. %). Table 4 shows the
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composition of trace elements in ppm. The content of trace elements Sc, La, As, Bi (detection limit 0.003 wt. %), Th, U, Nb, Mo, (d.l. 0.001 wt. %), Ga, Sn (d.l. 0.002 wt. %) and Sb (d.l. 0.01 wt. %) was below detection limit and is not given. The content above the average for the most significant components is in bold characters in both tables. Table 3. Chemical composition in wt. %. The content above the average for the most significant components is in bold characters. ALM-6A-3 ALM-6B-3 ALM-5 ALM-4 ALM-3 ALM-1 ALM-2B-2 ALM-2A-1 SiO2 37.5 44.9 40.6 43.7 33.8 30.7 44.1 42.7 Al2O3 9.14 11.5 8.72 9.63 7.37 7.07 12.2 11.7 Fe2O3 5.09 6.98 7.23 8.20 4.34 3.66 4.37 4.17 TiO2 0.40 0.49 0.41 0.40 0.34 0.31 0.47 0.44 CaO 15.2 8.30 11.0 8.69 22.8 24.3 12.8 13.8 MgO 7.43 7.12 8.46 7.36 3.55 4.54 4.29 4.16 Na2O 0.79 1.24 0.85 1.05 0.87 0.72 1.70 1.67 K2O 2.50 2.91 2.68 2.87 2.17 1.91 2.06 1.96 P2O5 0.16 0.18 0.25 0.15 0.13 0.14 0.11 0.09 MnO 0.24 0.14 0.16 0.14 0.39 0.33 0.18 0.15 S 0.08 0.09 0.07 0.08 0.04 0.05 0.09 0.24 Cl 0.31 0.74 0.35 0.55 0.40 0.36 0.34 0.34 C 4.85 2.94 4.09 3.21 5.58 6.08 2.74 2.93 Table 4. Trace element content in mg/kg. The content above the average for the most significant components is in bold characters. ALM-6A-3 ALM-6B-3 ALM-5 ALM-4 ALM-3 ALM-1 ALM-2B-2 ALM-2A-1 V 140 140 170 160 170 120 130 110 Cr 60 80 80 80 90 60 50 50 Ni 40 50 40 40 50 40 30 30 Cu 50 70 60 80 140 100 50 40 Zn 100 130 120 100 80 80 100 90 Rb 110 140 110 130 90 80 70 70 Sr 140 130 130 150 280 290 210 230 Y 30 20 30 20 20 20 20 10 Zr 90 100 100 100 80 70 80 90 Ce 40 50 50 50 50 50 30 30 Pb 130 180 210 230 290 280 170 150 Ba 630 800 950 750 240 120 510 360
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3.4 Mössbauer spectrometry - Analyses For this kind of study with various phyllosilicate minerals (a mix of kaolinite, smectite, illite) Mössbauer spectra indicates only the Fe(II)/Fe(Tot) ratio, but no Fe (II) and Fe(III) distribution in tetrahedral or/octahedral sites. All the analyzed samples (Table 5) present similar Mössbauer parameters with [Fe2+ / (Fe2+ + Fe3+)] ratio very closed to 0.25. Table 5. Mössbauer parameters of samples ALM-1, ALM-2A, ALM-2B and ALM-3 at 80 °K in ±4 mm/s
∗ δ is relative to Fe-metal and Γ = 0.35 mm/s is fixed for all the doublets
Fe2+ Fe3+ Fe2+ δ∗
(mm/s) ∆
(mm/s) % δ∗ (mm/s)
∆ (mm/s) % (Fe2+ +Fe3+)
ALM-1 0.425 0.308 47 1.126 2.120 4 0.563 0.993 11 1.233 2.680 22 0.288 1.059 16
0.26
ALM-2A 0.439 0.290 45 1.158 2.070 6 0.625 1.012 9 1.265 2.624 19 0.356 1.048 21
0.25
ALM-2B 0.416 0.288 37 1.176 2.082 7 0.738 0.712 10 1.289 2.600 21 0.283 1.005 25
0.28
ALM-3 0.366 0.315 31 1.254 1.824 3 0.663 0.642 16 1.291 2.505 20 0.237 0.872 30
0.23
Spectra recorded between -14 and +14 mm/s give evidence of no presence of iron oxy-hydroxides and only spectra between -4 and +4 mm/s are presented. The spectra of 4 samples at 80 °K (Figs. 6, 7, 8, and 9) are similar with a large band located at 0.5 mm/s and a small intensity band located at 2.5 mm/s. The spectra show a shoulder on the side of the large band located at -0.2 mm/s. The decompositions in Lorentzian doublets (Figs. 6, 7, 8, and 9) are performed with various contributions corresponding to (i) Fe2+ with large quadrupole splitting (two doublets) probably in octahedral sites with same Mössbauer parameters encountered in most ferrous phyllosilicates (ii) Fe3+ with quadrupole splitting between 0.25 and 1 mm/s corresponding to typical Fe3+ of phyllosilicates like chlorites, smectites and illites, specially the high doublet with δ = 0.4 mm/s and ∆ = 0.3 mm/s characteristic of octahedral Fe3+ in smectites.
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ALM1
0.987
0.989
0.991
0.993
0.995
0.997
0.999
1.001
-4 -3 -2 -1 0 1 2 3 4
Velocity (mm/s)
Abs
orba
nce
Fe3+
Fe3+
Fe2+
Fe2+
Experimental
Fitted
Fig. 6. Experimental and fitted Mössbauer spectra of ALM-1 at 80 °K.
Fe3+
Fe3+
ALM2A
0.9845
0.9865
0.9885
0.9905
0.9925
0.9945
0.9965
0.9985
1.0005
-4 -3 -2 -1 0 1 2 3 4
Velocity (mm/s)
Abs
orba
nce
Experimental
Fitted
Fe3+
Fe3+
Fe2+
Fe2+
Fig. 7. Experimental and fitted Mössbauer spectra of ALM-2A at 80 °K.
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ALM2B
0.963
0.968
0.973
0.978
0.983
0.988
0.993
0.998
1.003
-4 -3 -2 -1 0 1 2 3 4
Velocity (mm/s)
Abs
orba
nce
Experimental
FittedFe3+
Fe3+
Fe2+
Fe2+
Fig. 8. Experimental and fitted Mössbauer spectra of ALM-2B at 80 °K.
ALM3
0.957
0.962
0.967
0.972
0.977
0.982
0.987
0.992
0.997
1.002
-4 -3 -2 -1 0 1 2 3 4
Velocity (mm/s)
Abs
orba
nce
Fe3+
Fe3+
Fe2+
Fe2+
Experimental
Fitted
Fe3+
Fe3+
Fig. 9. Experimental and fitted Mössbauer spectra of ALM-3 at 80 °K.
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4 DISCUSSION The main chemical composition of the samples reflects somehow their mineral composition. The samples 6A and 6B are richer in dolomite [CaMg(CO3)2] than samples 2A and 2B and that is also the content of MgO for these samples. The higher content of K2O in the samples 6A and 6B may be due also to the relatively higher content of illite in these samples. No iron oxide minerals were identified by XRD in any of the samples. Typical reddish hematite pigment was observed in light-coloured mineral grains in sample 6A, less in sample 6B. The highest content in iron of the samples ALM-4, ALM-5, and ALM-6 (A and B) in Table 3 is in accordance with the reddish colour of these samples. It is possible that iron is released from the smectite lattice and iron oxide formed. The release of ferric iron is not well understood, but the transformation of smectite to kaolinite and/or illite during weathering is a possible reason (Srodon 1999). In our case it is difficult to explain why this process has happened in samples at such small interval of occurrence. In the field no physical contact (fissure, fault) was observed between reddish and greyish samples. Another line of reasoning is the influence of the pore structure of the original bentonite in the outcrop: the more connected internal porosity, the more possibility of drainage and weathering. The higher calcite content in sample ALM-2 (A and B) is not reflected in a much higher content of Ca in the chemical composition, but the high values of Ca in samples ALM-3 and ALM-1 is most possibly due to the content of calcite. The occurrence of traces of other carbonates may as well be possible as reflected in the high content of C, MnO (rhodocrosite MnCO3), Sr (strontianite SrCO3), and Pb (cerussite PbCO3) in these samples. A small amount of gypsum (CaSO4) was found in sample ALM-6. Barite (BaSO4) may be found as well in the same sample and in samples ALM-5 and ALM-4 as seen in the higher content of Ba with respect to the greyish samples. 4.1 Comparison of the samples in this study to other bentonite samples
from Serrata de Nijar Quartz, plagioclase and phyllosilicates (smectite, illite, kaolinite) have been reported in bentonites from Serrata de Nijar (Reyes et al. 1987). Mica, calcite and K-feldspar occur as traces. The total content of phyllosilicates is 88 % in a range of 72-96, whereas the content in the samples in this study stays below 50 %. Dolomite has not been reported for bentonites in Serrata de Nijar and quartz and plagioclase content stays well below 2 %. The chemical composition of the bentonites in Serrata de Nijar is listed in Table 6. The SiO2 and Al2O3 content in the samples of this study (Table 3) are lower than for the samples in Table 6. The iron and sodium oxides content is within the range of the Fe2O3 and Na2O content of the bentonites in Serrata de Nijar. Magnesium and calcium oxides
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are much higher in the samples of this study due to the high contents of calcite and dolomite. The trace element content of the samples in this study is within the range for the bentonite samples in Table 6. Table 6. Chemical composition (after Reyes et al. 1987) and trace element content (after Linares et al. 1993) from bentonites from Serrata de Nijar and Cabo de Gata respectively. Chemical composition (wt. %) Trace element (ppm)
Mean Range Mean Range SiO2 62.14 52-68 V 10 0-160 Al2O3 18.95 15-23 Cr 8 0-40 Fe2O3 4.47 1-13 Ni 15 0-100 TiO2 0.31 0-0.5 Cu 10 2-35 CaO 1.80 1-5 Rb 30 5-100 MgO 3.27 1-6 Sr 150 10-400 Na2O 2.35 0-4 Zr 160 30-300 K2O 0.78 0-2 Pb 25 2-100 CO2 0.18 0-3 Ba 110 3-600
4.2 Estimation of the site Collado del Aire as a natural analogue on the influence of
iron on the properties of bentonite A systematic graduation in the iron content in bentonite from any fracture or discontinuity was the main requirement of a site to be chosen as a suitable analogue to bentonite-iron interactions. Main fractures or discontinuities were not found in the outcrop in Collado del Aire, but as expected by the colouring of the outcrop, the iron content in the reddish samples is higher than in greyish ones. There is no gradual increase, but a sudden change in the content of iron from sample ALM-3 to sample ALM-4 in a distance of about 15 cm. The mean smectite (swelling clay) content is greater in the reddish samples than in the greyish ones, a fact to be taken into account if the swelling properties of the samples were measured. The swellability of smectite depends on the number of layers participating in free swelling and not on the separation distance between the individual clay layers (Stucki et al. 2002). If the smectite amount could be controlled before swelling test and there were not other differences in the smectite of the samples than the interlayer cation(s), then a sound comparison between the swellability of the samples could be done. The main argument against the use of the outcrop as a natural analogue is the dissimilarity between the mineralogical content of the samples in the study and the mineralogical content of any of the bentonites (e.g., MX-80, Kunigel, etc.) thought to be used in nuclear waste disposal.
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5 CONCLUSIONS In the samples analysed, the content of iron was greater (almost two-fold) in the reddish samples than in the greyish ones. The origin of the hematite pigment is unclear at this stage. The migration of structural Fe from smectite is one possibility. This could also create defects in the clay structure. Arguments favouring the use of the outcrop as a natural analogue to study iron-bentonite interactions and the influence of iron in the properties of bentonite are the clear difference in and sudden change in the Fe content and that this change is due most probably to low-temperature processes. The main arguments against the use of the outcrop as a natural analogue are the dissimilarity between the mineral content of these samples and the bentonite to be used in nuclear waste disposal. Especially the smectite content, which is one main factor affecting the basic swelling property of bentonite, is so low that the use of the samples for further measurements (e.g. swelling pressure) would be associated with large uncertainties. The estimation of the processes that have influenced the analysed mineralogy and chemistry of the samples in the outcrop would need of further examination of the surroundings of the outcrop. 5.1 Further studies with the samples used in this study Two groups of bentonite samples could be formed with the natural analogue samples used in this study. One group could be could be the mix of the reddish samples (ALM-6, ALM-5, ALM-4) and the other one the mix of the greyish samples (ALM-3, ALM-1, ALM-2). Then a rich smectite fraction could be separated from each group. The so selected smectite fraction could be further characterised for the mode of iron occurrence in it and the influence of iron in the swelling properties of the selected smectite fractions.
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5 REFERENCES Bhide, V.G., 1973. Mössbauer Effect and its Applications, Tata MCGraw-Hill. Greenwood, N.N. and Gibb T.C., 1971. Mössbauer Spectroscopy, Chapman & Hall. Linares, J., Huertas, F., Reyes, E., Caballero, E., Barahona, E., Guardiola, J.L., Yañez, J., Romero, E., Delgado, A., Rodríguez, J and Martín-Vivaldi, M.T., 1993. Investigación de bentonitas como materiales de sellado para almacenamiento de residuos radiactivos de alta actividad. Enresa Publ. Tec. 01/93, 324 p. Rancourt, D.G., Dang, M.Z., and Lalonde A.E., 1992. Mössbauer spectroscopy of tetrahedral Fe3+ in trioctahedral micas. American Mineralogist 77, 34-43. Rancourt, D.G., McDonald, A.M., Lalonde, A.E., and Ping, J.-Y., 1993. Mössbauer absorber thickness for accurate site populations in iron bearing minerals. American Mineralogist 78, 1-7. Rancourt, D.G., 1994a. Mössbauer spectroscopy of minerals I. Inadequacy of Lorentzian-line doublets in fitting spectra arising from quadrupole splitting distributions. Physics and Chemistry of Minerals 21, 244-249. Rancourt, D.G., 1994b. Mössbauer spectroscopy of minerals II. Problem of resolving cis and trans octahedral Fe(II) sites. Physics and Chemistry of Minerals 21, 250-257. Rancourt, D.G., Ping, J.-Y., and Berman R.G., 1994. Mössbauer spectroscopy of minerals III. Octahedral-site Fe+2 quadrupole splitting distributions in the phlogopite-annite series. Physics and Chemistry of Minerals 21, 258-267. Reyes, E., Caballero, E., Huertas, F. and Linares, J., 1987. Bentonite deposits from cabo de gata region, Almería, Spain. Field book guide. Euroclay-87, Excursion A: 9-32. Srodon, J., 1999. Nature of mixed-layer clays and mechanisms of their formation and alteration. Annu. Rev. Earth Planet. Sci., 27, 19–53. Stucki, J.W., Lee, K., Zhang, L. and Larson, R.A., 2002. Effects of iron oxidation state on the surface and structural properties of smectites. Pure Appl. Chem., 74, 2145-2158.
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APPENDIX A A/1 – FTIR spectra for samples 2A and 2B A/2 – FTIR spectra for samples 6A and 6B
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Appendix A/1
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Appendix A/2
