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Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 191–196

SEM evidence of structural re-arrangement from gellingto aggregation in Birdwood kaolinite

Marek Zbik ∗Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Warendi Road, Mawson Lakes, SA 5095, Australia

Received 10 November 2005; received in revised form 29 March 2006; accepted 3 April 2006Available online 15 April 2006

bstract

Birdwood kaolinite from South Australia displays extremely different colloidal stability and particle interactions in comparison with many otheraolinites. Birdwood kaolinite gels at very low solid loadings in aqueous suspension and creams easily on top of the dispersion upon addition ofcationic surfactant (CTAB) and aeration by shacking. The particles have both high surface area and cation exchange capacity as well as X-rayiffraction patterns suggestive of the presence of low level of interstratifications by smectite-like layers in this kaolinite structure. To find theossible cause of this behaviour, the microstructure of Birdwood kaolinite was studied using a cryo-SEM system. SEM observations of aqueoususpension in this kaolinite show voluminous three-dimensional microstructure. This gel structure is composed of individual kaolonite plateletsonnected at the edges forming an expanded cellular network where the particle basal faces seem to repeal each other. This structure collapses afterntroduction of CTAB into aqueous suspension. CTAB-treated kaolonite suspension shows compact aggregates, and after aeration by shacking

roduce microbubbles of air with kaolonite platelets adhered to the bubble walls. This structural re-arrangement after addition of CTAB cane explained by significant charge reduction on negatively charged basal planes of Birdwood kaolonite which allow particles to form compactggregates. 2006 Elsevier B.V. All rights reserved.

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eywords: Kaolinite; Birdwood; Colloids; Gelation; Clay aggregation

. Introduction

Effective flocculation and dewatering of clay-based slurryparticularly kaolinite and smectite) is becoming increasinglyrgent in view of the growing demand for high-density disposalf mining slurries, tailings and other mineral wastes. Becauselay particles are extremely small (often submicron in diameter)atural separation by sedimentation under gravity is ineffective.o increase rate of sedimentation, particles which are submicron

n size should be aggregated.To aggregate the clay suspension is not a trivial subject and

ften uncommon as kaolinite gelling behaviour prevents aggre-ation. The Birdwood kaolinite from South Australia has beenescribed in [1–3] as easy to gel in natural pH [1], and it was

emonstrated that particles in such gels are in constant contactith each other by creating three-dimensional spanned spongy

tructures. According to recent studies [3] submicron kaolinite

∗ Tel.: +61 8 83023688; fax: +61 8 83023683.E-mail address: Marek.Zbik@unisa.edu.au.

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latelets connecting each other in edge to edge (EE) orientationuild highly porous cellular structure. This was obtained fromlectron microscopy study of the structure of the gel formed byirdwood kaolinite obtained using a cryoscopic technique [3].he existence of similar network in kaolinites from coal tailingsas been recognised in [4,5] as the major cause of difficulties inocculating kaolinite suspensions.

The aim of the present work is to investigate the three-imensional microstructure transformation and particle aggre-ation after adding positively charged surfactant CTAB.

. Materials and methods

The material used for this study was commercially availablerom Commercial Minerals Limited CML K15GM kaoliniterom Birdwood quarry in South Australia.

For SEM microstructural studies dry kaolinite particles wereixed with aqueous electrolyte at high solid content by shaking

he suspension in a SPEX mill for 5 min. The slurries were theniluted to the required solids concentration (4 wt.%) and stirred

192 M. Zbik / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 191–196

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ig. 1. The bench test shows Birdwood kaolinite aqueous 0.01 M NaCl suspeignificantly alters in studied kaolinite behaviour (room temperature, ∼8 pH).

or 2 h using a magnetic stirrer. The aqueous electrolyte used forhe suspension was 0.01 M NaCl in deionised water. Suspensionas investigated in natural pH 8. The cryo-transfer method of

ample preparation and cryo-SEM (Philips XL30) used here wasescribed in [3]. X-ray diffraction was performed in the HUBERiffractometer with Guinier Camera 670. The radiation appliedas Co K�I from a long fine focus Co tube operating at −35 kV

nd 34 mA.To demonstrate difference in kaolinite behaviour, kaolinite

as dispersed in a 0.01 M NaCl aqueous solution with additionf negatively and positively charged surfactants, at a kaolinitery mass content of 4 wt.%. Negatively charged sodium dodecylulphate (SDS) and positively charged cetyltrimethylammoniumromide (CTAB) surfactants were added to the suspensions atwo dosages of 0.8 mM (∼10 mg/g of dry kaolinite) and 8 mM∼100 mg/g of dry kaolinite). The kaolinite suspensions werehaken and left to settle for 20 h before a photo (Fig. 1) wasaken.

Specific surface was measured based on BET, N2 sorptionnd ethylene glycol desorption methods [6]; cation exchangeapacity (CEC) was measured using an Alpkem segmented flowolorimeter model Flow Solution IV and Winflow software.mmonia was determined using a narrow band pass filter at40 nm and chloride using a narrow band pass filter at 480 nm.he electrokinetic potential (zeta potential) was measured usingcoustoSizer for 6 wt.% kaolinite suspension with background

lectrolyte 0.01 M NaCl and with 10 mg/g of CTAB surfactant.rain size distribution was measured using Malvern Instru-ents, Mastersizer.

. Results and discussion

The grain size investigation of studied bulk kaolinite samplehow that 10% of particles [D(0.1)] have diameters smaller than.52 �m, 50% of particles [D(0.5)] have diameters smaller than

.62 �m and 90% of particles [D(0.9)] have diameters smallerhan 20.6 �m. After CTAB addition significant aggregation wasoticed, especially in the lower particle diameters and show(0.1) = 1.46 �m, D(0.5) = 6.97 �m and D(0.9) = 16.93 �m in

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s (A) and with surfactant added (SDS, B and C; and CTAB, D and E) which

he bulk sample. The specific surface area found by BET,2 sorption (Commercial Minerals Limited) has been deter-ined as 19.7 m2/g, which suggests that very disperse particles

re present. Specific surface measured using ethylene glycolethod shows 25 m2/g (bulk sample) which is much higher

han expected for kaolinite [7]. Cation exchange capacity at.5 cmol/kg (bulk sample) and 20 cmol/kg (below 1 �m frac-ion) is also larger than determined for kaolinite. Both elevatedalues of CEC and the specific surface suggest some interstrat-fication of kaolinite by smectite type layers as found by Mand Eggleton [8]. Similar interstratified high CEC soil kaolin-tes which occur in Queensland were reported by Churchman etl. [9].

Differences in behaviour of the Birdwood kaolinite samplesn different solutions as shown in bench trial (Fig. 1) are appar-nt. This kaolinite, suspended in aqueous 0.01 M NaCl solutionFig. 1A) shows division by a mudline with a clear supernatantolution and sediment bed of about 45% by total volume. Thisoluminous sediment suggests a large extended network struc-ure as was described in [3]. The addition of a small amount10 mg/g) of a negatively charged surfactant (SDS) slightly com-resses the bed by about 10% (Fig. 1B). Such decrease in bedeight can be the result of positive charge neutralisation onaolinite edges, which slightly compacts the porous network.urther addition of SDS (100 mg/g) produces almost fully dis-ersed stable suspension with some larger particles settling outf suspension (Fig. 1C). The bed height of suspension is aboutvol.% and consists of coarse kaolinite crystals and stacks set-

ling out of suspension. The formation of a stable sol on theddition of a large amount of SDS suggests that the kaoliniteas developed a high (presumably negative) surface charge afterdsorption the SDS on particles surface.

Addition of the positively charged surfactant (CTAB) in lowoncentration shows a dramatic difference. Birdwood kaoliniten this solution develops a “cream” of about 16 vol.% at the top

f suspension as shown in (Fig. 1D). Such creaming may bexplained by adsorption of CTAB to negatively charged basalurfaces and subsequent hydrophobisation of mineral surfaces.his explanation needs assumption of presence of the permanent

Physicochem. Eng. Aspects 287 (2006) 191–196 193

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M. Zbik / Colloids and Surfaces A:

egative charge on the Birdwood kaolinite basal faces. Shak-ng during suspension preparation in a SPEX mill may deliverir bubbles to the hydrophobised mineral-suspension and causeineral grains to float and cream at the top of the vessel. Further

ddition of CTAB (high concentration) results in the formationf stable sols and some sediment (Fig. 1E). The formation ofstable sol on the addition of a larger amount of CTAB sug-

ests that kaolinite has developed a high (presumably positive)urface charge under the influence of CTAB. In high concen-ration of CTAB compact beds of sediment about 9 vol.% arebserved which is more than that observed in similar concentra-ions of SDS. This may happen because in addition to the largeraolinite particles, there may also be large aggregates increasingediment bed thickness.

The differences in the sedimentation behaviour between thetudied kaolinite samples are most probably caused by differ-nces in texture pattern when networking in a three-dimensionaltructure. Consequently the mutual space orientation of kaolinitearticles dispersed in water (NaCl solution) and in low concen-ration of CTAB has been studied using SEM.

The electrokinetic (zeta) potential measured for both,ntreated and CTAB-treated kaolinite samples are shown inig. 2. The untreated kaolinite sample displays negative valuesf Zeta potential with pH-dependent profile due to OH− groupsccupying platelet edges in high pH. However, pH dependence isow in comparison to other high crystalline kaolinites, and smalleta potential variances profile suggesting dominance of theasal planes. This may be an effect of the platelets’ high aspectatio where the area of the edges contribution to the total areas much smaller in comparison with better crystalline kaolinites10]. The CTAB-treated kaolinite sample display mostly posi-ive zeta potential values with IEPS 8.24 pH.

The electron-microscopic study of the colloidal fraction ofirdwood kaolinite reveals that the sample consists of pseudoexagonal euhedral crystals with diameters down to ∼50 nmFig. 3). The crystals are typically thin and flexible plates ofery high aspect ratio ∼12. AFM investigations of this frac-

ion of Birdwood kaolinite show that crystals are on average60 nm in diameter and about 13 nm thick [3]. A few grape-haped anatase aggregates and a few tubular halloysite parti-les were also observed. The amorphous and semi-amorphous

ig. 2. Zeta potential in function of pH of Birdwood untreated and CTAB-treatedaolinite suspensions 6 wt.%.

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olloidal fraction on 100,000 times magnification. Kaolinite from Birdwoodouth Australia exhibits thin, sometimes curved, pseudo hexagonal euhedralrystals and few grapelike shape anatase aggregates.

aterial reported in [1] can also been seen as bright dots in vit-ified ice in most high resolution crio-SEM micrographs. In theigh resolution micrographs these extremely dispersed particlesan be recognised as flakes of diameters up to 20 nm, and displayl peak in EDAX analyses. These extremely disperse; almost

morphous minerals are probable the hydrous aluminium oxidesesponsible for the low intensity broad reflection at around 4.9 An the X-ray diffraction pattern (Fig. 4). Similar compounds haveeen reported from Lindner et al. [11]. Large kaolinite plateletsnd kaolinite stacks (books) have also been observed in SEM.hese large particles are the coarsest fraction of the examinedample and may contribute up to 10 wt.% of dry mass of theirdwood kaolinite. From these SEM investigations three dif-

erent mineral phases have been observed: (a) well-crystallisedarge kaolin flakes and books (a few �m in lateral dimension);b) poorly crystallised, submicron in size kaolin flakes and tubes;nd (c) nanometers size flakes. All these constituents differ iniameters: (a) ∼1–10 �m; (b) 50–200 nm; and (c) up to 20 nm.ubmicron kaolinite flakes (50–200 nm) are the dominant con-tituent of the studied kaolinite sample and has a major influence

n the physical behaviour of this kaolin.

X-ray diffraction of the Birdwood kaolinite has been dis-ussed in [1] and shows a strong asymmetry on the low angle

Fig. 4. XRD pattern of powder sample recorded using Co K� radiation.

194 M. Zbik / Colloids and Surfaces A: Physicoch

Fig. 5. SEM micrographs of the Birdwood kaolinite in dilute aqueous suspensionshows: (A) spanned network of gelled particles in mostly edge to edge (EE)platelets and chains orientation, nano-size colloids are visible in dark vitrifiedwbe

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tibpformed. This creaming may be explained by adsorption of pos-itively charged CTAB attracted by negatively charged basalsurfaces, leaving hydrocarbon chains in the bulk of the solu-tion and outside the mineral interface. This makes the particle

ater and (B) larger magnification reveal that individual platelets are connectedy edges and build expanded cellular network where platelets seems to repealingach other by basal surfaces.

ide of the 0 0 1 peak. It is suggested that interstratified smectite-ike layers may be present in this kaolinite. XRD of Bird-ood kaolinite also displays poorly crystalline kaolinite with aroad X-ray peak indicating that an amorphous compound maye present. Present electrono-microscopical study confirm thisnding.

Birdwood kaolinite has been investigated before using cryo-ransfer techniques and cellular structure with mostly (EE) par-icle orientation as reported in [3]. Present electron microscopybservation of the clay suspension structure confirms that dis-rete particles link together forming a coagulated spanned net-ork (Fig. 5A) that extends throughout all the suspension via

lay platelets networking in (EE) orientation as shown in SEMicrograph (Fig. 5B). These highly porous single plate-like

article and chain associations form a spongy structure wherendividual platelets form apparent cells a few �m in diameter.uch gel structured suspensions as observed in Birdwood kaoli-ite by SEM are difficult to explain on the basis of DLVO ast was discussed in [2]. Hydrophobic like attraction between

ough, aerated edges may be the dominant mechanism in addi-ion to hydrogen bonding between OH− group and oxygen fromroken bonds on crystal edges. As shown in (Fig. 5) basallatelets look like repelling each other which in effect build

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em. Eng. Aspects 287 (2006) 191–196

n expanded spongy network of individual platelets contactingach other in edge to edge arrangement. Similar cellular struc-ure as observed in Birdwood kaolinite, was also reported froma-montmorillonite aqueous dilute suspensions [12]. This typef structure has not been reported from Georgia kaolinite (USA)here much less voluminous, mostly EF and FF oriented card-ouse aggregates have been observed [3]. The expanded cellulartructure observed in Birdwood kaolinite suspension may formhen mineral platelets are attracted by edges while simultane-usly repelling each other by faces. This may work assuming thatlatelets carry a negative permanent charge on their basal sur-aces. This negative charge, about −18 mV (from zeta potentialeasurements) in pH ∼8 (suspension studied), may be attributed

o either isomorphous replacement of Si4+ by Al3+ or position-ng of the electro-negative oxygen atoms above the surface [13].f platelets are joined by edges and repelled by basal surfaceshis will build voluminous, expanded and highly elastic cellulartructures such are observed in the studied Birdwood kaoliniteuspension. Weak positive charges on the edges which resultrom a low edge area contribution may not be sufficient enougho connect particles in orientation EF. The negatively chargedDS addition to suspension as observed in Fig. 1B may resultrom low edge surface contribution to the total surface area inhe studied kaolinite.

Addition of CTAB produces a dramatic change to particleutual arrangement observed in the Birdwood kaolinite sus-

ension. As shown in the bench trail (Fig. 1) the most apparenteature was flotation and creaming of the studied kaolinite, buthe most spectacular change cannot be seen by the naked eye.

This creamed suspension was investigated using the cryo-ransfer technique. Air bubbles of micro-meter size are commonn SEM images. The hydrophobised kaolinite platelets cover allubble surfaces as shown in Fig. 6. Air bubbles carry kaolonitelatelets towards the water surface where the creamed layer

ig. 6. SEM micrographs of the Birdwood kaolin incrusting the air bubble sur-ace after treatment in CTAB. Positively charged surfactant adsorbs to negativelyharged kaolinite basal surfaces and hydrophobise kaolinite platelets. The nano-ize colloids are visible in dark vitrified water.

M. Zbik / Colloids and Surfaces A: Physi

Fig. 7. SEM micrographs of aggregate in CTAB-treated Birdwood kaolin, Thiscompact aggregate with particle orientation edge to face (EF) and face to face(fC

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urfaces hydrophobic and cause attachment to air bubbles whichnitiates flotation.

Particles which are not attached to bubbles show remark-ble difference in the aggregate structure (Fig. 7). As shown inhis micrograph, in CTAB-treated kaolinite most particles buildompact aggregates with edge to face (EF) and face to face (FF)article arrangements, instead of spanned networks as observedn untreated Birdwood kaolinite suspensions. Such an arrange-

ent is much less voluminous and particles in the observed flocre much closer together. Such close packing of kaolinite crys-als can only be possible after neutralisation of negative chargeshich are present on untreated kaolinite basal surfaces. Sucheutralization of the negative charge as seen in Fig. 2, whereeta potential in pH 8 (in which suspensions were studied) haseen lowered to almost zero.

The presence of compact larger coagulated aggregates, a fewicrometers in diameter, shown in (Fig. 7) are common in allEM micrographs from the CTAB-treated Birdwood kaolin-

te. Such large and compact aggregates without support of thehree-dimensional network should freely settle. Degassed sus-ension which consists of such compact aggregates should settleuickly and should form compacted sediment beds in contrasto untreated kaolinite.

Within the context of the SEM observations this behaviouran be interpreted as follows. At low solids loading, individ-al particles and aggregates are fully dispersed and settle freelyccordingly to Stokes’ law. When solid loading increases iteaches a critical particle concentration (gel point) at which thelosest particles interact and start to form a three-dimensionaltructure. In the case of the studied untreated Birdwood kaoli-ite, in solid loading above 4 wt.%, individual particles inter-ct immediately after mixing cease. Clay particles attractingach other towards their edges and repelling their basal planes

hus building an expanded, extremely voluminous cellular net-ork composed of chain-like platelet assemblages. Such an

xtended network may fill the entire volume of a vessel. In suchcase the suspension is gelled; there is no free settling in this

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cochem. Eng. Aspects 287 (2006) 191–196 195

ystem and further compacting may proceed slowly by structurale-arrangement. Kotlyar et al. [4] studied similar fast gellingaolinite using 2H NMR method and found that the fraction con-aining particles <200 nm, gelation was instantaneous and liquidithin the flocs is immobilised. Accordingly to van Olphen [14]all particles like observed in present study flocculate more

apidly and this behaviour may result from low absolute valuesf the repulsive potential for such particles. In [14] also has beenound that flocculation behaviour of dense clay dispersion is saltoncentration and particle size dependent.

The CTAB-treated kaolinite demonstrates compact aggre-ates due to compensation of the negative charges on theaolonite basal surfaces. In effect this treated kaolonite is notndergoing gelation, and forms more compact aggregates byreferring EF and FF orientation and settling faster. As a resultf this aggregate formation the spanned three-dimensional par-icle network collapses to form faster settling aggregates. Thebove differences in gelling properties may be explained heren the basis of the differences in orientation between individualarticles.

Because the Birdwood kaolinite platelets are thin (<20 nm)nd its specific surface as well as CEC is unusually high, it maye similar to described highly negatively charged kaolinite inter-tratified by smectite-like layers [9]. This type of interstratifiedaolinite has a few relatively high-charged smectite-like layersn one or both sides of the stack. This negative charge on basallanes repels neighbouring platelets and in effect build a highlyxtended and voluminous structure. Similar behaviour in kaoli-ite from oil sand tailings has been described in [15] where theeported bulk surface area is far larger than any possible contri-ution from kaolinite itself, and a low degree of interstratifiedmectite in this kaolinite has been suggested.

. Conclusion

Kaolinite sample from Birdwood (South Australia) consistsf well-crystallised larger kaolinite crystals and stacks fewicrometers in diameter, poorly crystallised kaolinite platelets

ubmicron in diameter and nano-particles of amorphic con-tituent. The highly disperse poorly crystallised submicron inize kaolinite is the dominant phase in the studied sample andetermines its physical behaviour.

The gel is similar in structure to that reported for freeze drieda-montmorillonite. Adding a positively charged surfactantTAB reduces the repulsive force between the basal surfaces.he surfaces also become hydrophobic and with aeration, theenser gel formed can be creamed off the surface. By contrast,ddition of negatively charged surfactant (SDS) causes edges ofarticles to become more negatively charged while the basal sur-aces remain hydrophilic and gel formation is inhibited. Theseesults together with the high CEC and the very high aspect ratiof the finest particles present indicate that Birdwood kaolinites likely to be interstratified by kaolinite/montmorillonite-like

ayers. Presence of this type of interstratifications may generateermanent negative charge on the basal surfaces.

Presence of this negative charge on kaolinite basal surfacess responsible for building expanded highly porous cellular

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tructure where individual, submicron in size platelets are con-ected by edges and repel each other by basal surfaces. Such par-icle arrangements build highly voluminous, three-dimensional,panned particle network which causing gelation.

Addition of positively charged polymer neutralise the nega-ive charge on platelet surfaces that cause particle agglomeration.his agglomeration aids the phase separation and prevents theuild-up of the spanned network that causes suspension to geln low solid content. Hydrophobised kaolinite platelets, aeratedy shaking in a SPEX mill, were attached to the air microbublesnd formed a macroscopic creaming blanket on top of the vessel.

Technological processes developed to aggregate such highlyelling suspensions (smectite and interstratified clay minerals)ay benefit by taking the above findings into consideration.

cknowledgements

The author would like to acknowledge the staff at Adelaideicroscopy, the University of Adelaide, Dr. Oladipo Omotoso

rom University of Edmonton, and Dr. William W. Emerson andr. Victor Gostin for their assistance in manuscript preparation

nd for useful comments and discussion.

eferences

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em. Eng. Aspects 287 (2006) 191–196

[2] M. Zbik, R.G. Horn, Colloids Surf. A 222 (2003) 323–328.[3] M. Zbik, Clay Sci. 12 (Suppl. 2) (2006) 31–36.[4] L.S. Kotlyar, B.D. Sparks, Y. LePage, J.R. Clay, Minerals 33 (1998)

103–107.[5] M. Zbik, P. Self, R.St.C. Smart, Coagulation edge to edge contacts

between clay particles in dilute aqueous suspension; the major obsta-cle of fine clay waste dewatering, abstracts and poster presented onEUROCLAY 1999, Krakow Poland, 1999, p. 149.

[6] K.G. Tiller, L.H. Holmes, Aust. J. Soil Sci. 28 (1990) 1–26.[7] R.E. Grim, Clay Mineralogy, McGraw-Hill, 1968, p. 596.[8] C. Ma, R.A. Eggleton, Clays Clay Minerals 47 (2) (1999) 181–

191.[9] G.J. Churchman, P.G. Slade, P.G. Self, L.J. Janik, Aust. J. Soil Res. 32

(1994) 805–822.10] M. Zbik, R.St.C. Smart, Clays for our future, in: H. Kodama, A.R.

Mermut, J.K. Torrance (Eds.), Proceedings of the 11th InternationalClay Conference, Ottawa, 15–21 June 1997, Ottawa, Canada, 1999, pp.361–366.

11] Clays for our future, in: G.-G. Lindner, H. Nakazawa, S. Hayashi (Eds.),Proceedings of the 11th International Clay Conference, Ottawa, 15–21June 1997, Ottawa, Canada 1999, pp. 457–459.

12] R.St.C. Smart, M. Zbik, G.E. Morris, in: J.S. Laskowski (Ed.),43rd Annual Conference of Metallurgists of CIM, 2004, pp. 215–228.

13] A.R. Gerson, in: G. Cisneros, J.A. Cogordan, M. Castro, C. Wang(Eds.), Computation Chemistry and Chemical Engineering, World Sci-entific Press, pp. 227–235.

14] H. van Olphen, An Introduction to Clay Colloid Chemistry, GriegerPublishing Company, USA.

15] H. Kaminsky, T. Etsell, D.G. Ivey, O. Omotoso, Interstratified materialin oil sand tailings, in: Program with Abstracts of The 13th InternationalClay Conference, August 21–27, Tokyo, 2005, p. 64.