Conadian MinerologistVol. 25, pp, -347:352 (1987)

TRANSFORMATIONS OF AI-INTERLAYERED MONTMORILLONITE UPON AGING

KAZUE TAZAKIDepartnent of Geologlt, University of Western Ontorio, London, Ontario N6A 587

ABSTRACT

Al-interlayered montmorillonite, gibbsite and boehmitehave been synthesized at room temperature and l00oC. TheAl-interlaygred montmorillonite, which shows a basal spac-ing of 13 A after heating at 6@oC, is the most stable atroom temperature for 2 weeks. Over a longer period, gibbs-

- ite at room temperature or'boehmite at l@oC grows fromthe Al-interlayered montmorillonite, The hexagonal platesof gibbsite and the latlr of boehmite are usually associatedwith, andgrow on, films of the montmorillonite. Gibbsiteand boehmite develop well in the X and Y directions, buthave a limited thickness. At room temperature, tleSiO2,/Al2O3 ratio of the film ranges from 0.9 to 1.25,which is close to that of dioctahedral chlorite and differ-ent from that of the film at 1@"C (range from 1.8 to 2.7)..The gibbsite and boehmite have a SiO2/Al2O3 ratio of 0.19and 0.56, respectively, which suggests that they grew onthe surface of the Al-interlayered montmorillonite films.The pH, CEC, and surface area changed witl agkrg, whichsuggests that Al-interlayered montmorillonite is zufficiently 'unstable to peel off the Al-interlayer after longer periodsat room temperature and l@oc.

Key,:words: Al-intedayered montnaorillonite, gibbsite, boeh-mite, transmission electron microscopy, energy-dispersion analysis.

SoMMAIRE

On a synthdtis6 montmorillonite interstratifiee avecAI(OH):; gibbsite, et boehmite.d-tempdrature ambiante ete 100'C, La montmorillonite ainsi algmingusg, qui mon-tre un espacement interfoliaire de 13 A apr0s chauffage A600oC, wt la plus stable i temp6rature de la pibce pour deuxsemaines, A plus long terme, c'est la gibbsite i temp€ra-ture ambiante ou boehmite a lm'C qui crolt aux d€pensde cette montmorillonite. La gibbsite a une morphologiehexggonale, et la boehmite se prdsente en lattes. Les deuxmontrent une association avec les films de montmorillo-nite alumineuse, et croissent sur la surface de ces filns.Gibbsiteet boehmite sont bien-d€velopp€es dans les direc-tions Xet 7, mais sont d'6paisseur restreinte. Les films dtempfrature ambiante ont un rapport SiO2/Al2O3 entre 0.9et 1.25, qui est proche. de la valeur attendue d'une chloritedioctaddrique, et qui diffdre de celle pour le m€me mat6-

TABTE I. CHARACTERIZATION OF 'IHE PRODUCTS }IIT}I AGING

X-MY DIFFRAfiION TIANSMISSION ELECTRON MICROSCOPY

daysNON hEAtEd 4.85fi 6.2 ! MORPHOLOGY LAIERAL SIZE THICKNESS SUrf

treat glyc 600oc albbs boehr TTfm --Tex-TeI6

frex -ETfr

T1lm--5ex--la Ef' pt cEc area

25.C 1

I

t 4

28

44

99

100.c I

16 .37 A 16 .67 A t 3 .2 A' f6.99

17.67 9.7-13

1 5.78 19.21 1 0.28 few

13.81 19.21 10.16 comn

13.81 19.21 9.83 abund

l2. l l 20.08 9.83

13.00 20.08 t0.00

abund(0. 9- l .25) @

abund

?7-60 R

42 A 38-eo fi

6.5 0 286.33

6.0 20.44 193.22

5.9 22.30 ' t92.55

5.2 26.43 294.82

4.6 33.98 384.44

4.3 52.07 506.72

4.1 56.49 515.51

6.7 21.31 526.35

3.2 51.37 473.71

3.0 48,50 453.71

40 A 3.4 87.66 szt.z4

40 A 3.3 63.58 573.59

4l A 3.0 70.02 639,23

3 .5 71 .33 584 ,11

17.67 17.67 9.6-13 - abund tr rare

19.21 22,10 9.&l l t r abund t r rare 0.06 !n

com'n fe{ rar€ 0.06-0.1

comn rare 0.1-0.12(0. 1 e)conm rare

10.40 A 17.67 A 14 I - conn rr tr(vv)

11.95 18.80 9,41 t r com tr t r

11.05 18.41 9,21 comn conn tr conm

11.05 19.21 9.61 com comn tr comn 0.05 urn 0.1-0.16

conm(r .08)comn

I

l 4

28

44

83

oo

11.33 18.80 9.83 cornn ffl rare comr

comn few rare abund(1 .8 -2 .7 ) ( 0 .56 )

abund felf rare abund

0.08-0.2

0.2 urn 3? A

Abbrevlatlons: nontreat: nontreatment, glyc glycerol, gibbs 9lbbslte, boehn boehqite, hex hexagonal, surf surface, tr!!ace' cotnn cofi lon, abund abundant. Uni.ts: CEC ln meq/100 g, surface ar€a ln m2l9. 0 Si02/Alr03 deter:nined by energy-dlspersion analysls. (Resu]ts are quoted in parenthesei).

347

348 THE CANADIAN MINERALOGIST

riau chauffd e 100'C (entre 1.8 el 2.7). La gibbsite et laboehmite montrent un rapport SiO2/Al2O3 de 0.19 et 0.56,respectivement, ce qui fait penser que ces phases croissentsur la surface des films de montmorillonite i interstratifi-cations d'aluminium. Les changements pendant la matu-ration (pH, capacitd d'6change des cations, aire de surface)seraient une indication que telle montmorillonite est suffi-sanrment instable pour que la couche riche en hydroxided'aluminium soit rejetde avec le temps e tempdratureambiante et a lm"C.

(Traduit par la Rddaction)

Mots-cl4s: montmorillonite, interstratifications d'alumi-nium, gibbsite, microscopie dlectronique par transmis-sion, analyse par dispersion d'6nergie.

INTRODUCTION

The fact that aluminum hydroxide can enter in theinterlayer of montmorillonite to form Al-interlayeredmontmorillonite is well known. Investigators haveprepared aluminum-interlayered complexes by sever-

Frc. l. Electron micrographs of aged products at roomtemperature. Arrow shows hexagonal gibbsite. A. Afteragtng for 28 days; B. after aging for z[4 days.

al methods and have studied their properties exten-sively, e.9., surface area, cation-exchange capacityand activation energy ofchloritization (Shen & Rich1962, Barnhisel & Rich 1966, Brydon & Kodama1966, Sawhney 1968, Barnhisel 1969, Lahav et al.1978, Brindley & Kao 1980). But there are very fewchemical data on micromorphology of such mont-morillonite and associated products. The object ofthis paper is to illustrate, by transmission electronmicrographs (TEM) with energy-dispersion analyses(EDE, the stages of formation of the Al-interlayeredmontmorillonite, gibbsite and boehmite, which havedeveloped on the surface of films.

MATBRIALS AND METHODS

Montmorillonite (Wyoming), obtained from Fish-er Scientifis Company of Canada, was saturated withNa, suspended in water, and stored as 290 stock sus-pensions. Solutions of 0.54 M NaOH (107.33 mL)and 0.20 M AlCl3 (107.33 mL) were added slowlv(drop-wise) to tle clay suspension' and stirred cons-tantly and vigorously. The suspensions, with anOH/AI ratio of 2.7, were prepared in this manner,and were kept in teflon containers at room temper'ature (about 25'C) and at l@oC. The suspensionswere occasionally shaken aud part of the samplewithdrawn after intervals of l, 8, 14,28, M,84 and99 days for analysis.

X-ray powder dffiaction (XRD)

The clay suspension (about 2 mL) was centrifugedto collect the mineral colloids (about 0.5 g) for 1G-20minutes after washing with water to remove free salt.The samples were dried in air for one day. Humidi-ty around the specimens was not controlled duringanatysis. XRD analysis was carried out with a Rigakuinstrument under ordinary operating conditions.Heat treatment at 6([oC and treatment with ethy-lene glycerol are essential to establish the presenceof Al-interlayered montmorillonite.

Differential thermal analysis @fA)

DTA was carried out with a Rigaku instrumentunder ordinary operating conditions. The presenceof gibbsite and boehmite was confirmed by charac-teristic endothermic peaks; XRD cannot be used toascertain the presence of a small amount of alumi-num oxides.

Transmission electron microscopy (TEM)

After washing with water to remove free salt, themineral colloids became good suspensions; artificialmodes of aggregation were avoided in preparing the

TRANSFORMATIONS OF AI-INTERLAYERED MONTMORILLONITE

TEM samples. The clay suspensions spread well overthe TEM grids.

Electron-optical studies.were done with a PhilipsEM 400 transmission electron microscope equippedwith an EDX glW/@ analyzer, operating at 100 kVacceleration voltage, spot size2 pm, condenser aper-ture 100 pm, objective aperture 30 pm for images,and at 120 kV acceleration voltage, spot size 2000A, condenser aperture 50 pm, selected aperture 150pm, objective aperture 10 pm for electron-microdiffraction pattems. Polystyrene latex particlsof 0.109 pm diameter were used to standardize themeasurement of thickness by tungsten shadow.

The EDX 9100/60 analyzer contains an anticon-tamination trap cooled with liquid nitrogen and aberyllium specimen-holder to reduce backgroundnoise. The following conditions were used through-out the study: condenser aperture l@ pm; no objec-tive aperture, no diffraction aperture, tilting angleof the specimen holder toward the detectlr 2loaccelerating volrage fOO fv, tpot rir";d,i [, ;r:sion current 13 A. Under these conditions, AlKcr X-ray counts (1.41 to 1.51 kU and SiKa X-ray counts(l .66 to 1.76 kV) were measured in a 50-second pointanalysis on each morphologically distinct particle.Background subtraction was made in the semiquan-titative analysis mode in the EDX system by usingthe following 4 points: l.l l, 1,46, 1.72 and 2.36 kY .The Si/Al intensity ratio was determined by com-paring the signal with that ofpure and standard kao-lin KGa-l (Washington Co., Georgia).

Cation-exchange capocity (CEC) andsurface-area analysis

The CEC of the products was determined by thesilver thiourea method of Chhabra et al. (197 5), Thesurface area of these products was determined by theethylene glycol monothyl ether method of Carter etol. (1965).

Fta. 2. Electroo micrographs of aged products at 100'C.A. After aging for 28 days; B. after aging for 44 days.

to lg.2l A wm ethylene glycerol treatment, whichindicates typical montmorillonite.

After only one day at 100oC, the Al-interlayeredmontmorillonite was formed; but at longer periodq,instead of gibbsite, boehmite of basal spacing 6.2 Aand montmorillonite were observed (Table l).

The products aged for 99 days at room tempera-ture and lffioC were fufiher confirmed by DTA. Theendothermic peaks at 270 and{X)'C correspond togibbsite and boehmite, respectively. Other endother-mic peaks at about 100, 660, 940 and 960oC andexothermic peaks at 970 and 990oC correspond tomontmorillonite.

Obsemotions by electron microscopy

The montmorillonite starting material shows onlyfilm morphology, whereas the product held at roomtemperature and lfr)oC shows the following threekinds of morphology: film, hexagonal plate and lath.After aging for 2 weeks at room temperature, filmswere observed, but they possess a different value ofSiO2,/Al2O3, which ranges from 0.9 to 1.25. After

RESULTS AND DISCUSSIoN

XRD

X-ray data for the products collected at room tem-perature and aged I to 99 days are shown in Tablel. After the montmorillonite was aged for 2 ryeeks,a basal spacing between 16.37 and 17.67 A wasobserved. This spacing changes slightly with ethyl-ene glyceroltreatment; it changes to 13 A after heat-ing at 600oC and may then be called Al-interlayeredmontmorillonite. Furthermore, montmorillonite inLhe solution aged more than 28 days grves the 4.85A Uasat spacing of gibbsite. Although X-ray diffrac-tion qf gibbsite aged for 99 days showp the strong4.85 A peak, the basal spacing (13.31 A) expanded

350 THE CANADIAN MINERALOGIST

Ftc. 3. Electron mioograph of tungsten-shadowed parti-cles after aging 83 days at 100"C, with calibration specimen of Latex (diameter 1090 A).

aging 28 to 44 days at room temperature, fiIms werestill observed (Fig. l); there are no fikns that con-sist of hydrous aluminum oxide only. Hexagonal par-ticles (Fig. 1B) that consist of hydrous aluminumoxide are usually associated with films at room tem-perature. At l00oc, films were commonly observedafter aging for 28 days (Fig. 2A), whereas films arescarce after aging for M days, but lath-shaped par-ticles are commonly formed on the films (Fig. 2B).The number of hexagonal particles at room temper-ature and lath-shaped particles at l00oC increaseswith aging (Table l).

The crystal size and thickness are measured by thetungsten-shadow method with polystyrene latex par-ticles (Fig. 3). Relationships between morphology,crystal size, crystal thickness, quantity of crystals,and amount of aglng are summarized in Table l. Thelateral size of hexagonal plates and laths increaseswith aging, and these become relatively homogene-ous under the same conditions. The thickness of thefilms and hexagonal particles is variable, whereas tlethickness ofthe laths has not changed with aging at1000c.

These values on crystal size and thickness suggestthat hexagonal particles of gibbsite developed wellin the X and Y directions, but were limited in theZ direction, and therefore appear as hexagonalplates. Lath-shaped particles of boehmite also areof limited thickness. Note that the hexagonal andlath-shaped particles always form on the surface ofthe films, which suggests that aluminum hydroxideonce situated between montmorillonite layers laterdissociated and precipitated as AI(OH)3 orAIO(OH) on the surface of montmorillonite layers.The EDX data support these observations.

The electron-microdiffraction patterns for eachhexagonal and lath-shaped single crystal (Fig. 4) con-form with these observations (Figs. l, 2, 3). Gibb-site in hexagonal crystals was disqinguished by astrong diffraction-maximum at 4.4 A, along With anisolated major diffraction-maximum at 2.2 A (Fig,4A). Boehmite of the lath habit (orthorhombic) wasdetected by the appearance ofthe set ofthree majordiffraction-maxima at 1.83 (c), l.a2 @) and l.lj A(b). The vertical direction of the lath-shape{ crys-tals corresponds to the diffraction spot at 1.4 A (Fig.48).

Point analysis by EDX

By using a TEM equipped with an energy-dispersion electron-microprobe analyzer (EDX)'genetic relationships between morphology and chem-ical composition of the samples collected at roomtemperature, aged 1 and 83 days, and at lfi)oC, aged83 days, were investigated Clable l). These obser-vations reveal that the chemical compositions of thereaction products, which have three kinds of mor-phology, are all different. The EDX data contrib-ute to the identification of minerals of the same orsimilar morphology, such as films under differentconditions, and also establish the chemical purity ofthe gibbsite and boehmite.

The chemical composition of the films at roomtemperature shows that the SiO2/Al2O3 value rangesfrom 0.9 to 1.25; this finding is in agreement withthe values for dioctahedral chlorite, which rangesfrom 0.6 to 1.2 (Brydon et al. 1961, Weaver & Pol-lard 1973, Sudo & Shimoda 1978). The films aged83 days at 100'C gave a different SiO2/Al2O3 value'in the range from 1.8 to 2.7 , closer to the value ofstarting montmorillonite, in the range from 2.1 to2.8, than that of dioctahedral chlorite.

The hexagonal and lath-shaped particles, rich inAl, range from 0.05 to 0,25 and from 0.45 to 0.70in SiO2/Al2O, value, respectively. Their averagesare 0.19 and 0.56, respectively (Iable l). A smallamount of SiO2 in the hexagonal or lath-shapedparticles reflects the chemical composition of thefilms underneath the particles (see Figs. lB, 28) thathave not individually formed.

The EDX data also support the X-ray results,which identify the products as Al-interlayered mont-morillonite, montmorillonite, gibbsite and boehmite.This effect might suggest that the suspensions witha OH/AI value of 2.7 are immediately contributedto the formation of the dioctahedral chlorite or Al-interlayered montmorillonite, which is unstableenough to peel off after longer period because thealuminum hydroxide minerals are formed on the sur-face. The CEC data (Table l) are in agreement withthe "pillar" concept. The CEC of the Na-montmorillonite, which is about 80 meq/100 g,decreases to 0-20 meq/l@ g as a result of the for-mation of the Al-interlayered montmorillonite. Thesurface area for Na-montmorillonite, which is abour650 fr/g, decreases to 200-300 m2,/g upon deposi-tion of Al-interlayers. The surface area of the agedAl-interlayered montmorillonite with gibbsite orboehmite is over 350 r*/g during the pH decreasefrom 6-7 to 3-4 with aging. Aluminum hydroxidecannot fill up the vacant interlayer of montmoril-lonite under these conditions, but aluminum hydrox-ide partly enters the interlayer to make the pillar com-plex or cross-linked montmorillonite (Lahav et al.1978, Pinnavaia 1983) as an Al-chlorite-like mineral.

AcrNowLroceMENTs

I thank Dr. M. Tsuji, Dr. S. Saka and Dr. D.A.Goving of the Pulp and Paper Research Institute ofCanada, McGill University, for permitting me to usethe Philips EM 4(D transmission electron microscopeequipped with an EDX 9100/60 analyzer and fortheir helpful discussions. I thank Dr. W.S. Fyfe, ofthe University of Western Ontario, Drs. A.J. Sethiand R.N. Yong, Geotectrnical Research Centre,McGill University, for helpful commentsi this projectwas partly supported by their grauts. I acknowledgethe helpful and informative reviews of Dr. R. Mar-tin, Dr. H. Kodama, and an anonymous referee.

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351

FIc. 4. Electron-microdiffraction patterns. A. Hexagonalcrystal of gibbsite after aging 83 days at room temper-ature; B. lath-shaped crystal of boehmite after aging28 days at l@oC.

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352 THE CANADIAN MINERALOGIST

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Received February 19, 1986, revised manuscriptaccepted July jl, 1986.