Montmorillonite - High Temperature Reactions and Classification

7/22/2019 Montmorillonite - High Temperature Reactions and Classification

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THE AMERICAN MINERALOGIST, VOL. 46, NOVEMBER_DECEMBER, 1967

MONTMORILLONITE: HIGH TEMPERATURE REACTIONSAND CLASSIFICATIONR. E. Gnru eNp GBoncBS urBrcKr,* University J lli,noi.s,Urbana, llinois.Assrnecr

About forty samples of the montmorillonite group of clay minerals were heated to anelevated temperature (1400' c.) and the phase transformations studied by continuousr-ray diffraction. chemical, cation exchange, difierential thermal, infra-red, and opticaldata were obtained also on the samoles.All of the analytical data indicaie that the dioctahedral montmorillonites do not forma single continuous isomorphic series. Two difierent aluminous types have been found,cheto- and wyoming-types, which differ primarily in the population of their octahedrallayers. AIso, it is suggested that some (probably a small number) of the silica-tetrahedraare inverted in the cheto-type montmorillonite. cation exchange capacity and otherproperties are also not the same for the two types.some bentonites are mixtures of discrete particles of the two types which can be sep-arated by particle size fractionation.The high-temperature phase transformations of montmorillonite show large varia-tions depending on the composition and structure of the original material. TLe trans-formations are discussed in detail.

fNrnooucrrowThe object of the investigation reported herein was to study the succes-sive structural changestaking place when members of ths montmoril-

cates.The major technique used was continuous high temper ature r-raydiffraction using a spectrometer. This involved mounting a furnace inthe position of the specimenholder in the r-ray unit with some mannerof controlling and recording the temperature of the furnace. Thismethod has advantages in comparison with a technique that involvesheating, then cooling, followed by *-ray analysis in that it eliminates

* currently Director of Geologibal Research, socidt6 Nationale de pdtroles d'Aquitaine,Pau, France.1329


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1330 R. E, GRIM AND G. KULBICKIanalyses carried to about 1400o C., infra-red absorption data, and op-tical determinations. Complete silicate analyses and cation exchangecapacitieswere obtained for the samples.

The montmorillonite clay minerals are very common, being found inmany soils, sediments, and hydrothermal alteration products' They aregenerally the dominant constituents of bentonites. The minerals of thisgroup have a unique, so-calledexpandable attice, which has a variablec-axisdimension depending on the thickness of layers of water moleculesbetweensilicate ayers. The structure suggested y Hofmann, Endell, andWilm (1933),Fig. 1, s made up of si l icate ayersconsistingof two sil icatetrahedral sheets tied together through a central sheet containingaluminum andfor magnesium, iron, and occasionally other elements noctahedral coordination. The silicate ayers are continuous in the o and bdirections and stackedone above another in the c direction with variablewater layers between them. As first emphasizedby Marshall (1935) andHendricks (1942), a wide variety of substitutions in octahedral andtetrahedral positions are possible within the structure, and they alwaysleave it with a net negative charge which is satisfied externally by cat-ions which are exchangeable.The foregoing structure is not accepted by all investigators. Thus,Deuei el ol. (1950) believe that they have evidence that some of thetetrahedra of the silica sheetsare inverted-an idea suggestedearlier byEdelman and Favajee (1940). Other ideas concerning the montmoril-lonite structure have been published (McConnell, 1950), but the con-cept originating with Hofmann et at . (1933) is generally accepted asdepicting the most probable framework of the mineral.The substitution of various cations for aluminum in octahedral co-ordination can be essentially complete, in which casespecificnames areapplied for example, nontronite (iron), saponite (magnesium)' Ross andHendricks (1945) have shown that there is considerable variation in

of the mineral, and McAtee (1958) concluded that the Wyoming bento-nites he studied contained a sodium montmorillonite fraction and acalcium-magnesium montmorillonite fraction, and that these fractionswere a consequence f differences n isomorphic substitution within themontmorillonite crystal lattice. One of the objectives of the present


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MONTMORILLONITE 1331study was to investigate the possible mixing of aluminous montmoril-lonites n bentonites.

The montmorillonite minerals have interesting plastic, colloidal, andother properties which frequently are quite different from one sampleof the mineral to another. Thesedifferencescannot in many casesnow beexplained either by difierences n the composition of the exchangeablecations or of the silicate layer, or by present concepts of the structure.Thus, some montmorillonites have catalytic properties towards certainorganic substances,whereasothers do not. It follows that there is much

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1332 R. E. GRIM AND G. KULBICKIto be learned concerning the structure and compositional variations inthis important group of minerals.

PnocBp,unBOver a period of many years, f-ray difiraction diagrams of powdersand oriented aggregatesand differential thermal analyseshave been ob-

tained for several hundred samplesof bbntonites and other clays con-taining montmorillonite from all over the world that are in the Universityof Illinois collections.Preliminary studies of the high temperature reac-tions by continuous r-ray difiraction techniques were made also onmany of these samples.Based on thesedata, about forty sampleswereselected or the present study which appeared to be substantially puremontmorillonite or montmorillonite plus small amounts of qttartz orcristoballite. These sampleswere selectedalso to represent he variationsin characteristics shown in the preliminary analyses.No claim is madethat al l types of montmorillonite are represented,but it is believed thatthere is good coverageof the aluminous variety.

Kulbicki and Grim (1957) have shown that the high temperaturephasesdevelopedon heating montmorillonite are nfluencedgreatly by thenature of the exchangecation composition. To study the relation of themontmorillonite itself to high temperature reactions it was necessary oprepare all samples with the same exchangeablecation composition. Itwas also deemed necessary o use material of about the same particlesize, and in some cases o purify the samples.Accordingly, the followingpreparation procedurewas followed for all samples:The clays were dispersed n deionizedwater without the use of a chem-ical additive. If dispersion was difficult, the initial water was extractedthrough a porcelain filter candle, and new water added until dispersionwas attained. The portion of the suspensioncontaining the less.Ihantwo micron fraction was separated by repeated decantations. Hydro-chloric acid in concentrations kept less than 0.1 normal was added tothe suspension ontaining the less han two micron particles. Within tento twenty minutes after the addition of the acid, filtration of the clayon Buchner funnels was started. The clay was washed with acid of thesame concentration until the total amount of acid to which the clay wassubjected equalled about five times that necessary or complete cationexchange.This was followed by washing with deionized water until theconcentration of salts in the wash water was about 1 part in 100,000'A concentrated slurry of the material as prepared above was used toprepare oriented aggregates(Grim, 1934) for way and optical study.The remainder of the sample was dried at room temperature for the otheranalyses. n caseswhere he influence of addedcationswere o be studied,


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MONTMORILLONITE 1333the preparedclay was not dried. Also, the cation exchange apacitiesweremade on sampleswhich had not been acid treated.

For high temperature r-ray diffraction study, oriented aggregatesam-ples were prepared on platinum plates. The furnace used was of the de-sign described y Kulbicki and Grim (1957).Runs were made with con-tinuous heating at various rates, and also by soaking the samplesat vari-ous temperatures. fn many casessupplementary data were obtained byheating the samples n an electric furnace to various temperatures, airquenching, and then obtaining powder camera diffraction data.

Differential thermal analyses were made in a furnace with platinumwire as the heating element of the general design of Grim and Rowland(1942) with a platinum block as the sample holder. The analysesweremadeup to about 1400oC.

The other analysesweremade by standard and well known procedures.LocarroN oF SAMPLES rulrBo

The location of each sample studied in detail is given in Table 1. Nomention is made of the stratigraphic position or geologic setting of thesamples although pertinent information for most of the samples hasbeen obtained either by field studiesof one of us (R.E.G.) or from theIiterature. Possiblecorrelations of the character of the montmorillonitewith its occurrenceand mode of formation will be consideredseparatelyin a later report. All samples of the aluminous montmorillonites exceptpossibly6,23, and 31 are bentonites n that their origin is by the altera-tion of volcanic ash in situ. The origin of the exceptions and the othermontmorillonite samples s not established.

Hrcn TBITpBRATUREnasB DBvBropuBNrFigures 2 to 7 show the high temperature phasesdevelopedwhen each

of the samples s heated to a temperature causing he beginning of fusion.In these igures he intensity of a characteristic diffraction line is plottedagainst the temperature of the sample.

The data reported in Figs. 2 to 7 were obtained on samples whosetemperature was continuously increased at a rate of 5o C. per minute'Other heating rates were used also on various samples,but the rate of5o C. per minute was most satisfactory to permit the detection of the firstappearanceof a new phaseand to record its development.Differential thermal analyses to 1000o C. were made on all samplesand for many of them the analyseswere carried to 1400oC. The resultsof these analysesare given in Figs. 8 to 13.

The high temperature data show that all of the montmorillonites do


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1334 R. L,. GRIM AND G, KULBICKInot develop he samecrystall inephaseson heating. A study of the dataindicate that the highly aluminoussamples nvestigatedcan be groupedinto several ypes based on the characteristichigh temperaturephasesdeveloped, or as mixtures of these types. The types will be discussedseparately.

TAsr,n1.LoclrroN or Seupr,nsThe aluminous montmorillonites are listed according to types as determined by thepresent studv.

Cheto-type montmorillonites Mixtures of Cheto- and Wyoming-type1. Cheto, Arizona montmorillonites2. Otay, California 24. Pembina, Manitoba, Canada3. Burrera, Jachal, San Juan, Argentina 25. Polkville, Mississippi4. El Retamito, Retamito, San Juan, 26 . Grand Junction, ColoradoArgentina 27. Fadh,, Mostaganem, Algeria5. Mario Don Fernando, Retamito, San 28. Taourirt, Morocco

Juan, Argentina 29. Marnia, Algeria6. Tatatilla, Vera Cruz, Mexico 30. Marnia, Algeria7. Itoigawa, Niigata Prefecture, Japan 31. Montmorillon, France

Wyoming-type montmorillonites 32. Yokote, Alita Prefecture, Japan8. Hojun Mine, Gumma Prefecture, Miscellaneous aluminous montmorillonitesJapair 33. Colony, Wyoming9. Tala, Heras, Mendoza, Argentina 34. Amory, Mississippi

10 . Crook County, Wyoming 35. Weston County, Wyoming11. Rokkaku, Yamagata Piefecture, Ja- 36 . Humber River, New South Wales,pan Australia12. Amory, Mississippi Iron-rich montmorillonite13 . Santa Elena, Potrerillos, Mendoza, 37. Aberdeen, Mississippi

Argentina 38. Santa Rosalina, Baja California14. San Gabriel, Potrerillos, Mendoza, NontroniteArgentina 39. Manito, Washington

15. Emilia, Calingasta, San Juan, Ar- Hectoritegentina 40 Hector, California16. Sin Procedencia, Argentina Saponite

Wyoming-typemontmorillonitescontaining 41 Ksabi,Moroccofree silica Talc

17 . Usui Mine, Gumma Pre{ecture, Ja - 42 . Gouverneur, New Yorkpan18 Yakote, Akita Prefecture, Japan19. Rokkaku, Yamagata Prefecture,

Japan20. Wayne, Alberta, Canada21. Dorothy, Alberta, Canada22. Cole Mine, Gonzales County, Texas23. Cala Aqua Mine, Island of Ponza,

Italy


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Frc. 2. High temperature phases de-veloped on heating Cheto-type mont-morilionites; Samples l-7; Q, BetaQua:tz; C, Beta Cristobalite; K, Cor-dierite; F, Feldspar.

MONTMORILLONITE 1335

60 0 8O 0 IOOO l2O0 40 OTemPero lure "C

Frc. 3. High temperature phases de-veloped on heating Wyoming-type mont-morillonites; Samples 8-16; C, Beta Cris-tobalite; M, Mullite.

Cheto-TypeThis type is so named because amples rom the Cheto bentonite pro-ducing area in Arizonashow very well its characteristics.Figure 2 illus-trates the high temperature n-ray dif fraction data for samples 1-7


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1336 R. E. GRIM AND G. KULBICKI

Frc. 4. High temperature phases de-veloped on heating samples containingWyoming-type montmorillonite plus freesilica; Samples 17-23; C, Beta Cris-tobalite; M, Mullite.

Fro. 5. Iligh temperature phases de-veloped on heating samples containing amixture of Cheto-type and Wyoming-type 32montmorillonites; Samples 24-32; Q,Beta Quartz; C, Beta Cristobalite; M,Mullite: K. Cordierite.showing the development of beta qrartz, beta cristobalite, and cordier-ite, which are the characterist ic high temperature crystalline phases orCheto-type montmorillonite. Figure 8 shows the differential thermalanalytical curves for the same samples.

The montmorillonite structure is preserved o 850o-900oC. where it islost abruptly in a temperature interval of about 50' C. There does notseem o be any change n the intensity of the basal orders of the mont-morillonite prior to the loss of structure.

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MONTMORILINNITE 1337The first high temperature phase to appear is beta quartz betweenabout 900" C. and 1000' C. It develops at a temperature 50o to I25ohigher than that for the loss of the montmorillonite structure. Duringthe intervening interval the samples show no w-ray difrruction effects.The beta quartz developsrapidly as shown by the rapid increase n itsdiffraction intensity. The difiraction data indicate cell dimensionsslightlyIarger (0.1 A) than the values n the literature, suggesting he possibilityof some stuffing of the lattice.Beta cristobaliteappearsabruptly usually at 1100oC. and developsrapidly. The beta qtartz phase disappearsas the cristobalite develops,

indicating a phase inversion. Sample 1 is exceptional in showing thedevelopmentof cristobalite beginning before 1000" C. and before the

Frc. 6. High temperature phasesde-veloped on heating misceilaneous alumi-nous montmorillonites; Samples 33-36; Q,Beta Quartz; C, Beta Cristobalite; M,Mullite.

Frc. 7. High temperature phases de-veloped on heating iron-rich montmoril-lonites (37 and 38), nontronite (39), hec-torite (40), saponite (41), and talc (42);Q, Beta Quartz; C, Beta Cristobalite; K,Cordierite; E, Enstatite; C-E, Clinoen-statite.


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R, D. GRIM AND G. RULBICKI

Frc. 8. Differential thermal curves of Frc. 9. Difierential thermal curves ofCheto-type montmorillonites; Samples Wyoming-type montmorillonitesl Sam-l-7. ples 8-16.qvartz starts to disappear. This same sample also yields a feldspar be-tweenabout 1000o nd 1100' C.

Cordierite appearsat 1200'-1300oC. at about the temperature thatcristobalite begins o disappear. ts difiraction effects ncrease n intensityas those of cristobalite decrease.The samplesstart to fuse between 1400oand 1500' C. during whichinterval all of the diffraction effectsdisappear.

The differential thermal analytical (DTA) curves in I'ig. 8 show con-siderable variation in intensity of the initial endothermic peak due tolossof adsorbedwater, but no attempt has been made to study possiblecauses f this variation. Somesamples xhibit a singleendothermic eac-tion between600o nd 700' C. correspondingo the ossof hydroxyl water.Other samplesexhibit a double endothermic eaction n the range 450o


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MONTMORILI.ONITE 1339

Ftc. 10. Differential thermal curves ofsamples containing Wyoming-type mont-morillonite plus free silica; Samples17-23.

Frc. 11. Differential thermal curves ofsamples containing a mixture of Cheto-type and Wyoming-type montmoril-ionites; Samples 24 32.

to 700oC. In al l cases he endothermic eactionsdue to lossof hydroxylwater are of sl ight ntensity. A comparisonof Figs. 2 and 8 show hat thestructureof the montmori l lonite s not lost with the lossof hydroxyls. Itis significant that there is no important change n the r-ray diffractiondata for the (002)reflections ccompanying he lossof hydroxyls.The DTA curvesshow a rather intenseendothermic eactionbetween850 and 900oC., which is the interval in which the structureof the mont-mori l lonite s lost. This endothermicpeak s fol lowed after an interval o f50" to 150' C. by a sharp exothermic reaction which can be correlatedwith the appearanceof beta quartz.A secondexothermic eaction appearsat about 1100' C. which prob-ably is a consequence f the formation of cristobalite.The DTA curvesabove about 1200' C. are too complex o be interpreted with certainty.


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1340 R. E. GRIM AND G. KULBICKIHowever, here s a suggestion f an exothermicpeak between1200o nd1300' C. which may correspond to the development of cordierite, anendothermicpeak at about 1250oC. probably due to the break up ofcristobalite,and another endothermicpeak just short of 1400' C. at thetemperature of the beginning of the fusion of the samples.

One of the samples(17) contained cristobalite that could not be sepa-rated by fractionation from the montmorillonite. An inspection of thehigh temperature phase development and the DTA data shows no sig-nificant difference from samples of the same type without the excesssil ica.Wyoming-TypeMany samples of bentonite from Wyoming are composedof mont-morillonite with the characteristicsof this type, hence he name.

Figure 3 shows he high temperature phasesof samples8-16 composedof Wyoming-type montmorillonite. The characteristic high temperaturephases are cristobalite and mullite. Figure 9 illustrates the DTA datacharacteristic of this type of montmorillonite.The montmorillonite on heating in the range of 600o o 700oC. showsgenerally a decrease n the intensity of the (001) reflection, an increasein the intensity of the (003) reflection and no significant change n theintensity of the (002) reflection.

Frc. 12. Difierential thermal curves ofmiscellaneous aluminous montmorillon-ites; Samples33-36.

Frc. 13. ,*",;;,;"rmal curvesoriron-rich montmorillonite (37 and 38),nontronite (39), hectorite (40), saponite(41)and talc (42).


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MONTMORILLONITE T34IThe montmorillonite structure is lost at 900o o 950' C. and no r-ray

diffraction effects are noted again for a temperature interval of 200o o250oC., ,i ,.e.,unti lheating s carried to 1100" o 1150oC. at which tem-perature cristobalite and mullite appear at about the same time. Both ofthese phases persist until 1400' to 1500oC. when the material fuses.Frequently the cristobalite begins o disappearat about 1300' C. whereasthe mullite generally persists unchangeduntil near the fusion tempera-ture. However, the intensity of the mullite reflections s never very greatindicating that this phase is never very abundant and/or very wellcrystallized. The lat tice dimensions of the mullite vary slightly frompublished values for the pure mineral suggesting some replacements ordefects.

The characteristics of the initial endothermic peaks on the DTAcurves will not be consideredherein. The curves all show a lairly intenseendothermic reaction between 600o and 700" C. due to the loss of hy-droxyl water. Some of the samples show another endothermic peak be-tween 500" and 600o C. making a dual peak for the dehydroxylations. Ingeneral these dehydroxylat ion endothermic peaks are more intense forthe Wyoming-type than for the Cheto-type montmorillonites. The *-raydata show that the structure of the montmorillonite persists hrough theloss of hydroxyls but that some structural changes ake place which areadequate to causechanges n the relative intensities of the basal spac-ings. The DTA curves show an endothermic reaction of variable intensityat about 900" C., which is the temperature at which the difiractioneffects from montmorillonite disappear. This endothermic peak is fol-lowed immediately by an exothermic reaction and it is of special nterestthat there is no crystalline phaseshown at this temperature by the r-raydifiraction data, i..e., his thermal reaction occurs at a temperature in-terval in which there are no r-ray reflections.The DTA curve shows an exothermic reaction (sometimesmore thanone) at 1100o o 1200oC.which is the temperature at which mullite andcristobalite appear. The DTA curves beyond this temperature are quiteirregular and variable, and cannot be interpreted.Wyoming-Type With ErcessSilica

Many of the samples containing the Wyoming-type montmorillonitealso had quartz andfor cristobalite in small amounts (less han t57d inparticles so small that they could not be separatedby fractionation fromthe montmorillonite. It is interesting that only one sample (#7) of Cheto-type montmorillonite was found with such free silica. The results of thehigh temperature diffraction studies of the Wyoming type samples withexcesssilica (numbers 17-23) are given in Fig. 4. DTA curves for the


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t342 R. D. GRIM AND G. KULBICKIsame samplesare given in Fig. 10. The difiraction efiects of the quartzare not showri in Figure 4 since they did not influence the high tempera-ture phasedevelopment.A comparisonof Figs. 3 and 4 show that the excess i l ica had no de-tectableeffect on the developmentof high temperaturephases.A com-parison of the DTA curves n Figs. 9 and 10 show no large differences.The thermal reactions, especially the exothermic ones, are usually rela-tively less ntense in the sampleswith excess ilica. Also it is interesting,although he possible ignificances not presentlyknown, that noneof thesampleswith excesssilica show a double peak for the loss of hydroxyJwater.Mixl,wresof Cheto-and,Wyoming-Types

Figure5 showshigh temperaturephasedata for samples numbers24-32) which exhibit characteristics f both of the Cheto- and Wyoming-types. Samples27, 28 and 30 show cristobalite and mull i te which arecharacteristic f the Wyoming-typeplus cordieritewhich is characteristicof the Cheto-type.The DTA curves,Figure 11,of thesesamesamples rel ike those of the Wyoming type and it seems ikely that the dominantcomponentof thesesamples s Wyoming-type montmori l lonite.Samples24,25,26 and 29 show beta quartz and cristobalitehigh tem-perature phases ike those of the Cheto-typeplus mull i te. In samples24and26 cordieritehasnot developed. hesesamples xcept129show DTAcurves characteristicof the Cheto-type montmori l lonite and it seemslikely that this type is dominant in these samples.The DTA curve ofsample29 resemblesmore those of the Wyoming-type than the Cheto-type, however, he first exothermicpeak is unusually broad and couldwell be interpreted as a composite of the peaks in the Wyoming- andCheto-types.The suggested nterpretation is that sample 29 containsroughly equal amounts of Cheto- and Wyoming-type montmori l lonites.It is of interest that the sample nvestigated rom the type locality atMontmori l lon, France (131) is a mixture of the Che to- and Wyoming-types.

Sample32 is composed f a mixture of the montmori l lonite types pluscristobalite which could not be separatedby fractionation. The hightemperature phase development showsnothing unique. The exothermicreaction at about 900' C. seemsunusually broad and may again be inter-preted as due to the mixing of about equal parts of the two types ofmontmorillonite.

It is recognized hat another possible nterpretation is that thesedatado not indicate mixtures of two types of montmorillonite, but rathervariations in the composition within a single type. It will be shown pres-


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MONTMORILI,ONITE 1343ently that there is strong additional evidence for mixing of types, al-though there s undoubtedly somevariation in compositionwithin eachtype.MiscellaneousSamples

It is not meant to imply that all aluminous montmorillonites belong tothe two classesndicated above. Figures 6 and 12 present high tempera-ture phase and DTA data for samples hat do not fit exactly in either ofthese wo categories. hus samples33, 34, 35 and 36 have DTA curvesIike those of the Wyoming-type, and the high temperature phase char-acteristics are also like those of the Wyoming-type except for the smallamount ol quarLz orming just prior to the development of cristobalite.It is expected herefore, that these sampleswould not differ in any verysubstantialway from thoseof the Wyoming type. A possible xplanationfor the presenceof the beta quartz phase will be presented later in theDiscussion.

Figures7 and 13 presenthigh temperaturephaseDTA data for a fewmontmori l loniteswith a high ron content (37,38), a sampleof nontronite(39), sampleswith a high magnesiumcontent (40, 41), and a sampleoftalc (42). Sampleswith increasing eplacementsof aluminum by iron(samples37,38 and 39), show the absence f mull i te at high tempera-tures. n sampleswith abundant ron (38,39), cristobalite s the only hightemperature phase.The destruction of the montmorillonite lattice tendsto be at lower temperatures 800o o 900' C.) in the iron-rich samples scompared to the aluminous types. Also the cristobalite disappears inallyat a siightly lower temperature in the iron-rich montmorillonites. TheDTA curves or thesehigh iron samplesshow a lower temperature for theendothermic dehydroxylation peaks than is the case or the aluminoustypes. AIso the endothermic peak for the loss of structure is at a rela-tively lower temperature in the iron-rich types. In the nontronite sample(39) there s no peakaccompanying he lossof structure,perhapsbecauseof a gradual destruction of the structure which is in accordancewith thex-ray data shown in Fig. 7. The DTA curves for the iron-rich samplesshow an exothermic reaction between 800oand 900' C. which is not ac-companied by any crystalline phase detectableby *-ray difiraction. Nodefinite explanation can be ofiered, but it is the authors' opinion that itrepresents he nucleation of a phasewith a silica type crystallization. Theslight secondexothermic reaction just short of 1200' C. is at the tempera-ture at which cristobalite appears in high temperature diffraction data'The DTA curvesabove 1200oC. are o complex o be nterpreted.In the caseof hectorite (40) the structure is lost gradually from about800' C. to 1000oC. Enstatite appearsas soonas the structure of hector-


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t3M R. E. GRIM AND G. KULBICKIite begins to disappear and the maximum diffraction of enstatite is at-tained while the hectorite is still producing considerablediffraction in-tensity. At about 1125" C. enstatite changes o clinoenstatite. No otherhigh temperature phases were evident. The endothermic reactions atabout 800oC. and 1200' C. are in the range of the loss of montmorillonitestructure and the formation of enstatite, and the inversion of the enstatiteto cl inoenstatite, espectively.The saponitesample (41) loses ts struc-ture from about 800oC. to 875' C. without a correspondingDTA peak naccordancewith its trioctahedal structure. Enstatite begins to form at aslightly lower temperature than that of the final loss of the saponitestructure with no correspondingDTA peak. The intensity of the enstatitedifiraction continues to increase to the highest temperature attained,1500'C. Cristobalitebegins o form at about 1150'C. and the intensityof its difiraction effectsalso continue to increase o the highest tempera-ture attained. The DTA curve for saponiteshowsonly one moderately in-tensethermal reaction, the exothermic reaction at about 975" C., and nodefinite correlation is possible between the DTA curve and the hightemperature phase data.The talc sample (42) loses ts structure between about 700oC. and880" C. Enstatite appearsat a slightly lower temperature than that of thefinal loss of the talc structure and continues to diffract with moderateintensity up to the highest temperature attained, 1500" C. Cristobaliteappears irst at 1450' C. with very minor diffracting intensity which is,however, increasing at 1500oC. The DTA curve for talc showsa singlethermal reaction, an endothermic one just short of 1000" C., which can-not be correlated with any of the high temperature phasereactions.

Cupurcar ANervsBsChemical analyses of all samples together with structural formulaecomputations according to the method of Ross and Hendricks (1944)are given in Tables 2 to 6.The computed compositions of octahedral cations for all the mont-morillonite samplesare plotted in Fig. 14.The chemical compositions of the aluminous samples fall into twogroupscorresponding o those derived from the high temperature diffrac-tion and DTA data. The samplesclassedas Cheto-type, Table 2, have

less than 5/6 of tetrahedral silicon replaced by aluminum ; 25 to 35/6 oIoctahedral aluminum replaced by magnesium; and 5/s or less of theoctahedral positions populated by iron.In general the Wyoming-type montmorillonites, Table 3, show aboutthe same amount of the tetrahedral si l icon replacedby aluminum, al-though in some samples he amount of tetrahedral aluminum is greater


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? :


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1346 R. E. GRIM AND G, KULBICKIthan is the Cheto-type samples. In the Wyoming-type montmorillonite5 to 10/6 of the octahedralaluminum is replaced y magnesium less hanfor the Cheto-type); and 5 to more than l|/p of the octahedralpositior:sare populated by iron which is more than for the Cheto-type.The totaloctahedralpopulation is generally 2 or slightly less n the Wyoming-typesamples,whereas or the Cheto-type this value is generally sl ightlygreater than 2.The analysesof the Wyoming-type sample containing free silica show,Table 4, the expected elatively large amount of SiOz.Samplescomposedof mixtures of Cheto-and Wyoming-type mont-mori l loniteshave chemicalcompositions,Table 5, that are intermediatebetween he pure Wyoming- and Cheto-types.Samples33,34,35 and 36, which difier from the Wyoming-type sam-ples becausea small amount of beta qvaftz formed as an initial high tem-peraturephase,and which are isted among he Miscellaneous amples nTable 6, have chemical compositions similar to those of the Wyoming-type samples,Fig. 14.Samples37 and 38, Table 6, show some characteristicsof both theCheto- and Wyoming types in the larger replacement of octahedral

E C h e t o - t y p e w i t h - t r e e s i l i c oo Wyoming-typso wyoning-typ8 withfr.r ! i l icov Mixlurc3 ot Che lo ondWyoning- yp3e Mirlurar ot Choto ond Wyo-hing-types wilh free si l icor Miscel loneous nontmoii l lonifes

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Frc. 14. Computed octahedral cation compositions of the montmorillonites.


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t3+7ONTMORILLONITE

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r

I- l *4 l a- l - INA : ." - AH X

' R-l 4U I

N

N

a

N


20/41

R. E. GRIM AND G. KULBICKI348

- l - l - l - l l - l -t H t 4 t $ t t 6 i oI I I I d l " i

; l ; l

I rl er lc ls ls la le le lt : o I " . ; " ; d I - i l d 1 " . ;

=.,lelrlrl t ' l l l

l - j l +l l


21/41

MONTMORILLONITEaluminum by magnesium of the former and the relatively large replace-ment of tetrahedral silicon by aluminum of the latter. In addition thesesampleshave a larger amount of iron than the Cheto- and Wyoming-typesamples.

The sampleof nontronite (39) has only a small amount of replacementof magnesium or iron in octahedral positions. The nontronite sample aswell as the samples with considerable eplacement of the aluminum byiron in octahedral positions (37 and 38) are dioctahedral forms, but thetotal population of octahedralpositions is appreciably in excess f 2.

The samplesof hectorite (40) and saponite (41) both are trioctahedralforms in which magnesium s the dominant component of the octahedralIayer. It is interesting in both of these samples the tetrahedral layershave very little replacementof silicon by aluminum and in each case hetotal population of octahedral positions is slightly in excess f 3.

CarroN ExcuaxcB CePecrrYThe cation exchangecapacities of the samples determined and com-puted are listed in Table 7. The determined capacitiesof the Cheto-type

montmorillonite ranges between 114 and 133 milliequivalents per 100

t349

SamnleN. . lJetermrned Computed

Cheto-type133 168rr4 160118 151125 182t25 t6 l122

Wyoming-type

samPle Determined ComputedMixture of Cheto- and

Wyoming-types1a

4.)6

90959289160160

10690rr7106938882

2526272829303032

33343536383940

v5958792106959298t23

1099 l899896ttl110109102

89101112I Jt415I O

151126126129t2690

(contains reesilica)

118110109103.)/Miscellaneous Samples

Tasln 7. Cnrror Excrrl'r.rcnCellcrtv rN Mrr,r,requrvAr,ENTsER100 Gn.cMs


22/41

1350 R. E. GRIM AND G. KULBICKIgram, whereas he capacitiesof the Wyoming-type samples anges rom89 to 111mil l iequivalentsper 100gram. It is interesting hat there s nooverlap in the capacitiesbetween thesetypes of montmorillonites.The relatively higher exchangecapacity of the Cheto-type is in agree-ment with a greater total replacementand higher net negative charge onthe lattice of this type of montmorillonite as compared to the Wyoming-type. In the Wyoming-type most of the charge is derived from replace-ments n tetrahedralpositions,but the total chargeon the lattice is lessthan for the Cheto-typemontmori l lonite.The sampleswhich are mixtures of the two types have cation exchangecapacities,which are intermediateas would be expected.The miscellaneous ampleswith the exceptionof 135 have capacitiessimilar to those of the wyoming-type montmori l lonite. The aluminoussamples n this miscellaneous roup (nos. 33, 34, 35 and 36) also havechemicalcompositionswhich are quite similar to those of the Wyom-ing-type. Miscellaneous amples38, 39 and 40 are high iron or magnes-ium montmori l lonite samples and data are not at hand to indicatewhether or not thesevalues have any general significance.For the Wyoming-type montmorillonite, there is reasonableagreementbetween he determinedand the computed cation exchangecapacities.However, for the Cheto-type, the computed values are uniformly higherthan the determined values by the order of about 35 milliequivalents per100 grams.The only explanation that can be ofieredfor th is lack of agree-ment in the Cheto-type is that it is a consequence f the preparation ofthe samples.The samples or chemicalanalyseswerepreparedusing acid,whereas or the exchangecapacity determinations the sampleswere un-treated. Perhaps he acid treatment removedsome cations rom withinthe lattice thereby increasing the computed value. If this is the explana-tion, it fol lows that the Cheto-type s more susceptible o leaching hanthe Wyoming-type montmorillonite.In the caseof mixtures, the computed values are in generalhigher thanthe determined values in accord with the presenceof some Cheto-typematerial.

The foregoing explanation is supported by the much higher computedvalues for the iron rich samples (38, 39) as compared to the determinedvalues. rt would be expected that the lattice iron would be relativelymoreaffectedby the acid treatment.

X-Rav Drllnacrrox or. UNr,rnerrSalrplnsPowderDiagrams

The diagrams for the Cheto-type montmorillonites as compared to theWyoming-type show somewhat better defined prism reflections, he (001)


23/41

MONTMORILLONITE 1351reflections are frequently sharper, and there is a more definite indicationof higher basal orders.

In the caseof mixtures of the two types of montmorillonite, the prismreflections are Iike those of the Wyoming type. The (001) reflections arevariable with similarities to both types represented n different samples.

The sample listed as containing free silica shows difiraction lines forcristobalite andf or quartz in addition to those from montmorillonite.Except for the expected eduction in intensity of the montmorillonite re-flections, here is no significant difierence n the patterns of thesesamplesas compared o thosewith l i tt le or no free si l ica.

Sample No. Computed Measured Sample No . Computed MeasuredCheto ype

12346Wyoming type89101 1t2

Wyomingtype

(cont.)13t+15T6Miscella-

neous types333435363738

8.9668 . 9 7 38.9818.9798.9878.9388.9338 . 9 M8 . 9 58.9568.979

9.009 . 0 18 . 9 88 . 9 79 . 0 08 . 9 58 9658 . 9 48 . 9 78 . 9 7

8 . 9 7 28 9668 . 9 7 58 98

8 9518 . 9 7 88 . 9 78.9588.9989 . 0 3

8 . 9 78 . 9 78 978 . 9 7

8 . 9 s8.9658 . 9 88 . 9 79 . 0 09 . 0 4

Miscellaneoussamples (33, 34, 35, 36) give diffraction data like thoseof the Wyoming-type. Sample 38 with a higher iron content is also likethat of the Wyoming type. The sample of nontronite gives poorer datathan the Wyoming-type in that the prism reflections are merely broadbands.

The values of bo determined from the (060) reflectionsobservedon thepowder diagrams and calculated according to the formula oI MacEwan(1951) for the Wyoming, Cheto, and miscellaneousypes are given inTable 8. All specimensappear to be dioctahedral montmorillonites' Theagreementbetween the observed and computed values is reasonablygood. The Wyoming-type samplesobservedvalues range from 8.94 to8.97.The miscel laneousamples 33,34,35,36) range rom 8.95 o 8.98.The values or the Cheto type range rom 8.97 to 9.01-higher than the

Tanr,r 8. Coulurno lNr Mrasunm Vlr-uos lon b


24/41

t352 R. E. GRIM AND G. KULBICKIWyoming type as would be expected becauseof the higher content ofmagnesium. An exception in the case of the Cheto-type is the Tatatillasample (6) for which the value is unusually low probably becauseof theextremely small iron content. The values for the sampleswith a high ironcontent (37, 38) are 9.00 to 9.04 which is in the expected ange. The

D e g r e e s 2 9Fig. 15. X-ray difiraction spectrograms of oriented aggregates of Cheto-type mont-morillonite (3 and 5), wyoming-type montmorillonite (8 and 11), and a mixture of thesetypes (26).

cOc


25/41

MONTMORILI,ONITE 13s3(060) reflection for most of the samples s fairly sharp but for others it isfairly broad, sometimes suggesting hat it is a complex of several reflec-tions. The samples composed of a single type of montmorillonite are ingeneral sharper than those composedof mixtures. This is true for all ofthe Cheto-type samples but not for all of the Wyoming-type samples.Further, a few of the samples composed of mixtures yield quite sharp(060) reflections. t may be concluded rom the foregoingstatements hatpowder difiraction data may suggest hat a given sample s composedofa particular type of montmorillonite or a mixture, but it is no more thana suggestion. The data seem to indicate that the population of cationpositions n a mass of montmorillon ite is more uniform in the Cheto-typethan in the Wyoming-type mineral.Oriented.A ggregoteDiagr ams

The Cheto-type samplesshow ntense sharp (001) reflectionsbut higherorders are always oI about uniform low intensity both with and withoutglycol treatment, Figs. 15 and 16. On the other hand, the Wyoming-typesamplesusually show sharp (001) reflectionsand alsosharp ntensehigherorders up to about (006). The sharp higher orders wereobtained from onesample without glycol treatment, but for the other samplesglycol treat-ment was necessary o developthem.The samplescontaining excess ilica were substantia lly like thoseof thepurer montmorillonites. The Wyoming-type with excesssilica show thesame sharp higher orders up to about (006) as those without the silica,indicating that the qtartz or cristobalite is present in discrete particleseven though it has been impossible to separate it from the montmoril-Ionite.

The samples composed of mixtures, following glycol treatment, showthe sharp higher orders Iike the Wyoming-type except that they aresomewhat less intense. The (00a) and (006) reflections are relativelyweaker than the (002), (003), and (005) reflectionsand this characteristicis more pronounced in the mixtures than in the pure Wyoming-typemineral.

The miscellaneous amplesof the aluminous montmorillonites show thedevelopment of higher orders comparable o the Wyoming type. The non-tronite sample also shows he developmentof higher orders,but as wouldbe expected the relative intensities are difierent from those of thealuminous samples.

It is planned to consider urther the difiraction characteristicsof thesetypes of montmorillonite in a later paper. ft appears,however, that thedata warrant the conclusion hat the Wyoming-type is composedof unitsilicate layers lesswell bonded together than the Cheto-type, so that themontmorillonite can be more completely dispersed eading to more uni-


26/41

1354 R. E" GRIM AND G. KULBICKI

D e g r e e s2 9Frc. 16. X-ray diffraction spectrograms of oriented aggregates, glycol treated, ofCheto-type montmorillonite (3 and 5), Wyoming-type montmorillonite (8 and 11), and amixture of these types (26).


27/41

MONTMORILLONITE

Ftc. 17. Electron micrographs (carbon replicas) of Wyoming-typemontmorillonite (10), 6500X.

Ftc. 18 . Electron micrographs (carbon replicas) of Cheto-typemontmorillonite (6), 6500X.

1355


28/41

1356 R. E. GRIM AND G. KULBICKIformly oriented aggregate flakes which are more thoroughly and com-pletely penetratable by the glycol. This is in accord with the relativelylow cation exchangecapacity and hence ower charge on the lattice of theWyoming type which in turn is the consequence f a relatively smalleramount of substitution within the lattice of the Wyoming-type mont-morillonite.

Er-ncrnoN DrnlnacrtoN AND MrcnoscopvThe electron diffraction and microscopiccharacteristicsof the samples

were not investigated exhaustibly but only to determine if there wereob-vious differences corresponding to the Wyoming- and Cheto-types ofmontmorillonite. No such differencescould be found in the diffractiondata.Electron micrographs using the carbon replica technique indicate thatthe Wyoming type, Fig. 17, s composedof such extremely small particlesthat there is no suggestionof individual particles in the micrographs. Onthe other hand, micrographs of the Cheto-type, Fig. 18 and the samplefrom Montmorillon, Fig. 19, which is a mixture of types, have a granularappearancesuggestingsomewhat coarserparticles. It is not felt that theelectron micrographs presentunequivocal evidence or separating he twotypes. However, the characteristicsof the electron micrographs are gen-

Frc. 19. Electron micrographs (carbon replicas) of samples composed of mixtureof Wyoming- and Cheto-t1pe montmorillonite, (31), 6500X.


29/41

MONTMORILLONITE I J J /erally in accord with the characteristics of the two types of aluminousmontmorillonite derived from the other data.Electron micrographs of the samesamplesafter heating to a tempera-ture just adequate o destroy the montmori l lonite structure (900" C.+)showedno significant differencesas compared o the unfired samples.

Frc. 20. Infra-red absorption curves of montmorillonites: Cheto-type (1 and 2),Wyoming-type (8), Wyoming-type plus qtartz (20). All data from films except separatecurves 600-1300 cm-i lrom KBr oeliets.

INrna-nBo AN,lrvsBsInfra-red absorption curveswere obtained for a seriesof samplesshow-ing differences in chemical composition, fi-ray diffraction, and DTAcharacteristics by Dr. J. M. Serratosa of the Illinois State GeologicalSurvey.The authorsare ndebted o Dr. Serratosaor the interpretationof the infra-reddata which arepresentedn Figs. 20 to 23.Films composed of particles with parallel orientation of the basalcleavageplanes were prepared by evaporating a suspension on plasticslides;when dried the films are easily separatedwith ethyl alcohol. These


30/41

1358 R. E. GRIM AND G. KULBICKI

Frc. 21. Infra-red absorption curves of iron-rich montmorillonite (37), nontronite(39), hectorite (40), and saponite (41). All data from films except separate curves600-1300 cm r from KBr pellets.films were heated to 300oC. and protected with fluorolube oil in order toavoid rehydration. Infra-red spectra were obtained for different inci-dent angles.Serratosa nd Bradley (1958)have shown hat amongmicasand related crystallizations trioctahedral compositions exhibit an OHbond axis normal to the cleavage lake with an infra-red absorption fre-quency near 3700 cm.-1, but that the dioctahedralcompositions xhibitOH bond axes near the plane of the cleavage lake and of lesserabsorp-tion frequency.Determination of the direction of the OH bond axis byobtained spectra of oriented aggregates or different incident anglestherefore provides a means of identifying the dioctahedral or triocta-hedral nature of such crystallization.In the 3700cm.-r regionall of the samples f montmori l loniteand non-tronite examinedshoweda strong absorption or normal incidencewithlittle sensitivity to the orientation of the flake, thereby indicating thatthe crystal l ization n thesesamples s dioctahedral. t must be noted thatit is uncertainwhethera mixture of dioctahedraland trioctahedral orms

z

3


31/41

MONTMORILLONITE

Fro.22. Infra-red absorption cuives of sample 27 composedof a mixture of Cheto-and Wyoming-type montmorillonite, and after heating to temperatures indicated.

could be detectedwith the equipment used (NaCl prism), i f one of thecomponentswas present n small amounts. There is therefore, he pos-sibility that a small amount of trioctahedral material may be present inthesesamples. t is of interest that the maximum absorption of the mont-mori l lonitescorrespondso that of muscovite 3640cm.-1), whereas hatof the nontronites, s lower (3600-3610 m.-1).

The samplesof saponiteand hectorite, Fig. 21, showan absorptionathigher requencies 3700-3710 m.-t) which increasesmarkedly with theincidenceangle thereby indicating the trioctahedral nature of thesemin-erals.The efiect s morepronounced n the saponite han in the hectorite,probably because f the substitution of someOH by F in the hectorite.

In the 2000-600cm.-l region the sampleswere examined as films with-out any protection, and as disseminatedKBr pellets n concentrationsof 0.3 to 0.8 per cent. All the samples howa band at 1625cm.-l charac-teristic of the adsorbed water (deformation frequency of the H-O-Hvibration). There is strong absorption at 1000-1050cm.-1 associatedwith the Si-O bonds, and a medium band between1075and 1125cm.-lwhich cannot be explained. Also in the montmorillonites and nontronitesthere s a relatively weak band at 840-850cm.-l. The absorptionat 775and 800 cm.-l is due to qtrartz impurity.

In montmorillonites with aluminum as a principal cation in octahedralpositions there is a relatively strong band at 920 cm.-1. In the nontronitesthis band is not present,but instead here s an absorptionat 820 cm.-r,the shift in frequency being produced by the substitution of iron for

3


32/41

R. E. GRIM AND G. KULBICKI

^'i:'.,,O',


33/41

MONTMORILLONITE 136Iacter of the aggregatesand also, f possible, he optical properties of themontmorillonites.In general the Wyoming-type montmorillonite shows much betteraggregateorientation than the cheto-type. The individual particles n thewyoming-type aggregates re often too small to be seen ndividually andthe aggregatehas the appearanceof the fragment of a single crystal. onthe other hand, individual particles of the cheto-type are easily visible inthe aggregate which has a granular appearance. The uniformity oforientation of the individuals is much less n the Cheto-type than in theWyoming-type. The particles of the Wyoming-type are not only smaller,but have aggregated ogether so perfectly that something akin to crystalgrowth has taken place.As would be expected, the aggregatesof the samples which are mix-tures of the wyoming- and cheto-types are variable. some are about likethose of the wyoming-type whereasother are distinctly granurar. Theaggregatesof the miscellaneous ype of montmorillonites are more likethoseof the Wyoming-type than the Cheto-type. Many of these samplesprovide aggregateswhich are composedof extremely small particles witha very high degreeof uniformity of orientation. The sample of hectoritegives particularly excellent aggregates.The optical properties were studied to determine if there was any con-sistent difierencebetween values or the Cheto- and Wyoming-types. Nosuchdifferenceswere found unequivocally and perhapsnone are to be ex-pected since,as Rossand Hendricks (1945) have shown, the indices varywith the iron content and as Mehmel (1937) has shown, the indices alsovary with the content of magnesium. As both the iron and magnesiumvary within the types, the influence of this variable might well concealany variation between types. However, the data suggest hat where thecomposition is similar the wyoming type has slightly higher indices.Thus, sample 8, which is a Wyoming-type with a low iron content has ahigher ndice (B:1.530) than sample4 of the Cheto-type with a sl ightlyhigher iron content (B:1.520). No satisfactoryexplanationcan at themoment be offered or a possibleconsistentdifference n optical propertiesfrom one type to the other.

Porassruu aNn MacNpsrulr TnBarunNtSamples of the various types of montmorillonite were treated withKCI (1N) and then washed until free of chloride. Difiraction diagramswere obtained on oriented slidesafter air drying with and without glycoltreatment, and after oven drying and glycol treatment.Wyoming-type samples,Fig.24, collapsed o a c ax is spacing of abouttl .7 A on air drying. Both the air dried and oven dried (at 100' C.for 2hours) expanded to about 17.4A following glycol treatment. Therefore


34/41

1362 R. E, GRIM AND G, KALBICKI

6coc

Eoc

t oDeg rees 2 O

t oD e g r c e s 2 O

Frc.24. X-ray diffraction data for wy- Frc. 25. X-ray difiraction data for che-oming-type montmorillonite (10) treated to-type montmorillonite (1) treated withwith KCl. A-air dried, B-air dried and KCl. A-air dried, B-air dried and glycolglycol treated, C-dried at 100o C. Iot 2 treated, C-dried at 100" C. for 2 hours andhours and glycol treated. glycol treated.it may be concluded that no permanent collapseof the structure or re-tardation of expansionwas caused y potassium reatment of this type ofmontmorillonite.

The two samples f Cheto-type,Figs.25and 26,so reatedcoliapsed nair drying to l2.t and 11.9 A. respectively. Following glycol treatmentthe air d.ried samples expanded to 15.5 and 14.7A, respectively' Glycoltreatment of the oven dried samplesproducematerial which expanded oabout 15.5 and 14 A, respectively.Results fol lowing the treatment ofpotassium chloride are therefore different for the two types of mont-morillonite. This matter is being investigated further. Presentdata show


35/41

MONTMORILLONITE

D e g r e e s 2 Ol o

D e g r e e s 2 9Frc.26. X-ray diffraction data fo r Che- Frc.27. X_ray diffraction data for iron_to-type montmorillonite (3) treated with rich montmoriilonite (32) treated with KCl.KCl. A-air dried, B-air dried and glycol A-air dried, B-air dried and glycoltreated, c-dried at 100' c. fo r 2 hours treated, c-dried at 100oc. for 2 hours andand glycol treated. glycol treated.

that the results are not the same for all montmorillonites derived frombentonites,and that the processmust be used with caution in distin-guishing expandable clay minerals derived from different parent ma-terials (e.g.degradedmicasversusmontmori l lonites rom bentonites) rtseemsworthwhiie to emphasize hat the type of montmorillonite whichshowsno effect of potassium reatment, a.a.Wyoming type, is the onewith a lower chargeon the lattice and a lower cation exchange apacity.The type with a greaterchargeon the lattice shows etardation and re-duction in amount of expansion, i.e. apparently in this type of mont-mori l lonite enoughpotassium s held between he sil icate ayers to pre-vent completeexpansion.Miscellaneousample37 ollowingpotassium reatment shows,Figs.26-27, a c-axis pacingoI 12.5A without glycol treatment and 13.6A aftergiycol treatment. After oven drying the sample shows an expansion withglycol to only 11.6A. |Itr is samplehas a larger amount of replacementwithin the octahedralpositions han the Cheto-type samples, nd as ex-pectedpotassium reatment causes greater eduction n expansion hanfor the Cheto-typesamples.The same sampleswere treated with magnesium chloride (1N) andthen washed ree of chloride. The results were the same for all the sam-

1363

coc

COc


36/41

t364 R, E, GRIM AND G. KULBICKIples. Diffraction patterns obtained after a ir drying show a c-axisspacingof 13.8A. Following glycol treatment, the air dried samples nd samplesoven dried at 100oC. Ior 2 hours expanded o t7.2 A. Thus the mag-nesium treatment has no effect in reducing the amount of expansion ofthe montmorillonites studied. The difference n the effect of potassiumand magnesium s expected,since egardlessof the charge,at least withinthe limits of the samples nvestigated, the size and coordination char-

600 800 rooo 1200 1400T e m p e r o f u r eo C

Fro. 28. High temperature phases developed by particle size frac-tions of a sample (29) composed of a mixture of Cheto- and Wyoming-type montmorillonites. A, Fraction ( l micron;8, Fraction2-l micron;Q, Beta Quaftz; C, Beta Cristobalite; M, Mullite; K, Cordierite.

acteristics of the magnesium on would not cause t to aid in restrictingthe expansionof dried montmorillonites. Further magnesiumtreatmentis a sa{er way to distinguish expandablematerial derived from chlorite ascompared to montmorillonite derived from volcanic ash than is potas-sium treatment in distinguishing expandabie material derived from micaas compared o montmori l lonites rom bentonites.FnecrrowauoN ol rnB S.qMprB ouposrn oF MrxruRE or.TypESSample 29, indicated as a mixture of the Wyoming- and Cheto-typemontmorillonite was fractionated by centrifuging a dilute suspensionofthe minus 2 micron component into the fractions containing particles of2- 1 micronsand minus 1 micron. It can be seen rom Fig.28 that the finerfraction gave high temperature phase characteristics of the Wyoming-


37/41

MONTMORILLONITE 1365type, whereas the coarser fraction exhibited such characteristics of theCheto-type. It may be concluded hat in this sampleat least the mixtureis one of discrete particles. Also, as expected, the Wyoming-type dis-persed nto smaller particles than did the Cheto-type montmorillonite.

DrscussroNThe present nvestigation indicates that dioctahedral montmorillonitesdo not form a single continuous isomorphic series. Two difierent alumi-nous types have been found, Cheto- and Wyoming-types, which difierprimarily in the population of the octahedral layer; notably in the rela-

tively higher amount of magnesium n the Cheto samples.It is noteworthy that the addition of magnesiumto the Wyoming-typemontmorillonite does not cause the development of high temperaturephasescharacteristic of the Cheto-type (unpublished data). Also repeatedleaching of the samples with HCI in order to remove most of the octa-hedral cations did not change the high temperature reactions of eithertype. It seems, herefore, that there are structural as well as composi-

tional difierencesbetween he two types. The analytical data suggest hatthese differencesare as follows:The Cheto-type has relatively more substitution of aluminum by mag-nesium in the octahedral layer causing a greater charge on the lattice.Further, the replacementsare relatively more regular i.e. the position ofthe magnesiums s in a fairly definite pattern in the Cheto-type. If themagnesiumswere randomly distributed, the particles would have someMg-rich areasas well as someMg-poor areas.Therefore, they would havehigh-temperature phasesof both types; indeed, this never happens withproperly sized and purified fractions. Also it is thought that some (prob-ably a small number) of the silica tetrahedra are inverted in the Cheto-type. There doesnot seem o be a difference n the population of the tetra-hedral positions in the two types.Figure 29 shows an ideal arrangement of octahedral cations with onefourth of the aluminums replaced by magnesium. This is close to theaverage Cheto-type composition and it seems easonable o think thatthis type of montmorillonite has the magnesiums arranged in such apattern. Considering this pattern the typical properties of the Cheto-type montmorillonite can be explained.Thus, the exchangesites being ona hexagonalnet, the samekind of symmetry can be expected n the stack-ing of the elementary silica layers. Someof the exchangeable ations canalso act as bondsbetween he layers. The net result of these actors wouldbe the Iarger particles characteristic of the Cheto-type, the more difficultcomplete dispersion,and the developmentof a mica-like structure follow-ing potassium treatment. On the contrary, in the Wyoming-type mont-


38/41

1366 R. E, GRIM AND G, KULBICKI

Frc' 29. Probable arrangement of octahedral cations in cheto-type montmorillonite,showing suggested hexagonal arrangement of exchange sites. Large circles-Mg, smallcircles-Al.

moril lonite, the exchange itesare randomly distr ibuted, and no regularstacking and bonding of the sil icate ayers s to be expected.As a con-sequence,hydration and dispersion of the individual layers is relativelyvery easy.The Cheto-typeshowsa greater ossof water in the 100-500oC. tem-perature interval which can be accounted for by some inversion of thesilica tetrahedra. The hydroxyls of the exposed tips of the tetrahedrawould probably be lost within this temperature nterval.No specificexplanation can be ofiered for the variation in the dehy-droxylation characteristics, .e . dual versus single endothermic peak ac-companying he reaction and the variation in intensity of the reaction.Samples with higher iron and magnesium contents have reactions of

lesser ntensity, so that variations in composition are a factor but struc-tural attributes also are probably important. ft seems ikely that a dualpeak meanssomesort of mixing of layers.The intensities of the (001) reflections do not change on dehydroxyla-tion of the Cheto-type sampleswhereas he relative intensitiesof thesereflections do change for the Wyoming-type. It would seem ikely thatthere would be less structural adjustment in the better crystal l izedCheto-type with its regularity of substitution in the octahedral positionsand hence esschange n the intensity of the basal reflectionsaccompany-ing the loss of hydroxyls.The endothermicpeak at about 900' C. varies n intensity and over aconsiderable temperature interval and is probably a matter of theabruptnessof the loss of the montmori l lonite structure causing t. Forthe Cheto-type the reaction is generally relatively intense in accordance


39/41

MONTMORILLONITE 1367with the better crystallinity and hence he probable more abrupt loss ofstructure. The reaction for the Wyoming-type may be large or small,probably due to small variations in crystallinity and probably also tovariations in composition. The intensity of the reaction decreases s theiron content increases,and in very iron-rich samples t is about absent.The presenceof iron thus favors a gradual loss of the montmorillonitestructure.

Electron micrographs show that the loss of the difiraction character-isticsat about 900oC. is not accompanied y the complete ossof the ex-ternal morphology, i.e. the flake shape of the units is still preserved.Data are not unequivocal, but the external form seems o be better pre-served in the Cheto type than in the Wyoming-type montmorillonite.The reaction cannot be a complete structural breakdown but ratherone n which the ayer character s retainedprobably with lack of stackingorder and some distortion in the a and b direction.The formation of beta quartz from the Cheto-type probably involveswhole reorganizationof adja-qentetrahedral layers. rt is thought that thepresenceof some nverted tetrahedra would favor the formation of thisqrartz phasewhich is not in the temperature domain of the formation ofbeta quartz as indicated in silica equilibrium diagrams. The postulatedabsenceof inverted silica in Wyoming-type would explain the absenceofa qtrartz phase rom these montmorillonites (i t has beenpointed out thatthere is no difierence n the composition of the tetrahedral positions n thetwo types). The difierences n the temperature interval for various Cheto-type sarnplesbetween the loss of montmorillonite difiraction and theformation of beta qtartz and the variation in the intensity of the reac-tion accompanying the formation of quartz may be explained by varia-tions in the amount of inverted tetrahedra and the consequentvariationin the easeof formation of beta qttartz.The Wyoming-type montmorillonite showsa long temperature intervalbetweenthe lossof montmorillonite difiraction and the formation of anyhigh temperature phase.The absenceof any inverted tetrahedra makesdifficult the development of new phases.Cristobalite appearsat a lowertemperature in the Cheto- as compared to the Wyoming-type samples.That is, it appearsat a lower temperature when formed by the inversionof beta qtartz than when it developsdirectly from the silica of the mont-morillonite structure.At about 1200" C. mullite forms from the Wyoming-type samples.This phase does not form from the Cheto-type mineral as apparentlymagnesium n amounts in excess f about I-2/6 MgO prevent the forma-tion of mullite. AIso, in the iron-rich samples,mullite doesnot appear sothat small amounts of iron also block the formation of mullite. The


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1368 R. E. GRIM AND G. RULBICKImullite that does form from the Wyoming-type montmorillonite prob-ably is not pure aluminum silicate as the lattice parameters are slightlydifferent from published values of pure material-in many instances tprobably has about all the impurities that the structure will tolerate.

The intense exothermic peak shown on the differential thermal curvesof the Wyoming-type samples at about 1000oC. is not accompaniedbyany crystalline phase detectable by r-ray diffraction. This reaction isinterpreted as a consequence f a shift in bonding within the structureprobably from face sharing octahedral units oI dehydrated montmoril-lonite to the more stable edge sharing units which prevail in the hightemperature structures that may form. This bonding shift is but one stepin the development of new high temperature phases.A secondstep is themigration of cations into proper positions on a scale eading to crystalgrowth of a size detectable by *-rays. Higher temperature (about 1200oC.) is required to provide sufficient mobility of the cations so that thisgrowth can take place. This second step may never take place if thecomposition is not proper for the specificnetwork to form or if cationsare presentwhich block the growth or break up the network at relativelyIow temperatures. The potassium ion for example substantially inhibitsthe formation of any high temperature phasefrom the montmorilloniteminerals (Kulbicki and Grim, 1957).

The phases hat form at temperatures below about 1200oC., i..e. hebeta qtartz and cristobalite from it, form at this relatively low tempera-ture becauseof their structural relation to the silica part of the mont-morillonite structure. The first or nucleation stage of phases hat appearprominently above about 1200' C. is closely dependent on the structureof the original mineral. Thus the arrangement of the magnesiums n theoctahedral ayer of the Cheto-type is such that the nucleation of cordier-ite is favored. The development of the high temperature phasesbeyondthe nucleation stage s determined argely by the bulk composition of thematerial.

The presenceof cristobalite in the unfired clay has no efiect on theformation of high temperature cristobalite. This indicates that the newcristobalite is formed directly from montmorillonite rather than by acomplete breakdown of a montmorillonite structure and then a regroup-ing around the primary cristobalite. That is, the formation of the newcristobalite is a solid state reaction from the montmorillonite.The miscellaneous ypes of aluminous montmorillonites differ from theWyoming-type only in the formation of a small amount of beta qlrartz.This can be explainedby a varying small amount of inversion of the silicatetrahedra in these samples.

Nontronite and the very iron-rich montmorillonites only yield a silica


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MONTMORILLONITE r369phase (cristoballite) at elevated temperatures. Iron apparently in sub-stantial amounts blocks the development of any other crystalline phase.

For the trioctahedral magnesium-rich montmorillonites and talc,enstatite develops before the montmorillonite structure is completelylost and without any accompanying thermal reaction. This suggestsagradual breakdown of the structure of the original mineral with gradualgrowth from the debris of the enstatite-unlike the solid state reactionsfor the dioctahedral forms. In some cases,cristoballite developsat veryhigh temperatures from the left over silica. The reasonfor the develop-ment of little or no cristoballite in some cases hectorite) and much lorother minerals (saponite) s not clear. The elongatestructure of the hecto-rite as compared to the flake-shapeof the saponite may be significant'

RalnnnNcrsBvnwn, P. J. S. (1954), Some observations on montmorillonite-organic complexes: Proc.SeconrlNat. Cl'ay ConJerencePub. 327, U. S. Nat. Acail. oJ Sci ,241-253.Druer., H., I{unrn, G., aNl Inrnc, R. (1950), Organische Derivate von Tonmineralien:

HeIt:. Chim. Acta,38, 1229-1232.EorluAn, C. H., aNr Faw;nn, J. Cn. L. (1940), On the crystal structure of montmoril-lonite: Zeit. Krist., lO2, 417 431.Gnm, R. E. (1934), The petrographic study of clay minerals-A laboratory note: fown-Seil. Petrol., 4, 45-46.- AND RomAl+o, R. A. (1942), Differential thermal analyses of clay minerals and

other hydrous materials : Am. M inu atr.,27, 746-7 61, 801-818.HrNnnrcxs, S B. (1942), Lattice structure of clay minerals and some properties of clays:

f oul. Geol.,50, 276-Zn.HorulNr.r, U., Exnnr,r., K., lxn Wrr-u, D. (1933), Kristallstruktur und Quellung vonMontmorillonit : Z eit. Krist., 86, 340-348.JoNns, E. C. (1955), The reversible dehydroxylization of clay minerais: Proc. Thi'rd' Nat.Cl.ayConf , Publ'.395, U. S. Nat. Acad. oJ Sci.,66-72.Kulercrr, G., eNo Gmu, R. E. (1957), Etude des Reactions de Hautes Temperatures dansles Mineraux Argileux au Moyen des Rayons X: Bull. Soc. France Ceramique, 36,2r-28.MncEwew, D. M. C. (1951), The Montmorillonite minerals, "X-ray Identification and

Structure of the Clay Minerals." Monograph Min. Soc. Great Britain, 86-137.Mansner,r,, C. E. (1935), Layer lattices and base-exchangeclays: Zei't. Krist., 91,433-449-McAttr, J. L. (1958), Heterogeneity of montmorillonites: Proc. FiJth Nat. Cl'ay ConJ.,Publ.566, U. S. Nat. Acad..oJSci.,270-288.McCoNxor,r, D. (1950), The crystal chemistry of montmorillonite: Am. Mi.neral'., 35,

166-172.Mruurr-, M. (1937), Beitrage zur Frage des Wasserhaltes der MineraleKaolinit, Halloysit

und Montmorillorit: Chern. Erde, ll, l-16.Ross, C. S., aNo Hnxontcr

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Montmorillonite - High Temperature Reactions and Classification

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