DIFFERENTIAL THERMAL ANALYSIS OF CLAY MINERALSAND OTHER HYDROUS MATERIALS. PART 2.

Ralpn E. Gnru aNp Rrcnarrs A. Rowr,aNpt

(Continued, from page 761)

Materials that haw been called, beidellite

The samples represented by differential thermal curves in Fig. 8 havebeen classified as beidellite chiefly on the basis of their optical properties.Sample 8A has a curve like that of kaolinite, or a halloysite, because ofthe endothermic peak between 500'C. and 600oC., the sharp exothermicpeak, and the absence of a third endothermic peak. The initial endo-thermic peak suggests hydrated halloysite rather than kaolinite, and thehump at 400'C. reflects the presence of a ferric iron hydrate. The trace ofa peak between 600'C. and 700"C. indicates the presence of a smallamount of montmorillonite. X-ray diffraction analysis checks theseidentifications, showing the dominant constituent to be a two-layer typeclay mineral. The low silica-alumina molecular ratio shown by the chem-ical analysis (Table 4) is also in accord with this interpretation, but theKzO content suggests that the sample also contains illite.

Under the microscope this sample (8A) appears to be homogeneous ex-cept for pigmentary hydrated ferric iron oxide, and the optical propertiescan be c losely approximated ( f :1 .580,7- ,a: .025) . I t is c lear that astudy of the optical properties alone might lead easily to a misinterpreta-tion of the composition of the sample. A correlation of optical, chemical,r-ray, and thermal data for this and other samples shows clearly thatmixtures of clay minerals and hydroxides can be so closely intergrownthat the obtainable optical data for the mixtures suggest homogeneityand a composition other than the actual one. This does not reduce thevalue of optical data in clay mineral work but it does mean that suchdata must be used with caution, particularly when the determinations areincomplete and approximate.

The differential thermal curves of samples 88 and 8C are closely alike,and they indicate the presence of a small amount of montmorillonite anda dominant amount of illite, kaolinite, or a halloysite. Because of thesmallness of the third endothermic peak and the large size of the initialpeak, hydrated halloysite is suggested as a prominent constituent. Ther-ray difiraction patterns of these samples are very poor and indicateonly that they contain a small amount of an expanding-lattice type claymineral. The KzO content shown by the chemical analyses (Table 4)

t Petrographer and Assistant Petrographer, respectively, Illinois State Geological Sur-

ve:r, Urbana, Illinois.

801

802 RALPH E. GRIM AND RICHARDS A. ROIT/LAND

suggests that illite is an important constituent. It seems clear that thesesamples are mixtures of a halloysite and illite with a very small amountof montmorillonite. Only a mean index of refraction could be determinedfor the samples, and optical study alone would not disclose the truecomposition of the materials.

o oo eoo s .ooT E M P E R A T U R f . o€GRefs c

Frc. 8. Materials that have been called beidellite. Scale A.

A. Namiquipa, Chihuahua, Mexico, U. S. Nat. Mus. No. R7595.B. Fairview, Utah.C. Twin Falh, fdaho.D. Beidell, Colorado, U. S. Nat. Mus. No. 93239.E. Wagon Wheel Gap, Colorado, U. S. Nat. Mus. No. 94963.F. Nashville, Arkansas.

Sample 8D, crude material from the type beidellite locality, gave a

curve similar to 88 and 8C and it is concluded that the material is pri-

marily a mixture of a halloysite, illite and montmorillonite. The x-ray

diffraction pattern of this material was very poor and indicated only the

THERMAL ANALYSIS OF CLAY MINERALS

presence of some montmorillonite. A difierential thermal curve was alsoobtained for a sample in which an attempt was made to hand-pickhomogeneous material. The curve for the hand-picked material was thesame as the curve for the crude sample except that the size of the peakbetween 600' C. and 700o C. increased. An r-ray analysis of the hand-picked material showed more pronounced montmorillonite difiractionsthan did the crude sample. It seems clear that the material from Beidell,Colorado, even if purified by hand-picking, is a mixture and not a singlespecies. Again, on the basis of optical properties alone, one might con-clude that the hand-picked material was homogeneous.

The curve for sample 8E is similar to the curve for sample 8D exceptthat the endothermic peak at 600-700'C. is more pronounced, indicatinga greater proportion of montmorillonite. The curve for 8F is also similarexcept that the montmorillonite peak is still larger, indicating an evenlarger amount of this mineral. The diffraction pattern for sample 8Eshowed the montmorillonite lines more distinctly than in previous sam-ples of this series, and the montmorillonite lines in the pattern for sample8F were even more pronounced. The r-ray evidence is therefore in accord

Tlsrr 4. Cnelucar Anelvsns or Mernnrel,s Tner Hevr BBoN Clr-r,Bo Brrnnutrp

8F8E8D8C8B8A

SiOzAl2o3FezOaFeOMgoCaONarOKzOTiOzIgn. lossTotalHzO*HzO-SiOz/RzO:SiOr/AIrOs

43.7627 .886.42

1 . 2 3.90.30

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7 .6710.592 . 3 32 . 6 6

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100.4410.085 .992 . 9 0. ) - . )z

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Analyses 8B, 8C and 8E were made under the supervision of O. W. Rees, Ill. StateGeol. Survey.

8A Namiquipa, Chihuahua, Mexico. Analysis from W. F. Foshag, U. S. Nat. Museum.88 Fairview, Utah.8C Twin Falls, Idaho.8D Beidell, Colorado. Jour. Am. Cer. 50c.,9,94 (1926).8E Wagon Wheel Gap, Colorado.8F Nashville, Arkansas. Jour. Am. Cer.50c.9,94(1926).

804 RALPH E. GRIM AND RICHARDS A. ROWLAND

with the conclusions that samples 8E and 8F contain a relatively largeramount of montmorillonite than the other members of this series. Sample8F appears under the microscope to be a mixture, whereas sample 8E hasthe appearance of a single species. The optical properties of 8E can beaccurately determined (r:1.560, ^r-d:.030 (-)) and on these dataalone one might easily conclude that the material was a single species andnot a mixture.

The curves of all of the so-called beidellite samples show an endother-mic hump or a suggestion of one at about 400'C. to 500oC. This is inthe temperature range of the endothermic reaction of hydrated ferric ironoxides, and this is believed to be the proper interpretation since opticalstudy reveals the presence of some such material in these samples. How-ever, the study of so-called "nontronite" and "chloropal" (samples 124and 13D) suggests that some expandingJattice type clay minerals, prob-ably those containing ferric iron instead of aluminum, m&y show anendothermic reaction between 400"C. and 500'C. It is possible, there-fore, that the hump on the curves between 400'C. and 500'C. may re-flect the presence of a trace of this type expanding-lattice clay mineralas well as ferric iron hydrate.

The data herein presented do not necessarily mean that beidellite isnot a valid species. They do show that much of the material classed asbeidellite, including that from the type locality, is actually a mixture ofclay minerals and hydrated ferric iron oxide. Norton (22) presentedcurves for beidellite showing endothermic peaks between 500oC. and600"C. and at 690'C. but failed to suggest that the curves indicatedmixtures. Orcel and Caillere (25) and Jourdain (15) also presented curvesfor beidell ite showing endothermic peaks between 550oC. and 600oC.which they interpreted as due to kaolinite. It seems likely, on the basisof the present data, that the peak between 550oC. and 600oC. in manysamples is due to illite whereas in others it reflects the presence of kaolin-ite or a halloysite.

Prepared mirtures oJ montmorillonite and illite

Orcel (24) and Norton (22) have published curves of prepared mixturesof kaolinite and montmorillonite, but synthetic mixtures of other clayminerals have not been studied previously.

Differential thermal curves for illites and montmorillonite show endo-thermic peaks at 100-250'C. and about 900"C., and an exothermic peakfollowing the 900'C. endothermic peak. The substitution of one of theseclay minerals for another in a mixture is not reflected in any of the ther-mal reactions at these temperatures. Thermal curves for montmorilloniteshow an endothermic peak at about 700oC. and none between 500'C.

THERMAL ANALYSIS OF CLAY MINERALS

and 600"C., whereas i l l i te curves show an endothermic peak between500'C. and 600oC., and none at about 700oC. As shown in Fig' 9 the

substitution of montmorillonite for illite in a mixture is clearly reflectedquantitatively by the second endothermic peak.

l r l r l r l r l r t r l r l r l r l r l r t r I r I r I r I r I r I r I r I r Io roo 200 300 .@ goo coo 70 rD 90 l@

TEMPERAIURE - D€GRTES C

Frc. 9. Prepared mixtures of montmorillonite (5D) and

illite (4D). Scale A.

A. 9016 montmorillonite, lU/s illite.B. 7 5/6 rnontmorillonite, 2;570 illite.

C. 50/6 montmorillonite, 50/6 illite.

D. 25/ montmorillonite, 7 5/s illite.

B. l0le montmorillonite, 9016 illite.

F. 5/p montmorillonite, 9516 illite.

Prepared. mhctures of illite and. haolinite.

Illites show endothermic reactions at 100-250"C. and at about 900'C.

whereas kaolinite does not show a thermal reaction at either tempera-

805

806 RALPH E. GRIM AND RICHARDS A, ROWLAND

ture. The substitution of illite for kaolinite in mixtures is indicatedquantitatively by peaks representing these reactions in the curves asshown in Fig. 10. The final exothermic reactions are distinctive for eachof these clay minerals so that the substitution of illite for kaolinite is in-

l ' l r l r l r l r t r l r t r I r l r l r I r I r I r I r I r I r I r I r I r Io r@ 200 300 .oo loo 600 tE tdt 9@ rGt

TEMPERAIURC. DIGREES C

Fig. 10. Prepared mixtures of illite (4D) and kaolinite (3C). Scale B.

A,. 9 5/s illite, 5/6 kaolinite.

B. 9016 illite, 10le kaolinite.C. 7 57o illite, 25le kaolinite.D. 50/o illite, 50/p kaolinite.E. 25/6 illite, 7 5/ kaolinite.F. 10/6 illite, 90lp kaolinite.

dicated also by the portion of the curve showing these reactions. Bothkaolinite and illites show endothermic reactions between 500oC. and600'C. However, the mixing of kaolinite and illite is reflected to some ex-tent in the peak representing this reaction because of the much greatermagnitude of the reaction for kaolinite than for illites.

THERMAL ANALYYS OF CLAY MINERALS

M'i.s c ellane ous min er al s

In order to obtain the necessary background for the interpretation ofthe differential thermal analyses of clays, differential thermal analyseswere made of a large number of specimens of hydrous material. Many of

l r l r l r l r l r l r l r l r l r l r l r l r I r I r I r I r I r I r I r I r Io roo 200 300 400 500 600 700 €oo 9@ l@

I E M P € R A T U R C - O E C R S E S C

Frc. 11. Miscellaneous minerals. Scale A.A. Muscovite, University of Illinois collections.B. Talc, Vermont.C. Pyrophyllite, North Carolina.D. Pyrophyllite, Oreana, Nevada.E. Chlorite. Chester. Massachusetts.

these samples were obtained from the IJnited States National Museum,and the authors are indebted to Drs. W. F. Foshag and E. P. Hendersonfor their kindness in supplying them. Differential thermal curves of thesesamples are presented in Figs. ll to 14. In many samples the curves sug-gest that the species is not valid, but other characteristics of most of thesamples require study before this point can be settled definitely.

The curve for muscovite presented in Fig. 11A shows a broad endo-

808 RALPH E. GRIM AND RICHARDS A. ROWLAND

thermic reaction between 750'C. and 950oC., and small peaks from300'C. to 400"C. which may or may not be significant. This curve doesnot agree with those published for muscovite by Orcel (24) and Norton(22).It is quite possible that all the muscovite curves so far published arenot significant, because the coarse flakes permit only a small amount tobe packed loosely in the specimen holder. Consequently inherent limita-tions of the differential thermal method are magnified and reproducibleresults are difficult to obtain.

The curve for talc (sample 118) is in agreement with the curves forthis mineral published by Orcel (24) and Norton (22).

The curve for the pyrophyllite sample from North Carolina (sample11C) is believed to be significant for the mineral. The other sample ofpyrophyllite (11D) is not pure, and its curve is less significant. The curvefor sample l1C checks Orcel's (24) findings for pyrophyllite, but notNorton's (22).

Orcel (23) has published a large variety of curves for the chlorites butnone of them is l ike that shown for chlorite sample 11E. A study of manychlorites, combining r-ray, thermal, and chemical analytical data, isneeded before this group of minerals will be well understood.

Sample 12A, "nontronite" from San Luis, Potosi, Mexico, is shown bymicroscopic study to be composed of about thirty per cent quartz, tenper cent ferric iron hydrate, and the remainder a clay mineral that hasthe optical properties of nontronite. An r-ray difiraction analysis of thesample showed lines characteristic of clay minerals with an expandingIattice, in addition to lines for quartz and ferric iron hydrate. The difier-ential thermal curve is not exactly like the curves of known materials,and is particularly characteized by an endothermic peak between 400'C.and 500oC. Since the relative amount of ferric iron hydrate in the sampleis too small to cause this thermal effect, and quartz does not exhibit athermal reaction at this temperature, it seems likely that it results fromthe clay mineral with the expanding lattice. These data suggest that iron-rich clay minerals of the expanding lattice type exhibit a second endo-thermic reaction about 200oC. lower than the aluminous variety. Someferric iron hydrates show an endothermic reaction between 400"C. and500oC., and it may be difficult to determine if a peak in this temperatureinterval has resulted from a clay mineral or a ferric iron hydrate.

The difierential thermal curve of the "woody nontronite" (Fig. 128)indicates a mixture of illite, montmorillonite, and a ferric iron hydrate oran iron-rich expandingJattice type clay mineral. The optical characteris-tics of the sample (z:1.535, 'r- 'ot:.030) can be measured accurately,and again on the basis of optical data alone the sample might easily bedesignated as a single homogeneous mineral.

THERMAL ANALYSIS OF CLAY MINERALS 809

T E M P E R A T U R E . O E G R € E S C

Frc. 12. Miscellaneous materials. Scale A.A. "Nontronite,t' San Luis, Potosi, Mexico, U. S. Nat. Mus. No. R6685.3. "fioody nontronite," Howard County, Arkansas.C. White Hector clay, Hector, California.D. Bravaisite, Noyant Allier, France, U. S. Nat. Mus. No. 4918.B. "Smectite," Cilly, Austria, U. S. Nat. Mus. No. 13098.F. "Leverrierite," April Fool Hill, Manhattan, Nevada, U. S. Nat. Mus. No. 3819.G. "Leverrierite," Salida, Colorado, U. S. Nat. Mus. No. 94962.

The sample of white magnesium clay from near Hector, California,described by Foshag and Woodford (7), gives a unique curve (Fig. 12C).The curve is unlike the montmorillonite curves, suggesting that the ma-terial should not be classed in this group of clay minerals. Under themicroscope, the sample seems to be a single species.

l r l r l r l r l r l r l r l r l r l r l r I r I r I r I r I r I r I r I r I r IO IOO 2Oo 3OO .lOO 5OO 6{tO XtO COO 9@ l@

810 RALPH E. GRIM AND RICHARDS A, ROWLAND

Bravaisite from the type locality yields a difierential thermal curve(Fig. 12D) l ike that of the i l l i tes with an additional endothermic peak

close to 700"C. suggesting the presence of a small amount of montmoril-

lonite. The chemical composition of bravaisite (Table 5) and its opticalproperties (t:1.552, ^y - oL:.025 ( - )) are also similar to the i l l i tes.

Tlsro 5. Cnrurcer- ANar,vsns or Irr.rrns aNn Bnav.qlstrn.

4D t2D4C

SiOzAl2o3FezOsFeOMgoCaONazOKzOTiOzIgn. lossTotalHzO*HrO-SiOz/RzO:sioz/Al:or

52.2325 .854 . 0 4

2 . 6 90 . 6 00 . 3 36 . 5 60 . 3 77 .88

100.557 .881 . 1 33 . 1 33 . 4 3

51.2225.914 . 5 9r . 7 02 . 8 40 . 1 60 . 1 76 . 0 90 . 5 37 .49

r00. 707 . 1 41 . 4 53 . 0 23 . 3 6

54.642 6 . 2 9

1 . 5 10 . 3 83 .031 . 4 0o . r 25 . 9 40 . 4 56 . 6 8

100.446 . 6 14 . 2 83 . 4 13 . 5 3

Analyses made under the supervision of O. W. Rees, IIl. State Geol. Survey.

4C lllite, purified from shale, Alexander County, Illinois.

4D Illite, purified from underclay, Vermilion County, Illinois.

12D Bravaisite, Noyant Allier, France.

The differential thermal curve for the smectite sample (Fig. 12E) is

l ike that of montmoril lonite. The presence of a small amount of i l l i te is

suggested by the endothermic hump between 500oC. and 600oC., and

the exothermic reaction between 250'C. and 450'C. indicates the pres-

ence of organic material. This is in agreement with Kerr's (17) conclusion

that smectite is not a valid species because of its similarity to montmoril-

lonite.The curves for two samples labeled leverrierite (Figs. 12F and 12G)

are like those of montmorillonite. The second endothermic peak of sam-

ple 12F occurs at a temperature slightly above that usually found for

montmoril lonite, but probably not too high for that identif ication. Sam-

ple 12F has the optical properties of montmorillonite, whereas sample

12G appears to be only faintly anisotropic with a mean index of refraction

of about 1.470. On the basis of optical data the latter sample would be

classed as allophane, illustrating again the caution necessary in identify-

THERMAL ANALYilS OF CLAY MINERALS

ing clay minerals from a single criterion. The very low apparent birefrin-gence of sample 12G is probably due to a random orientation of the claymineral particles mixed with some isotropic material. X-ray diffraction

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TEMPERATURE - OEGRECS C

Frc. 13. Miscellaneous materials. Scale A.

A. Rectorite, Garland County, Arkansas, U. S. Nat. Mus. No. 80607.

B. Celadonite, Verona, Italy. U. S. Nat. Mus. No. 93080.

C. Attapulgite, Quincy, Florida.D. "Chloropal," Hungary, U, S. Nat. Mus. No. 80644.

E. Volkonskoite, near Sverdlosk, U. S. S. R. !

F. "Allophane," Bedford, Indiana, U. S. Nat. Mus. No. 95335.

G. "Allophane," Iyo, Japan, U. S. Nat. Mus. No. 93052.

8 1 1

812 RALPE E. GRIM AND RICHARDS A. ROWLAND

analyses of both samples gave good patterns characteristic of the expand-ingJattice type clay minerals. Samples of leverrierite studied by Rossand Kerr (29) proved to be kaolinite, and de Lapparent (19) has de-scribed leverrierite as an intergrowth of kaolinite and mica. Clearly theterm does not designate a valid mineral species.

The difierential thermal curve for the sample of rectorite (Fig. 13.A) isunique. Under the microscope the sample has the appearance of a halloy-site with some addiiional hydrated ferric iron oxide which may be re-sponsible for the slight endothermic hump in the curve at about 450'C.

The difierential curve for the celadonite sample (Fig. 13B) is uniqueexcept that the three endothermic peaks are somewhat similar to theendothermic peaks of the illites. The optical properties of the sample areIike those of glauconite. The sharp exothermic reaction at about 450'C.probably results from organic material.

Attapulgite gives a curve (Fig. 13C) unlike that of the other mineralsstudied and this supports the conclusion (1) that the material is a distinctspecies.

The differential thermal curve for the sample of "chloropal" (Fig. 13D)is much the same as the curve for the "nontronite" from San Luis, Po-tosi, Mexico. An r-ray analysis of the "chloropal" sample indicated thatthe prominent constituent has an expanding lattice. Therefore, this sam-ple provides further evidence that certain clay minerals with an expand-ing lattice may show an endothermic peak between 400'C. and 500oC.The samples of expanding lattice material with the endothermic peakbetween 400"C. and 500oC. show two exothermic peaks between 800oC.and 1000'C., and they do not show a third endothermic peak. fn all ofthese characteristics the curves are unlike those of the expanding-latticematerial with an endothermic peak at 600' to 700'C.

The curve for the sample of volkonskoite (l3E) is distinctive becauseit shows no endothermic reaction above 250oC., i.e., no thermal reactioncorresponding to a loss of lattice water. Under the microscope the vol-konskoite appears to be an amorphous substance with a pigmentary ma-terial distributed through it. The materjal is listed by Dana (5) as achrome-bearing clay.

The sample labeled "allophane" from Bedford, fndiana, (13F) gavea curve suggesting a mixture of kaolinite or a halloysite because of the525'C. endothermic and 950oC. exothermic peaks, gibbsite because ofthe 350'C. endothermic peak, and amorphous material. The 925'C.exothermic peak is difficult to explain but it is probably due to the amor-phous material. This and other samples suggest that amorphous hydrousaluminum silicate materials show only a single endothermic reaction

THEKMAL ANALYSIS OF CLAY MINERALS

I ' l r l ' l , l ' l r l r l r l ' l ' l r I r I r I r I r I r I r I r I r I r Io oo ato 300 400 soo aqt 700 800 900 l@o

TETIPERATURE . OECRCES C

Frc. 14. Miscellaneous materials. Scale A.A. Racewinite, Bingham, Utah, U. S. Nat. Mus. No. R4759.B. Dillnite, Schemnitz, Hungary, U. S. Nat. Mus. No. R4795.C. Severite, Elko, Nevada, U. S. Nat. Mus. No. 13511.D. Miloschite, Rudniak, Serbia, U. S. Nat. Mus. No. 48649.E. Collyrite, Fichtelgebirge, Bavaria, U. S. Nat. Mus. No. R4793.F. Cimolite, Sverdlosk, U. S. S. R., U. S. Nat. Mus. No. R4765.G. Newtonite, Newton, Pennsylvania, U. S, Nat, Mus, No. R4760.

813

814 RALPH E. GRIM AI{D RICHARDS A. ROWLAND

which occurs below about 250oC. Optical study of the sample indicates

a mixture containing a considerable amount of kaolinite or a halloysite.

The sample of "allophane" from fyo, Japan, gives a differential thermal

curve (Fig. 13G) Iike that of a halloysite except that the exothermic reac-

tion takes place at a lower temperature. The sample has the optical char-acteristics of a halloysite.

The sample of racewinite (1aA) from the type locality appears under

the microscope to be a mixture of amorphous material and carbonate,and this is the composition suggested by the curve. The endothermic peak

between 750oC. and 875'C. is in the range in which carbonates show en-

dothermic reactions,* and the endothermic reaction below 250oC. is in

the range of the thermal reaction of amorphous material.The sample of dillnite yielded a difierential thermal curve (Fig. 14B)

exactly like kaolinite, and the optical characteristics of the sample are

identical with those of kaolinite.The curve for the sample of severite (Fig. 1aC) is also similar to the

curve for kaolinite, except that the initial low temperature endothermicpeak suggests the presence also of a halloysite. The optical characteris-tics of this sample are like those of a halloysite or kaolinite.

The rniloschite sample gave a difierential thermal curve (Fig. 14D) in-

dicating the presence of kaolinite andf or a halloysite. The initial endo-

thermic peak is broader than that for hydrated halloysite and probably

indicates the presence of some amorphous material. Optical study of the

sample shows material with the optical properties of a halloysite pig-

mented with amorphous material. Miloschite is l isted by Dana (5) as a

chromiferous clay.The curve for the sample of collyrite (Fig. 1aE) is also like that for

kaolinite andf or a halloysite. The optical characteristics of the sampleare like those of a halloysite.

The differential thermal curve for the sample of cimolite (Fig. 14F) is

unlike the other curves. An *-ray diffraction analysis shows the presence

of alunite, but further study is required to determine if the curve is char-

acteristic of alunite.The curve for the sample of newtonite (Fig. 1aG) suggests the presence

of kaolinite andf or a halloysite because of the endothermic peak be-tween 500oC. and 600oC. and the sharp exothermic peak at about950'C. The endothermic peaks between 700oC. and 900oC. indicatethe presence of additional material which cannot be identified. Under themicroscope the sample has the optical characteristics of kaolinite. Ross

and Kerr (29) have shown that samples of newtonite studied by themwere actually kaolinite.

* Unpublished work by R. A. Rowland and F. L. Cuthbert: Illinois State Geological

Survey, Urbana, fll.

THERMAL ANALYSIS OF CLAY MINERALS 815

Suuuanv aNo Drscussrow

Kaolinite and halloysite type clay minerals are characterized by asharp endothermic reaction between 500'C. and 600"C., and by a veryabrupt exothermic reaction at about 950'C. Hydrated halloysite showsan additional sharp endothermic reaction between about 100oC. and200'c.

I l l i tes give endothermic peaks between 100oC. and 250oC., 500'C.and 650oC., and at about 900oC. and an exothermic peak immediatelyfollowing the third endothermic peak. Montmorillonite gives a curvesimilar to that for illites except that the initial endothermic curve islarger and the second endothermic reaction takes place at about 100'C.higher temperature (600'to 700'C.). For both of them the exothermicreaction shifts to a slightly lower temperature as the iron content in-creases.

The third endothermic reaction at about 900oC. has not been foundin two-layer-lattice clay minerals, and seems to be characteristic of thethree-Iayer types, i.e., illites and montmorillonite. Illites and montmoril-lonite are largely dehydrated in the temperature range of the secondendothermic peak, but their lattice structure is not destroyed until ahigher temperature is reached, and this is followed at once by the forma-tion of spinel. It is believed that the third endothermic peak correspondsto the final destruction of the lattice and the exothermic reaction to theformation of spinel.

fn the case of the three-layer clay minerals an endothermic peak be-tween 500"C. and 600"C. seems to be characteristic of the non-swellingtype (illites) and one between 600'C. to 700'C. appears to be character-istic of the swelling type (montmorillonite). No swelling material hasbeen found to have an endothermic peak between 500"C. and 600'C.and only one example of a non-swelling material (metabentonite, sample68) has been found to have an endothermic reaction between 600oC. and700'C. However, the sample of "nontronite" from Mexico (12A) and thesample of "chloropal" (13D) which gives a thermal curve similar to thatof "nontronite," suggest that iron-rich expandingJattice-type clay miner-als may show their second endothermic peak at a Iower temperature(400'C. to 500'C) than that of the illites, and in the same temperatureinterval in which the endothermic reaction in some ferric iron hydratestakes place.

Kaolinite and halloysites require considerably more energy for dehy-dration than illites or montmorillonite. The temperature difierence indi-cated for this reaction in kaolinite and halloysites is about ten times thatof the other clay minerals. Because of this large difierence, apparatus de-signed for kaolinite may not detect the thermal reactions of the otherclay minerals unless special provisions are made to vary the vertical ex-

816 RALPH E, GRIM AND RICHARDS A. ROWLAND

aggeration of the record of the thermal reactions by varying the resist-ance in the galvanometer circuit of the difierence thermocouple.

Pore water in the samples does not show in the thermal curves above100'C. Montmorillonite, illites, and hydrated halloysite all show initialendothermic reactions below about 200'C. It is suggested from informa-tion regarding the structure of montmorillonite that this water is held onthe basal planes of the unit cells and that it may have distinctive proper-ties. It corresponds to "planar water" as defined by Kelley (16) and hiscolleagues. The thermal data suggest that certain clay minerals, e.g., il-lites, can have some of this type of water without also having an expand-ing lattice.

The thermal evidence indicates that many bentonites and other claysthought to be composed of a single clay mineral are actually mixtures ofclay minerals probably closely intergrown. Many of the bentonites aremixtures of illite and montmorillonite; others are mixtures of kaoliniteandf or a halloysite and montmorillonite. fn some of these clays mont-morillonite does not seem to be the dominant constituent. X-ray andoptical dath are in accord with these conclusions.

A series of samples that have been classed as beidellite, including ma-terial from the type locality at Beidell, Colorado, gave differential ther-mal curves indicating that they are not composed of a single species, butare mixtures of clay minerals. X-ray diffraction data are not opposed tothis conclusion. Some of these mixtures yield optical data that suggesthomogeneous material and so might easily lead to erroneous identifica-tions. The sample from the Princess Mine, Namiquipa, Chihuahua, Mex-ico, is a particularly good example because c-ray and thermal data showdefinitely that it is a mixture primarily of kaolinite andf or a halloysiteand a ferric iron hydrate, whereas its apparent moderately high bire-fringence and indices of refraction would not suggest this identification.This does not reduce the value of optical studies in clay mineral investi-gations, but it does emphasize the fact that trustworthy clay mineralidentifications usually require the study of more than one set of proper-ties. Undoubtedly the literature is full of clay mineral identifications thatare inaccurate because they are based either on optical, r-ray, thermal,or chemical data, but not on a combination of several of these data.

Differential thermal analyses are presented for a series of additionalmiscellaneous hydrous minerals that were investigated primarily to pro-vide a background for the clay mineral study. These samples were ob-tained mainly from the collection of the U. S. National Museum. Someof the salient conclusions from this work are as follows:

Attapulgite and the white magnesium clay mineral from Hector, Cali-fornia, each show distinctive differential thermal curves unlike that of

THERMAL ANALYSIS OF CLAY MINERALS

montmorillonite or any other clay mineral. Bravaisite from the type lo-cality has the same thermal characteristics, optical properties, andchemical composition as the illites. A sample of celadonite also yielded adifferential curve similar to that of the illites. A sample of smectite hadthe thermal properties of montmorillonite, which is further evidence forKerr's (17) conclusion that smectite is montmoril lonite.

Two samples of leverrierite proved to be montmorillonite. Other sam-ples studied before by Ross and Kerr (29) and by de Lapparent (19) haveanother clay mineral composition so that the term can have no standingas a specific mineral name.

A sample of volkonskoite appeared from its curve to be a mixture ofamorphous material and a pigmentary material, perhaps chromic oxide.Similarly a sample of racewinite from the type locality seems to be a mix-ture of amorphous material and a carbonate.

One sample labeled allophane gave a differential thermal curve sug-gesting a mixture of gibbsite, amorphous material, and a halloysite. An-other sample of allophane from another locality proved to be chiefly ahalloysite.

Samples labeled respectively dillnite, severite, miloschite, and collyritegave differential thermal curves exactly like that of kaolinite andf or ahalloysite. Optical data confirm this classification of the material. A sam-ple of newtonite also gave a difierential thermal curve of the same type,but had in addition several small peaks indicating unidentified constitu-ents.

Sa,mples labeled cimolite and rectonite yielded differential thermalcurves different from those of any other materials studied.

Rprnnnncps

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818 RALPH E. GRIM AND RICHARDS A. ROWLAND

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