C'I,ATS O F T H E
2 :1
TI-PE
Hew oitly otic type of striictiiral i i t i i t is k i i o n - i i , i i i ivhicli t h e nliimina shwt is s\-tiimctric~allyrontlmisd rvith t i v o silic8:i slic~r,ts. T h i s i.s thc. striirtiirc of thr, niirns ant1 of thc c~los(~ly ix31:ttt~cl~-crniic~ilitc~s;. ' 1 ' 1 ~ hiiiiplest) mrnilx~r.+of this class of iiiinc.r:ils : i r ~ ~)yropttyllitc~, A\120z~ 1Si02.H20, :znd talr, lIg30:' 4Si02'H 2 0 . 'I'hrw arv c~oiiil)nr:iti~.c~ly w l l c~rystallizc~cl :i,s they o(3riir i n ii:itiirf>,and thcir striirtiirc~slia\.t. 1 ) w i i (l(lteriiiiit(v1( 8 ) . Hofiiiatiit, Endell, :ind li7iltii (13) first pointrtl out the rlow similarity i u x-ray cliagraiu l w t u w i i pyrophyllite (figiirc 2a) atid iiiotit~iiorillotiitr~ (nhiclt is the iiiajor c~oiistituciitof riiost hcntonitrs), the> r1iit.f tliffr~rrnc~e lieing that the. 1nttr.r .sho~vccla .siitglc liiir closc t o the rciitral 1ww1n \vhosc :i 1igii1a r (lefl e('t i o 11 var ied iii 1 - r m r1y rvi t h t ht, 111oist iir~ c o I i t ( > t i tof t hc. .< R iii ple . ?'hiis t h r y \\-ere led to the valiin1)lc cwnc*cptof n i i r~xpancling]:it t i w foriitctl of pyro~)hyl!itc.slic.cts whose distaiiw apart T.xrictl wit'li th(>\rRtcr coiiteiit of thc ralay. Bcirlcllitc, found in a fci\ hciitonitw and it1 ninny (slay soils, i v w aft mvarc1.s sho\\-11to gi1.e precisely thc saiiie x-ray liiic.s, iiic~ludi~ig the \.nrialilc onr (14). Griiiicr ha,< esteiidcd the series to include noutronitr, a clay iiiinc3ral in ~rhichferric iron Itas largely replactd aliiniiniim (9). Considering n o ~ vthe chemical formulas for thew miiiwals, the nearest simple approximations derived from thc chemical formiilxs \\-ere taken as follon-e: montmorillonite, IIgO ~ A ~ 1 2 5Si02 0 n . ~ n H z O ;l)eidellite, .&On.3Si02,n H 2 0; noiitronite, Fc20a.3Si02 .iiH20. In order to reconcile these and the :iiialyses on u-hicli they are ba.sed with the pyrophyllite iiiiits of structure, .4l2O3.4Si02.H20,the follon-ing replacements arc foiincl to he nrces.sary. They may occur independriitly or together. Three magnesium atoms (ha11 replace two aluminum atoms in the gihhsitc sltcet, as pointed out 131- Hofrnaiin, Endell, aitd lViltii (13). It is al,m possible for magnesium plus one equi\-alent of another cation to replace aluiiiiiiiiiii (19). Iron caii replace aliiiiiitiiiiii, the iron lwing either hi\-alent or trivalent. The former case is similar to that of magnesium, h i t docs riot appear t o be of great importancc in iiatiirally occurring clays. Replacement by
COLLOIDAL P R O P E R T I E S AND STRUCTURE O F CLAYS
939
phyllite unit. On the other hand the change from the non-exchangeable to the exchangeable form must be regarded as more deep-seated. Depending on circumstances, it may or may not involve actual breakage of the valency bonds holding the framework atoms together. An instance of very deep-seated changes accompanying the liberation of non-exchangeable magnesium is aff orded by the fine grinding experiments first performed by Kelley, Dore, and Brown (16). They showed t h a t the base-exchange capacity of many clays could be considerably increased by grinding. It was concluded ( l i ) that the essential part of the grinding process was to make the niagnesium more readily accessible by cleavage along or breakage across the structural planes of the clay. Xray studies indicated t h a t amorphous material accumulated as grinding proceeded, suggesting actual destruction of the crystal lattice (15). Optical and chemical evidence strongly supports this conclusion. h sample of Putnam clay (< 100 mp) was ground in a rubber-lined ball mill with agate balls for 168 hours. The electrical double refractions of the ground and unground materials were then compared. both before and after electrodialysis. Before electrodialysis the ratio of the electrical double refraction of ground t o unground material was 0.18; after electrodialysis it was 0.09. The base-exchange capacities as determined by the ammonium acetate method were '77 (unground) and 131 (ground) milliequivalents per 100 grams of ignited clay. Differences in chemical character were revealed by washing with N/10 acetic acid. The unground clay released only 0.067 per cent of sesquioxides (ignited basis) in three successive treatments and none thereafter. The ground clay released 10.38 per cent of sesquioxides in nineteen successive treatments, the last of which gave 0.26 per cent, i.e., more than the three treatments on the unground clay. Silica was also brought into solution. Thus the ground clay can no longer be regarded as a beidellite; its behavior approaches more nearly that of a permutite, mixed with a little unchanged beidellite. Fine grinding, therefore, does not simply cleave the crystals in their planes of weakness, but breaks and distorts every part of the lattice. The exchange capacity of the new material will depend on its particular molecular structure, and it need not bear any simple relationship to the value for the unground material. Besides these clays with an expanding lattice TW have indications of certain less reactive clays which more nearly approach the micas in chemical composition. Hofmann, Endell, and Wilm (14) found in certain clay soils a mineral similar to montmorillonite in its x-ray diagram but without the expanding lattice. Grim and Bray (4) have characterized a sericitelike mineral in shales and in clay soils derived from shales and have drawn attention to its importance. It is possible that these minerals may bear close relationship to the vermiculites investigated by Gruner (10).
940
C. E. MARSHALL
THE REACTIVE CLAYS AS COLLOIDAL ELECTROLYTES
This picture of a montmorillonite or beidellite crystal leads us t o anticipate some unusual properties. One might expect t h a t the distance apart of the structural pyrophyllite units would be influenced in several ways. It has been shown t h a t polar organic molecules can replace water between the sheets (14) and that non-polar liquids are unable to force the sheets apart. Experiments with different cations hare shown t h a t in equilibrium with liquid water the spacing between the sheets is nearly the same for the calcium and hydrogen clays (14 -4.U.) but much greater for the sodium clay (>23 A.U.). At lower vapor pressures the three ions give almost identical spacings (11). The ease with which the spacings can vary suggests t h a t materials may exist in JThich a t a given instant the spacings may have different values in the same crystal. That is, instead of having a single characteristic spacing there would be a distribution of spacings caused, for instance, by differing replacements and hence differing resultant charges on different layers. One can regard the resultant charge as holding the lattice together, this tendency being opposed by the disrupting action of the water dipoles. Comparing a fully hydrated with a dehydrated clay, it will be clear t h a t the sheets are much less firmly held together in the former. On the other hand, the orientation of the water dipoles in the intense ionic field can be regarded as a stabilizing influence in the hydrated system, since the orientated dipoles will tend to hold the sheets together more efficiently than would non-orientated molecules (26, 27). Considering this electrical field between the layers, we can form a close estimate of the surface density of charge by recalculation of the baseexchange capacity in terms of ions per unit cell. For beidellites the values range around 0.4 to 0.5, and for montmorillonites around 0.5 t o 0.7 equivalents per unit cell. I n calculating the density of charge these values should be halved, since each unit cell exposes two surfaces. The smallest value given above corresponds t o 4.45 X l O I 3 electrons per cm.2 and the largest to 7.8 X 1013;conversion to practical units gives 7.1 X lo4 and 12.4 X coulombs per cm.2,respectively. I n making these calculations complete dissociation of the exchangeable ions from the surface is assumed. The distribution of the positive dissociated ions in the water between the lattice sheets is unlikely to be uniform. If it were, then one could calculate the strength of the ionic solution betm-een the sheets to be about 2.3 N for the calcium montmorillonite investigated by Hofmann and Bilke ( l l ) , in which the exchange capacity is 0.69 per unit cell and the free space between the layers is 11.1A.U. (c = 20.7 A.V.). For the sodium clay, which gives a wider spacing, the value would be considerably less than this. The hydrogen clay, with practically the same spacing as the calcium clay, would probably not be completely dissociated, and in any case
COLLOIDAL P R O P E R T I E S A l i D STRUCTURE O F CLAYS
941
hydrogen clays behave as hydrogen-aluminum clays, owing to reaction of hydrogen ions with aluminum of the lattice (23). The electrical double layer surrounding the particle may be considered t o arise in two ways: (a) by dissociation from the plane surface of the lattice sheet, just as occurs between lattice sheets, and ( b ) by the unsaturation arising from the breakage of bonds at the edge of the sheets. Taking a sheet which is large in comparison with its thickness, then one can use the charge per unit cell as calculated from the base exchange and the potential of the electrical double layer, in order to calculate the thickness of the double layer normal to the plane of the sheet. Thus for sodium-putnam clay (beidellite) with an exchange capacity of 0.48 ion per unit cell, Baver (1) found a potential of 0.0454 volt. Using the condenser formula 4roe
{=-
D
and assuming the normal value for D, the effective thickness of the double layer is found to be 3.8 X cm. This, however, is to be regarded as an artificial value. since D should probably riot be 81, owing to the orientation of the water dipoles and, further, the assumption of complete dissociation may be incorrect. If the structure proposed for the reactive clays is correct, then each clay particle may be regarded as an electrical insulator in a 'direction normal to the plane of the sheets and a good conductor in the plane of the sheets. It is interesting to inquire whether there is any experimental evidence which would confirm these peculiar properties. It is known t h a t clay particles orientate themselves in the electric field (20) ; this orientation has been used in order to measure their double refraction. I n electrodialysis such an orientation occurs along with a compaction of the clay to form a solid mass on the anode membrane. When electrodialysis experiments are carried out with increasing quantities of clay, it is found t h a t the current passing shows an increase of many times its former value as soon as the clay has formed a solid structure between the fixed membranes. If this structure is broken by means of a glass rod, then the current suddenly falls off, although the total amount of conducting material remains the same.? Experiments have been carried out in order to determine whether this effect has any connection with the thixotropy of clays. Determinations of conductivity on a thixotropic sodium-putnam system only showed a slight increase (about 5 per cent) on setting. Thus in t h e thixotropic system there is little or no preferred orientation. The experiments on electrodialysis would lead one to conclude t h a t there the clay particles 2
Dr. R. Bradfield first told me of this last demonstration.
942
C . E . MARSHALL
arrange themselves with the plane of the sheets in the electrical lines of force. MECHANICAL PROPERTIES O F T H E CLAYS
I t will be clear from this description of clay structures that these may influence the bulk mechanical properties in several nays. I n both groups of clays the platy character will facilitate the arrangement of adjacent particles in quasi-parallel groups. I n addition, the high degree of hydration of the reactive clays may make it possible for comparatively slight mechanical forces to break the single crystals by the gliding of one plane over another. These effects have not yet received theoretical treatment, although they would appear to be of importance in studies of plasticity and cohesion. REFEREK-CES (1) BAVER, L. D . : Missouri Agr. Expt. S t a . Research Bull. 129 (1929). (2) EDELMAX, C . H . : Trans. Intern. Congr. Soil Sei., 3rd Congr., Vol. 111, p. 37 (1935). (3) FOSHAG. W.F., ASD WOODFORD, -4. G.: Am. Mineral. 21,238 (1936). (4) GRIN,R. E., . ~ N DBRAY,R. H.: J. Am. Ceram. Soc. 19, 307 (1936). ( 5 ) GRUNER, J. 11.: Z. Krist. 83,75 (1932). (6) GRUSER,J. W.:Z. Krist. 86,345 (1933). (7) GRUNER, J. R.:Z. Krist. 83,394 (1932) J. W.:Z. Krist. 88, 412 (1934). (8) GRUNER, J. W.:Am. Mineral. 20,475 (1935). (9) GRUNER, (10) GRUSER,J. W.: Am. Mineral. 19,557 (1934). (11) HOFMASK~ U., AKD BILKE,R.: Kolloid-Z. 77,238 (1936). (12) HOFMASN, I-., ENDELL, K . , A N D BILKE,W.: Z. Elektrochem. 41,469 (1935). (13) HOFMASS, L-., EXDELL, K., A N D ~ V I L MD.: , Z. Krist. 86, 340 (1933). U.! EXDELL,K . , ASD WILM,D . : Z . angew. Chem. 47,539 (1934). (14) HOFMASK, IT. P . : Trans. Intern. Congr. Soil Sei., 3rd Congr.. Vol. 111, p. 88 (15) KELLEY, (1935). (16) KELLEY,W.P., DORE,1T. H . , ASD BROWN, 9.11,: Soil Sci. 31,25 (1931). (17) KELLEY, W .P., .4ND JENNY, H . : Soil Sei. 41,367 (1936). (18) LARSEN, E. S., ASD STEIGER, G.: Am. J. Sci. 1 6 , l (1928). (19) >xARSH.4LL, C. E.: z. Krist. 91, 433 (1935). (20) MARSHALL, C . E . : Trans. Faraday Soc. 26, 173 (1930). (21) MEHMEL, 11.: Z. Krist. 90,35 (1935). L.: Proc. Natl. Acad. Sei. U. S. 16,123,578 (1930). (22) PACLING, H . . AND MARSIIALL, C . E.: J . Sor. Chem. Ind. 63, 750 (1934). (23) PAVER, (24) Ross, C. s.,BND & R R , P. F. : C , s. Geol. Survey, Prof. Paper 165-E (1931). (25) Ross,C . S., ASD KERR, P. F. : U. S. Geol. Survey, Prof. Paper 185-G (1934). E . W . : Phil. Trans. 233, 361 (1934). (26) RUSSELL, (27) WISTERKORN, H. F . : Soil Sci. 41,25 (1936).
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