Hydroxyl Orientation in Kaolinite, Dickite, and Nacrite 473

AmerictttrMineralogist. Volume 58,pages 471-479, 1973

HydroxylOrientation in Kaolinite,Dickite, andNacrite

R. F. GIrse,Jn. eNo P. Deru Departntentof GeologicalSciences, Stste Uniuersity of New York at Bufialo, P. O. Box U, StationB, Bufralo,New York

Abstract

The orientationsof the hydoxyl groups in the minerals kaolinite, dickite and nacrite have been determined by an iterative process based on the minimization of the total electrostatic energy of the respectivecrystal structures.The hydroxyl shared by the tetrahedral and octahedral sheetsis directed toward the empty octahedral sites in all three structures.The remaining three hydroxyls which form the surface of the octahedral sheet are nearly normal to the layer in dickite, two are nearly normal, and one is inclinedat an angle of 38" to the kaolin layer in nacrite,and in kaolinite two are nearly normal and the third is inclinedat an angle of 14" to the layer.In eachmineral, the hydroxylsforming the surfaceof the octahedralsheet are directedaway from the kaolin layer.

Introduction kaolin minerals (Table I ) reveal two distinct types The generalprinciples of the crystal structuresof of oxygen-hydroxyl distances: one group less than the kaolin minerals (Al:SizO;(OH)u) kaolinite, 3.0 A and the other group greater than 3.0 A. The dickite, and nacrite have been known for some time observation of the close proximity between OH and (Brindley and Robinson, 1946; Newnham and O of neighboringlayers had led to the proposalthat Brindley, 1956; Hendricks, 1938). Recent refine- there is a long hydrogen bond between each inter- ments of the structuresof kaolinite (Zvyagin, 1967; layer hydroxyl and the corresponding basal oxygen Drits and Kashaev, 1960), dickite (Newnham, in the next layer (Newnham, 1961; Grim, 1968; 1961), and nacrite (Blount et ql, 1969) show sub- Farmer, 1964; Farmer and Russell, 1964). The stantialdifierences in detail from the original descrip- alternateview that some of the interlayerhydroxyls tions. None of thesestudies provided direct informa- are involved in long hydrogen bonds and some are tion about the positions of the hydrogen atoms in not has also been proposed (Wada, 1967; Ledoux the structures. and White, 1964a; Serratosa et al, 1962, 1963; The kaolin layer consistsof a tetrahedral sheet Wolff, 1963). The location of the inner hydroxyl hy- with silicon filling the tetrahedralsites and an octa- drogen is also uncertain.On the basisof infrared ab- hedral sheet with aluminum filling two thirds of the sorption data, Serratosa e/ ql (1962, 1963) and octahedralsites. These two sheetsshare a common Wolff ( 1963) concludethat the inner OH is perpen- plane of oxygen/hydroxyl atoms. Of the four hy- dicular to the kaolin layer in kaolinite and is di- droxyls in the formula, one is in this sharedplane of rected toward the opening in the tetrahedral sheet atoms and will be designatedin this paper as the which is formed by a ring of six tetrahedra.Ledoux inner hydroxyl. The remaining three form the outer and White (1964a) propose an orientation toward surface of the octahedral sheet and are designated the empty octahedral site. inner surface hydrox-,-lsas distinct from hydroxyls The disagreementregarding the orientationof hy- on the surface of the clay crystal. All the kaolin droxyls in the kaolin minerals indicates the difficulty mineralscontain this layer, but the manner in which involved in interpreting infrared absorption spectra the layers are stacked differs for each. Hendricks and suggeststhat a different approach might be more ( 1938) noted the importanceof the juxtapositionof fruitful. It has been shown that the orientation of the basaloxygen atoms of the silicatetrahedra in one the hydrogen atoms of water molecules is such that layer and the hydroxyl oxygen atoms of the alumi- the electrostatic energy of the crystal is minimized num octahedrain the next layer. The distancesfor (Baur, 1965; Ladd, 1968). This has beenextended the three O-OH contacts between layers in the to the orientation of hydroxyls in simple compounds 471 472 R, F" CIESE, AND P. DATTA

TesrE.l. Oxygen-(Hydroxyl ) OxygenDistances between pairs of ions where both have known positional Kaolin Layers parameters,2) pairs where both haveunknown positional parameters,and 3 ) pairs where one ion is miner€ l O - O(H) distence re ference known and the other is not. keolinite o(f)"- o(g) 2.9OA Zvyagtn (1967) The location of the hydrogen ions involves loca- o(2) - o(e) 2,89 o(3) - o(7) 3.02 tion of the minimum of 6 as a function of the dicklte 0(2) - o(1) 2.95 Newnhm (1961) hydrogen ion positional parameters.Since' the O-H o(3) - o(3) 310 o(4) - o(2) 2.97 distance is fixed, as is the position of the oxygen, the positional parameters is best nacrite 0(2) - O(1) 3,06 Blount et a1 (1969) the variation of o(3) - o(2) 2.97 done in terms of spherical coordinates.For each o(4) - o(3) 2.98 hydroxyl oxygen ion, a referenceaxis is established hydroxyl oxygen atms which goes through the oxygen ion and is approximately perpendicularto the clay layers. This direction (Fig. I ) is then used as an axis for the genera- such as diaspore, AIOOH, and goethite, FeOOH tion of a cone of half angle o. The program computes (Giese et al, 1971) and complex silicatessuch as hydrogen atom positions which lie on the specified muscovite(Giese, l97l). This techniqueof locating cone at 0.97 A distancefrom the oxygen ion. The hydroxyl hydrogensby minimization of the electro- number and spacingbetween points on the cone as static energyhas been applied to the kaolin minerals well as the cone angle may be varied and the energy and the results are reported here. for each point can be calculated. The "correct" orientation for the OH is the minimum and is ob- Method tained by interpolation. If the crystal structure has If the atomic positional parametersof an ionic only one hydroxyl hydrogen in the asymmetricpart compound are known, then the electrostaticenergy of the unit cell, such as in 2M1 muscovite,the elec- is obtained from the relation (Sherman. 1932) trostatic energy 4 consistspredominantly of type I terms with a smaller contribution from type 3 and a: -if,;1?' (,) smaller still from type 2. As d approachesthe mini- 2 ",^ ?, r,i mum (by varying the hydroxyl hydrogen ion posi- where e is the charge on the electron,Z is the ionic tion), the terms of types 2 and 3 also approachthe charge, N is the number of atoms in the unit cell, correct values and the procedure is complete once and r is the interatomic distance. A more rapidly the minimum is found. converging algorithm has been described by Bertaut For the kaolin minerals where there are four (1952) and programmed for digital computer by hydrogenions in the asymmetricpart of the unit cell, Baur (1965). We have used a modificationof this the initial set of seventeenatoms consists of positions program. for thirteen non hydrogen atoms and four "guesses" In using equation ( 1) we assumethat 1 ) the non for the hydrogen atoms. For each hydrogen the hydrogen ion positional parametersare correct; 2) previously described procedure will yield a "mini- the ions have full charge; 3) the hydroxyl oxygens mum" which is a better approximation than the have been correctly identified;and 4) the O-H dis- original guess but is not correct since 6 contains tancesare known. There is no questionabout which terms of the second and third types which are still oxygens are the hydroxyl oxygens and, for the pur- inaccurate becausethey involve hydrogensnot yet posesof this study, the O-H distanceis held fixed at refined. But if the procedureis repeated,the values 0.97A. The positionalparameters for nacriteare from for the positional parametersof the four hydrogen Blount et al. ( 1969). those for dickite are from atomswill convergeto the correctones. This iterative Newnham (1961) with reference to the new unit approach, in practice, is terminated when shifts in cell of Bailey (1963), and those for kaolinite are the positionalparameters of the hydrogen atoms are from Zvyagin (1967). less than the standard deviations of the known In a situation where the positional parametersof heavier atoms. In the case of the kaolin minerals the non-hydrogenatoms but not the hydrogen atoms describedhere, four cycles were sufficientto locate are known, the electrostaticenergy (Eq. 1) consists the hydrogen atoms. The exact number dependsto of three types of terms. These terms result from 1) some extent on how correct the initial guessesare. HYDROXYL ORIENTATION IN KAOLINITE, DICKITE, AND NACRITE 473

Results Tesr-E,2. Fractional Positional Parametersfor the Hydro- gen The positional parameters for all the hydrogen Atoms in the Kaolin Minerals atoms are listed in Table 2 along with the angles made by the OH bondsrelative to the plane of the y z designation kaolin layer as well as the anglebetween the b-axis Kaolinite .273 .669 -.262 H(1) 77 -83 .727 .512 -.27L H(2) 77 L32 and the projectionof the OH bond onto the kaolin .853 .9L4 -. 159 H(3) L4 -152 layer.The interatomicdistances and anglesinvolving .633 .594 .120 H(6) l4 30 the hydrogenatoms are listedin Dickite .678 .565 .174 H(1) L2 30 Table 3, .066 .496 . 361 H(2) 76 -35 Randomerrors in the heavyatoms positions pro- .598 . 315 .361 H(3) 69 t74 .o79 .t72 .359 H(4) 66 81 duce approximateerrors in the hydroxyl orientation Nacri te .707 .454 .L72 H(1) I9 lrl determinedby the electrostaticenergy minimization .511 .7tO .329 H(2) 38 -62 processfor dickite,nacrite, .235 .662 .35I H(3) 79 3 and kaolinite,To assess 1r(4) these approximateerrors, one can look at changein * the angle between the kaolin layer and the 0-I1 bond

OH orientationas a function of changein the frac- ** the angle between the projecEion of the 0-H bond on the kaolin tional positionalparameters layer aDd the b axis. A negative sign indicates a counter- of nearby atoms.Since clock-ilise rotation. the standarddeviations in the fractionalcoordinates **x the nwber of hydrogen atms corresponds to the n@ber of the are similar for all three structures,only hydroxyl oxygen atm in the origlnal stlucture description dickite has (see Table l-).

been examinedin detail; the resultsshould be directly applicableto nacriteand kaolinite.The com- putational procedurewas to redeterminethe OH orientationafter changingone of the atomic positional parametersby one standard deviation as determinedfrom the structurerefinement process. Since,in dickite,all the inner surfacehydroxyls have ihe same environment,the calculationswere done for only one surfacehydroxyl, O(4)-H(4), and the innerhydroxyl, O(l)-H(1). The influenceof an atom on the electrostaticenergy decreases with increasinginteratomic distance, so that the calcula- tion has to be done only for nearestneighbor ions of each type and the hydroxyl group itself. The resultsare shownin Table 4. The anqularchanses

TesI-E 3. Distancesand Angles for the Interlayer Hydro- gensin the Kaolin Minerals

kaoLiniEe 0(r) - H(r) --- 0(8) 173" lu(1)- 0(8) I 93 A 0(2) - n(2) --- 0(9) 162 HYDROXYT OXYGEN R(2)- 0 (9) 0(3) - H(3) --- 0(7) 96" H(3)- 0(7) 276

0(2) - H(2) --- 0(1) 165. H(2)- 0(r) 2.00 A Ftc. l. The coordinate system used to find the electro- o(3) - n(3) --- 0(3) 170 H(3)- 0(3) static energy minimum for a hydroxyl group. The reference 0(4) - H(4) --- 0(2) 163 axis is arbitrarily chosen to be approximately normal to H(4)- 0(2) 2.O2 the kaolin layer and passesthrough the hydroxyl oxygen. By choosingvarious values 0(2) - H(2) --- 0(1) 109. for the angle a, the electrostatic H(2)- 0 (r) 2.60 A energy may be computed for a number of possible hydro- o(3) - H(3) --- o(2) 166 H(3)- 0(2) 2,02 gen positions, designatedby r's, and the minimum is ob- 0(4) - H(4) --- 0(3) t73" tained by interpolation. H(4)- 0(3) 2.O2 474 R. F. GIESE, AND P, DATTA

TlsI-E, 4. Angular Changes in the OH Orientation in mately alongthe c-axisin the samedirection as the Dickite due to a Shift in the Fractional Positional Parameters shift in O(2). Sincethe O-H distanceis not varied (by one standard deviation) of Neighboring Ions. during the computations,the orientationchange is

Shifted Change not as large as might be expected.Other apparent Hydroxyr QLz) //r D anomaliesare not as readily explained,as for ex- o(4) - H(4) r.rz i-1 A1(1) 2.0" 0.003A amplethe.r andy shiftsfor O(2)-H(2). A change 0.005 2.9 0.005 in the positionalparameters of one ion changesnot L.32 Si(2) 2.0 0 002 only the termsin equation( 1) due to the shiftedion 30 0.003 20 0 006 and the hydroxylhydrogen pair but all termswhich

30 0 .003 involve the shifted ion and thesealso contributeto 20 0 003 2.9 0.014 moving the electrostaticenergy minimum. It is clear from Table 4 that the inner surface o 69 oH(2) 2.0 0.001 H(2) 2.O 0.013 hydroxylsare more sensitivethan the inner hydroxyl 2.0 0.005 to smallshifts in the neighboringatoms. The average - 0(4) 2.O H(4) 20 angular changefor the surfacehydroxyls is 2.4o, 20 whichis sufficientlyaccurate to assurethat the orien- 0(1) - H(1) 7.32 05 0.002 averagechange in the 06 0.008 tations are meaningful.The 0.5 0. 003 inner hydroxylis 0.5" and is the sameorder of mag- 1.26 Si(2) 0.5 0.005 nitude as the standarddeviations for other bond 0.5 0.002 0.5 0 006 anglesin the structure.

0.5 0 .007 0.5 0.007 Hydroxyl Orientation 06 0 .009 Figure 2 is a projectionof the dickite structure The contribution to eq. 1 of the hydroxyl hydrogen and the ion which is shifted onto (001) showingthe aluminum atoms, their inner surface hydroxyls, the silicon The change ln che distance frm the hydroxyl hydro8en to the ion coordinating which is shifted. In the case of 0(2) - H(2), the distance is atomsin the next layer and their coordinatingbasal to 0(2) oxygenatoms. Al(l), its three hydroxyl oxygens, andthe hydroxylhydrogens H(2), H(3), andH(4) due to shifts in Al, Si and O are similar even though are centeredin the large almosthexagonal opening the chargeson these ions are very different. To assess in the baseof the silicatetrahedra and the immediate the relative influence of shifts in these ions on the environmentof each of the hydroxylsis very sim- orientation change of the hydroxyl, one can look at ilar; henceone would expectthem to behavesim- the contribution to equation ( I ) due to the inter- ilarly, which they do. All three O-H bondstilt action of each of these ions with the hydroxyl hydro- slightly toward the empty octahedralsite and, in gen. These values are listed in the first column of fact, the small deviationsfrom perpendicularityin Table 4 and suggest that the relative influences of view of the asymmetryof the dioctahedrallayer the standard deviation shifts of the nearest neighbor cations are surprising. ions are approximately the same. The higher charged A similar projectionof kaolinite (Fig. 3) shows ions are at greater distances from the hydroxyl hy- a lesssymmetrical arrangement. All threeOH bonds drogen than are the lower charged ions. The change still tilt in toward the empty octahedralsite, two in interionic distancebetween the hydroxyl hydrogen onlyslightly,H(2) and H(1), but thethird H(3) is and the shifted ion is also a factor since this changes almost horizontal. A possibleexplanation for the the denominator in equation ( I ) for the ion pair different behavior of H(3) is that the stackingof involved. The change in the interionic distance due kaolin layersis such that Si(l) in the next layer to the shifts in the fractional coordinates of the near- interfereswith and repulsesH(3). If the OH bond est neighbors about the two hydroxyl hydrogens are were to rotatetoward O(7) in the planebisecting listed in the last column of Table 4 and are approxi- Al(l) and Al(2) and containingO(3)-H(3), its mately the same with a few exceptions. The large distanceto Si(l) and Si(2) in the nextlayer would distance change due to the shift along the c-axis of decrease,which is not energeticallydesirable. If it O(2) has relatively less effect on the O(4)-H(4) were to rotate in the other direction,the distances orientation becausethe hydroxyl is oriented approxi- to the aluminumswould be shorterand this alsois a HYDROXYL ORIENTATION IN KAOLINITE, DICKITE, AND NACRITE 475

si(r)

H(2) H(3)| 51111i At(2)

si(2)

FIc. 2, The projectiononto (001) of the aluminum ions, their coordinating hydroxyls, and the silicon ions in tbe o next layer with their coordinating basal oxygen ions in tbe Frc. 3. A view of the kaolinite structureas in Figure 2. dickite structure. The open circles are oxygen ions, the larger solid circles are cations and the smaller solid circles are hydrogen ions, The dashed line representsthe possiblo hydrogen bonds as explained in the text,

less stable situation. It is also interestingto note that the O(3)-O(7) distanceis the longestof the threeinterlayer distances. A less clear cut caseis that of nacrite (Fig. a) whereH(3) and H(4) arenearly vertical but tilted slightly toward the vacant octahedralsite whereas H(2) is inclinedto the kaolin layer by 38". The environmentof O(2) is very similar to O(3) in terms of placement'ofcations and anionswith the exceptionthat the O(3)-O(2) distanceis 2.97A (2) o(l while the O(2)-O(1) distanceis 3.06A.There is, H(3) H(2). therefore,a strongerattractive interaction between H(3) and O(2) than betweenH(2) and O(1). si0) si(2) The origin of this longerdistance for O(3)-O(l ) Ar(t) At(2) lies in the buckling of the basesof the tetrahedral layerwhich raises O(1) aboveO(2) andO(3). The inner hydroxyl in eachstructure is effectively isolatedfrom influencesdue to stackingdifferences and,since the OH is directedtoward a largeopening in the structure,the distortionsin the kaolin layers in the differentstructures will have relativelylittle effect on the OH orientation.If the OH vectorsof Ftc. A view of the nacrite structure as in Fisure 2. 476 R. F. GIESE, AND P. DATTA the three structuresare plotted on a stereographic expectthat if thereare no outsideinfluences (hydro- projectionsuch that the inner hydroxylspoint in the gen bondingwould be one such influence),these same senseand the octahedrallayers are oriented two anglesshould be equal.The formationof a long similarly,an interestingpattern is evident.In Figure hydrogen bond or the close approachof another 5, a portion of the hydroxylsheet of dickiteis shown cationwould tend to makethe anglesunequal except with the inner surfacehydroxyls at the top of the in the rare circumstancewhen the perturbingion is octahedra(1, 2 and 4) and the oxygen-hydroxyl also symmetricallyoriented. A plot of the two sheetbelow (3 is the inner hydroxyl at the bottom AI-O-H angles,one along the x-axis (arbitrarily of the octahedra).The OH vectorsfor the hydroxyls the Al ( 1) angle)against the other,along the y-axis in the three structuresare shown in stereographic (arbitrarilythe Al(2) angle) would show, if this projection (Fig. 5, right) with the projectedpoint simplepicture is correct,the pointsdue to the inner being indicatedby the lower caseletter appropriate hydroxylsalong a 45' line of equal angles.The to the mineral and a number identifyingthe par- points for oxygenswhich are involved in hydrogen ticular hydrogen.Hydroxyls 2, 3 and 4 plot closely bonds would tend to lie off this 45" line' Figure 6 togetherfor the three structures.One would expect showsthis generalpicture is correct,The data points group 3 hydroxylsto be closelyaligned because, as for inner hydroxyls all lie on or near the line, the mentionedearlier, these are the innerhydroxyls and inner surface hydroxyls of kaolinite and nacrite are relatively unperturbedby the structuraldiffer- which are inclinedto the layer surfaceare very close encesbetween minerals. The closenessof groups2 to the line, and the other inner surfacehydroxyls and 4 must indicatethat thesehydroxyls have ap- which satisfymost closelythe requirementsfor hy- proximatelythe sameenvironment in all threestruc- drogenbond formation tend to lie off the line with tures and the difterencesin orientationin group I two of the dickite hydroxylsclosest. This is not sur- may be due to the difterencesin stacking and prisingin view of the symmetricalstacking arrange- variationsin hydroxyl-oxygendistances as indicated ment in that structure(Fig. 2). above. In summary,then, the hydroxylsof all threemin- There are differencesin the distortions in the erals tend to orient themselvesin a similar sense, kaolin layerin the threeminerals and in the stack- being influencedlargely by the atoms in the same ing of the layerswhich perturbthe orientationsof layer. The major differencesamong the structures the hydroxyls.These perturbations can be seenif is in one of the inner surfacehydroxyls, the one one looks at the anglesAI-O-H of which there are whichwith the inner hydroxylforms a sharededge in two for each hydroxyl.Hypothetically, one would the octahedralsheet, The differenceshere appearto be due to 1) the stackingof the next layer which may or may not presentinterfering cations, and 2) distortionsof the oxygensheets which make some OH-O distanceslarger than others.It should be pointed out that the contributionsto the electrostatic energyfrom next nearestneighbors and ions fartheraway are substantial.Thus it maybe an over simplificationto draw causeand effectrelationships just from inspectionas is done here.These observa- tions should be regardedas tentative until tested directly. Discussion

FIc, 5. A portion of the octahedralsheet in dickite is The questionof hydrogenbonding involving the shown on the left with the inner surfacehydroxyls on top. inner surfacehydroxyls and the basal tetrahedral The hydroxyls are arbitrarily numbered.On the right is a oxygensin the next layer is of great interest.It stereographicprojection of the OH bonds in all three kaolin bringsup, however,the questionof what evidenceis minerals-each indicatedby the first letter of the mineral the existenceof a long hy- name. Three of the hydroxyls form closely aligned groups requiredto demonstrate (2, 3 and 4) and in the other group are the hydroxyls drogenbond. A generalrule which is often cited is which have a differentorientation in each of the minerals. that OH-O contactsof lessthan about 3.0A which HYDROXYL ORIENTATION IN KAOLINITE, DICKITE, AND NACRITE 111 are not the edges of coordinationpolyhedra are r40 indicativeof a hydrogenbond (Hamilton and Ibers, 1968). In the caseof the kaolin minerals,all contactsof this natureare closeto or greaterthan 3.0A t30 (Table 1) so this is of little aid in decidingon hydrogenbonding. Another characteristicof hydro- - gen bondingis a tendencyfor the O-H. . . O angle ri r20 - YO to be in the region of 150' 180' althoughthere :-z is a smallnumber of observedcases for anglesdown S< to 110o (Hamilton and Ibers, 1968, p. 214). In a lro addition,the distancebetween the hydroxyl hydrogen and the acceptoroxygen should be less than the sum of the van der Waalsradii; in this casethe roo observedvalue of the sum is 1.7A and the calculated value is 2.6A, both i 0.1A (Hamilton and Ibers, 1968). If one appliesthese criteria to the too llo t20 130 140 data in Table 3, one can concludethat for kaolinite, H(3) is not involved in hydrogenbonding while Ar(l )-o- H H(1) and H(2) meet the requirementsas stated ANGTE aboveexcept for the observedH....O distances againstthe other for both of which are greaterthan 1.7A. In fact, all the Frc. 6. A plot of one AI-O-H angle all hydroxylsin kaolinite (k), dickite (d) and nacrite (n). hydroxyl hydrogensin the kaolin minerals have The subscript is the number used to identify the hydrogen H....O distancesgreater than this. Ignoringthis as described in the text. The fields marked YES contain requirementfor the moment, in dickite, all three hydroxyl hydrogenswhich participate in hydrogen bonding inner surfacehydrogens are involved in hydrogen and the area along the solid line and betweenthe two dashed bonded hydroxyl bonding,and in nacrite H(3) and H(4) are in- lines marked NO contains nonhydrogen hydrogens. volvedin hydrogenbonding while H(2) is not. If one requiresH.. ..O distancesto be 1.7A or less,then there is no hydrogenbonding between and the third was directed either toward the empty layers, and the questionis what holds the layers octahedral site (Ledoux and White 1964a) or else together.Theoretical treatments of hydrogenbond- perpendicular to and down toward the kaolin layer ing (Coulsenand Danielsson,1954) indicateit to so as to participate in a long hydrogen bond with havea largeelectrostatic component. Accepting this, the oxygens in the plane shared by the tetrahedral one could rephrasethe hydrogenbond questionas- and octahedralsheets (Wada, 1967). The kaolinite is there a net electrostaticattraction between kaolin model described here is in general agreementin that layers and, if the hydrogensare removed (for ex- two of the three inner surface hydroxyls are normal ample by substitutingF- for OH-), is there still to the layer and available for deuteration while the a net attractionor a repulsion?If the answerto the third is nearly in the hydroxyl sheet, making a small first questionis yes and to the secondis no, then angle away from the kaslin layer. It is unlikely that this would demonstratethat the presenceof the inner this latter hydroxyl is directed down toward the surface hydrogensis a necessaryfeature of the empty octahedral site at any great angle because kaolin structures.We are presentlyinvestigating this (Fig. 6) the inner hydroxyl is also directed in aspectof the problem. toward the empty site and the two hydrogen ions As indicatedearlier, there is little agreementon will repel each other. Several other investigators the interpretationof the IR studies in terms of have suggested a similar orientation based on IR hydroxyl orientation.The intercalationand deutera- studies of untreated kaolinite (Serratosa et al, 7962, tion experimentsof Wada (1967) and Ledoux and 1963; Farmer, 1964) but with less convincing evi- White (1964a, b) on kaoliniteindicated that of the dence. Kukovsky (1968, 1969) in a quantitative three unique inner surface hydroxyls, two were study of hydrogen bonding in kaolinite based on the nearly perpendicularto the kaolin layers and were relative intensities of the high and low frequency pointing toward the basaloxygens of the next layer hydroxyl stretching bands concluded that of the four 478 R. F. GIESE, AND P. DATTA hydroxyls,2.1 arc not involved in hydrogenbond- Conclusions presentedin this work has 2'0 ing. The model Under the assumptionsof full ionic charges,O-H H(6), which definitelydo not hydroxyls,H(3) and distanceof 0.97A, and the correctnessof the crystal andthis close agree- participatein hydrogenbonding structures of kaolinite, dickite and nacrite, it is for the correctnessof the ment is strong support possibleto arrive at modelsfor the hydroxyl orien- model. tations in theseminerals by an iterativemethod of experimentaldata on dickite There is much less minimizing the electrostaticenergy. In dickite, all check on the electrostatic and nacrite to use as a three inner surfacehydroxyls are nearly normal to model presentedhere energymodels. The dickite the kaolin layer and point toward the basaloxygens proposed Newnham(1961) agreeswith the one by in the next layer; in kaolinitetwo of the threeinner just sincehis assignment but this may be fortuitous surfacehydroxyls are similarly oriented while the questioned(Farmer, 1964). of IR bandshas been third is nearly parallel to the kaolin layer pointing ( the deuterationof a "dickite- Wada 1967) studied away from the octahedralsheet; in nacrite two of pure and suggested nacrite" rather than samples the inner surfacehydroxyls are normal and the third hydroxylsare that only two of threeinner surface is inclined at 52o to the kaolin layer. In all three, Our modelssuggest that, availablefor deuteration. the inner hydroxyl is directed toward the empty hydroxylsshould be in dickite, all inner surface octahedralsite and makesa small anglewith the O, while in nacrite,since one availablefor deuteration OH sheetto which it belonss. of the hydroxylsmakes an angleof 38' with the kaolin layer,it may not deuterateas readilyas the Acknowledgments other two which are nearly normal to the kaolin layer.The apparentdiscrepancy with Wada'swork We are grateful to Dr. V. C. Farmer for his comments may be a resultof the impurenature of his samples. on recent IR experimental work and their interpretation, and to Drs. J. L. White, G. W. Brindley,and V. C. Farmer Of moreinterest is the resultof Kukovsky'sstudy of for reviewing the manuscript. The research was supported dickite (as describedabove) in which he findsthat in part by the National Aeronauticsand SpaceAdministra- only 1.2 hydrogensare not hydrogenbonded. The tlon. presentmodel has only H(1), the inner hydroxyl hydrogen,definitely not hydrogenbonded. It is References unfortunate that Kukovsky did not also look at BAILEv,S. W. (1963) Polymorphismof the kaolin minerals. nacrite. Anter. Mineral. 4E, 1196-1209. The most recent IR work on kaolinite is by Brun, W. H. (1965) On hydrogenbonds in crystallinehy- Jacobs (I971). He concludesthat the transition drates. Acta Crystallogr. 19, 909-916. (1952) reseaux momentsfor the 3620, 3652 and 3670 cm-1bands Btnuul, F. L'energie electrostatiquede ioniques. l. Plty.s.Radiunt, L3, 499-505. show no pleochroismwhereas the 3700 cm-1does. BLouNT, A. M., I. M. THnrlDcoLD, AND S. W. Bntlev Hencethe first threehave transition moments nearly (1969) Refinementof the crystal structure of nacrite. parallelto the kaolinitelayer and the last is 65' Clays Clay Minerals, 17, 185-194. to the layer.In addition,partially deuterated samples BnrNolrv, G. W.. lwo K. RogrNsoN(1946) The structure of kaolinite. Mineral Mag. 27, 242-253. show that there is couplingbetween the inner sur- Coulsox, C. A., ,rNo U. DrNmlssoN (1954) Ionic and face hydroxyls and thereforethere is not a one-to- covalentcontributions to the hydrogen bond. Arkio lor one correspondencebetween absorption band and Fysit, Bd. 8,239-244. hydroxyl group, so that the orientationof the tran- Dnrrs, V. A., lxn A. A. Krssrtv (1960) An X-ray study sition momentneed not coincidewith the orienta- of a kaolinitesingle crystal. Kristallografiya, 5,224-227. (1964) absorption hydroxyl tion of any OH group.If this is true, it meansthat FARMER,V. C. Infrared of groupsin kaolinite.Science,145, 1189-1190. the determinationof OH orientationsfrom IR datais lNo J. D. Russrrl (1964) The infrared spectraof extremely difficult if not impossiblefor complex layer silicates.Spectrocltint. Acta, 20, ll49-1173. caseslike the kaolin minerals. Grrse, R. F. (1971) Hydroxyl orientationin muscoviteas A refinementof the kaolinitestructure by powder indicated by electrostaticenergy calculations.Science, 172,263-264. neutrondiffraction is being done (personalcommu- S. Wrnrn, lNo P. D.rru (1971) Electrostatic nication,Dr. S. Spooner)but the resultsat present energycalculations of diaspore(alpha AIOOH), geothite are only qualitativeand as yet do not provide a (alpha FeOOH) and groutite (alpha MnOOH). Z. testof the OH orientationsdescribed here. Kri stallogr. 134, 275-284. HYDROXYL ORIENTATION IN KAOLINITE, DICKITE, AND NACRITE 479

GnIu, R. E. (1968) Clay ntineralogv.McGraw-Hill Book ture and some remarkson polymorphismin kaolin min- Co., New York. erals. Mineral Mag. 32, 683-704. HeuIrroN, W. C., eNo J. IsEns (1968) Hydrogen bonding aNo G. W. Bnrxplry (1956) Structureof dickite. in solids. W. A. Benjamin, Inc. Acta Crystallogr. 9,759-764. HrNonIcxs, S. B. (1938) The crystal structureof nacrite Sennlros.e,J. M., A. Hrnelco, lNo J M. VrNls (1962) AlrO,,'2SiO,.2H,Oand the polyrnorphismof the kaolin Orientationof OH bondsin kaolinite.Nature, 195, 486- minerals. Z. Kristallogr. 100, 509-518. 487. Jecoss, H. (1971) Etude des ltydroxyle de la kaolinite par AND- (1963) Infrared study of the Spectroscopielalnlrarouge.Thesis,UniversityofLotrvain. OH groups in kaolin minerals.Proc. Int. Clut, Conl. l, Kuxovsxv, E. G. (1968) Ukranian cltemical magazinespe- 17-26. cial copy. Naukova Dumka, Kiev. SuEnIra,rN,I. (1932) Crystal energies of ionic compounds (1969) Alteration processesin clay minerals.C/c.r' and thermochemicalapplications. Cltem. Reu. 11,91-170. Minerals,8,234-237. W,ue, K. (1967) A studyof hyroxyl groupsin kaolinemin- Leoo, M. F. C. (1968) The locationof hydrogenatoms in erals utilizing selectivedeuteration and infrared spectro- crystallineionic hydrates.Z. Kristallogr. 126, 147-152. scopy.Clay Minera[s,7,51-61. Lrooux, R. L., euo J. L. Wnrrt (1964a) Infrared study of Worrr, R. G. (1963) Structuralaspects of kaolinite using the OH groupsin expandedkaolinite. Science, 143,244- infrared absorption.Amer. Mitrcral. 4E, 390-399. 246. ZwrctN, B. B. (1967) Electron-difftactionanalysis ol clat AND - (1964b) Infrared study of selective ntirteralstructures. Plenum Press,New York. 364 pp. deuteration of kaolinite and halloysite at room tempera- ture. Science,145, 47-49. Manuscript receiued,Nouentber 12, 1971:accepted NewNHrtu, R. E. (1961) A refinementof the dickite struc- lor publicatiort,lurunry 18,1973.