Hydroxyl Orientation in Kaolinite, Dickite, and Nacrite 473
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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 octa- hedral sheetsis directed toward the empty octahedral sites in all three structures.The remain- ing 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 inclined at 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 posi- tional 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 approxi- mately perpendicularto the clay layers. This direc- tion (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.