Of Kaolinite and Halloysite in Relation to Weathering Feldspars And

American Mineralogist, Volume 59, pages 365-371, 1974

Stabilitiesof Kaoliniteand Halloysitein Relationto Weathering of Feldsparsand Nephelinein AqueousSolution

WsN H. HulNc

Department of Geology, Uniuersity ol South Florida, Tarnpa, Florida 33620

Abstract

Newly designed stability diagrams, which consider Al as a mobile, reactive component (including hydrated Al+, AI(OH)'", AI,(OH)*,, AI(OH)*", and AI(OH)-+ species) as well as parameters,Na*, K*, H*, and H,SiOn,.show that in average river water halloysite and kaolinite are stable with respect to K-feldspar, albite, and nepheline. By contrast, in present sea water, kaolinite alone is stable in a system containing kaolins plus K-feldspar or albite, but both halloysite and kaolinite are stable in a system of kaolins and nepheline. The geologic observations that kaolinites become more abundant than halloysites through geologic time are interpreted from solution chemistry, Gibbs free energy, and activation energy. Halloysite may crystallize from a solution supersaturated with respect to halloysite, but kaolinite may crystallize under these same conditions or also from a solution saturated with respect to kaolinite. Moreover, any halloysite formed will tend to be spontaneously transformed to kaolinite (AG.'being negative). Rate of transformation, however, is governed by the activation energy (not Gibbs free energy), the temperature, and the rate of kaolinite precipitation from such nutrient solutions as may be supplied from dissolution of halloysite through diagenetic processes.Geologic occurrences of kaolinite and halloysite in modern and ancient weathered products are consistent with these theoretical interpretations.

Introduction interpret from thermodynamic and kinetic relation- ( kaolinite and halloysite Kaolin minerals have been formed in a number of ships 1) the stabilities of of nonJayer silicates, and (2) different ways in geological environments, as re- formed by weathering two minerals through viewed recently by Keller (1970). The mechanisms the relative abundancesof these geologic time. for the formation of kaolin minerals include direct judgment form in crystallization from solution, replacement, crystal- Final as to what minerals could if po- lization from colloidal gels, and weathering of layer nature could, of course, be better made all or non-layer silicates. Geologic occurrences of kaolin tentially stable minerals were considered simulta- minerals formed by weathering of nonJayer silicates, neously, rather than two at a time, as done here. particularly of feldspars, have been documented in This present paper thus represents a step toward Mesozoic weathered products in Minnesota (Par- that goal. ham, 1969), and in modern weathering products in Stability Diagrams the Southern Appalachian region (Sand, 1956), in Hawaii (Bates, 1962), in Hong Kong (Parham, Relative stabilities of aluminum silicates (includ- 1969), and in Mexico (Keller et al, l97l). Minato ing primary rock-forming and clay minerals) in and Utada (1972) even showed present-day forma- aqueous solution at 25"C, 1 atm. have been gen- tion of halloysite in Japan. Both kaolinite and hal- erally determined by thermodynamics and indicated loysite have been found in recent weathering pro- graphically in stability diagrams. These stability dia- ducts, but kaolinite predominates in the ancient grams are constructed primarily in terms of log of weathered products. Although many geological activities of K-/H- or Na./H- and log of activity of mechanisms have been suggestedfor the formation HlSiO4 in aqueous solution, assuming AlzOg as an of kaolinite and halloysite from weathering of non- inert component (Feth, Roberson, and Polyer, 1964; layer silicates, the relative abundancesof these two Garrels and Christ, 1965; Hess, 1966). Geologic minerals through geologic time, however, have not observations of kaolin deposits indicate that kaolin been fully explored. This paper therefore attempts to minerals can be formed by weathering of aluminum 365 366 WEN H. HUANG

solution may be present in varying proportions (Huang and Keller, 1972a), relative stabilities of aluminum silicates should be expressedin terms of activitiesof the various speciesof A1 (including Al3*, AI(OH)'., AI2(OH)4*2,AI(OH).2, and Al(OH)-a, all hydrated) in addition to those of HnSiO+,K*, Na*, and H-. The relativestability relationshipsin aqueous solutionsbetween kaolin minerals and the non-layer silicatesK-feldspar, albite, and nephelinecan be obtained from the following two relationships' (l) AG.' (standard free energy of reaction) : o o 2 AGr" (products)-2AGl (reactant);where AGl is standardfree energyof formation (the values from Robie and Wald' -\ of AGr" are obtained baum, 1968; Huang and Keller, 1972b; Hem et al,1973;Huang and Kiang, 1973;Huang and Keller, 1973b). (2) AG," : -13641o9 K; whereK : equilibrium liquids are Fra. 1. Stability diagram of K-feldspar-kaolinite and constant(activities ofpure solidsand K-feldspar-halloysite at 25'C and I atm., in which pAl- equalto 1). pHnSiO. is plotted against pH and pK*, (l) Equilibrium reactions between K-feldspar (AG,o : -892.6 kcal/mole)and kaolinite (AGro : -902.9 kcallmole) and halloysite(AG1" : -898.4 silicates either from solid-state transformation or kcal/mole) are written as follows: solution-precipitationprocess. In either process,sol- (a) KAISi,o, * Al'* + 5 H,o : Al,Sirou(oH)nf ute speciesof aluminumoccur (Keller, 1970; Hem K* + HnSiOn -l 2Il' log [Al3*] - log [HnSiOr] et al, 1973). Aluminum thereforeshould be con- : 5.89(or 9.19for halloysite)* loe [K- - 2pH] sidered as a mobile, active componentrather than (b) KAlsi3o8 + Al(oH)"* + 4 H,O : Al2Si'Os a fixed component in the stability diagrams of (OH). + K* + HnSiOn* H* log [AI(OH)'.] - aluminumsilicates (Huang, 1973;Htang and Keller, log : 0.89 (or 4.19 for halloysite) 1973a). [HnSiOn] { log [K.] - pH Becausedifferent Al speciesin a simple aqueous (c) KAISi'O' + AI(OH)*, * 3 H,O : Al,Si,Ou (oH)n * K* + H4SiO4log [Al(OH)*, - -0.14 . log [HnSiOJ: -3.16 (or for halloysite) f log [K.] (d) KAlsi3o" -l 2 H* + H,O + AI(OH).- Al,si,ou(oH)n * K* + H4SiOnlog [Al(oH)-r -13.23 - log [H4SiO4]: -16.53 (or for halloysite)* log [K.] + 2PII The stability rel,ationsof K-feldspar and kaolinite, and of K-feldspar and halloysite, can be expressed in terms of three measurableexperimental parameters,namely pAl ( -1og [Al] minus pHaSiO+( -log [HaSiO+1),pK' (-log [K.]), and pH (seeFig. 1). Figures 2a and 2b, respectively,are two isoplethic sectionsof stability diagramsfor present sea water Frc. 2. Isoplethic sections of the stability diagram (pK. = 2.21) andaverage riverwater (pK = 4'25) (K-feldspar-kaolinite and K-feldspar-halloysite) for environments.The activities of Al (speciesgen- (sea (river water) environ- marine water) and non-marine eralized),H4SiO4, K*, and H- in presentsea water ments. S and R are the compositions of sea water and average river water, respectively. (A) Marine (pK* ,= and averageriver water were calculated in earlier 2.21), (B) Non-marine(pK. - 4.25). papers (Huang, 1973; Huang and Keller, 1973a). STABILITIES OF KAOLINITE AND HALLOYSITE 367

As shown in Figure 2b where the activity of K. is * Na* + H4SiO4+ 2H* log [Al3*] - log - small, such as in river water (pK. 4.25), halloy- lH4SiO4l : 3.15 (or 6.46 for halloysite) + site and kaolinirteare stable with respect to K-feld- log [Na*] - pH spar. In presentsea water (Fig. 2a), kaolinite (not (b)NaAlSi3O, + AI(OH)'* + 4Il2O halloysite) is stable with respectto K-feldspar. The Al,si,o5(oH)4 + Na* + H4sio4 + H* occurrencesof kaolinite and halloysite from altera- log [AI(OH)'.] log [HnSiOn]: -1.84 tio,n of feldsparsin differenceplaces, for examplein (or 1.46 for halloysite)f log [Na*] - pH the Southern Appalachian region (Sand, 1956), (c)NaAlSi,O, + AI(OH)., + 3H,O could be due to' the differencesin the activities of Al,Si,OE(OH)n* Na* + H4SiO4log [Al(OH).J K*, Al, H4SiO4and H- in the environments.Sand - log [HnSiOJ: -5.89 (ot -2.59 for halloy- (1956) alsofound thatriaer terrqcesapparently are site) + log [Na*] favorable locations for extensivedeposition of hy- (d) NaAlSi,O. * 2H* + H,O * AI(OH)-4 : drated halloysite from alteration of feldspars, as is AI,Si,O5(OH).* Na* + H4SiOnlog[AI(OH)-,] consistentwith the theoretical predictions from the - log [H4SiO4]: -19.26 (or -15.96 for stability diagrams in Figure 2. Furthermore, the halloysite)* log [Na.] + 2pI{ diagrams show that halloysite, which is stable in Figure 3 is a three-dimensionalplot of stability river water, may become unstable and be trans- relationsof albite and kaolinite and of albite and formed to kaolinite when halloysite is transported halloysitein termsof the experimentalparameters- to marine environment. The stability fields are pAl, pHrSiO+,pN&*, and pH. Figures 4a and 4b are strongly controlled by pH below 4.2 or greater than two isoplethicsections at pNa. = 0.48, and 3.58, 6.7, but between4.2 and 6.7 they are independent for averagesea water and river water, respectively. of pH. The diagramsindicate that in non-marine environ- Three points should be noted, however,in apply- ment (averageriver water) halloysiteand kaolinite ing these stability diagramsto natural geologicsys- are stablewith respectto albite,whereas in marine tems: environmentkaolinite (not halloysite) is a stable (a) Sincenaturally occurringkaolinite and halloy- phase.This is consistentwith the resultsobtained site have a wide range of stabilities depending on from the K-feldspar-kaolinite-halloysitesystem as particle size,degree of crystallization,or hydration shownin Figure 2. (3) = (halloysite), etc, standardfree energy of formation Equilibriumreactions for nepheline(AGro for the minerals (AGt') will be varied, and sub- sequentlythe stability boundary betweenK-feldspar and kaolinite (or halloysite) will be modified,de- pendingon which.AGloare used.In this paper.AGlo are chosenfor those most stable clay minerals, and the diagrams should provide a base, and the sub- -8 sequentmodifications of the diagramsmay be neces- saryto suit for individualneeds. .4 -2 (b) Before applyingthe diagramsto natural sys- 0 A tems where the pH is higher than 5.6, activity of 2

H4SiO4 should be corrected for the effect of dis- q 1 solutionfrom total activityof Si (Huang and Keller, 6 1,973a). 8 t0 (c) Thesecalculations are made for fully dehy- .-\ l2 .\ drated halloysite becauseaGlo is available only t4 for dehydratedhalloysite. It has been recognized that typical halloysitemay be slightly hydrated. (2) Similarequilibrium reactions for albite (aGr" = -884.0 kcal/mole), and kaolinite and halloysite areshown as follows: Frc. 3. Stability diagram of albite-kaolinite and albite- halloysite at 25"C and I atm., in which pAl-pH,SiO, is (a) NaAlSirO, + Al3* + 5 H,O : AlqSi,O,(OH)r plotted againstpH and pNa*. 368 WEN H. HUANG

(c) NaAlSiOe * Al(OH)'t + HoSiOn Al,Si,O5(OH)n* Na* f H* log [AI(OH)'.] + log [HnSiOl : -17.31(or -14.01 for halloysite) + log [Na*] (d) NaAlSiO4 + AI(OH)-4 + H4SiOnI 2H* : Al,Si,Ob(oH)n* Na* + 3 H,O log [A(OH)-J * log [H4SiOJ : -30.68 (or -27.38 for halloysite)* log [Na*] + 2pH Figure 5 is a phase diagram between nepheline and kaolinite and betweennepheline and halloysite in terms of (pAl+pH1SiO4), pNa-, and pH. Two isoplethic sectionswhich show stability relations of thesethree mineralsin marine (pNa-=0.48) and non-marine environmentsare presentedin Figures t". ;. ;r;,.,n,. ,*,,"*'.r",0" .*o,,,tv aiueru- (ur- 6a and 6b. The diagramsshow clearly that in either bite-kaolinite and albite-halloysite) for marine (sea water) and non-marine (averageriver water) environments.S and environmenthalloysite and kaolinite are stable with R are the compositions of sea water and average river water respectto nepheline,independent of activity of Na* respectively.(A) Marine (pNa* = 0.a8); (B) Non-marine (from marineto non-marineenvironment). The fact - (pNa- 3.58). that nepheline from nepheline syenite in Arkansas has beenaltered to halloysite(Keller, personalcom- -469.7 kcal/mole) and kaolinite and halloysite are munication) is consistentwith the theoreticalresults expressedas follows: obtainedfrom such diagramswhere Al is considered (a) NaAlSiOn -l- A1'* + H4SiO4 + HrO a mobile, activecomponent. It shouldbe pointed out, A1,Si,O5(OH).* Na* + H* log [A13.] 4 however,that since the more disorderedhalloysite log [HnSiOn]: -8.26 (or -4.96 for halloysite) can probably form more rapidly than kaolinite (ki- * log [Na*] - 2pH neticconsideration, discussed later), suchdifferential (b) NaAtSioa * Al(OH)'* + H4SiO4 rates of formation may mask the presenceof any Al,Si,o5(oH)n * Na* f H* log [A(OH)'. + minor amount of kaolinite. log [HoSiOn]: -13.26 (or -9.96 for halloy- (4) Stability relations for albite and Na-mont- site) * 1og[Na.] - pH morillonite, and K-feldspar and K-mica which are expressedin terms of pAl, pHaSiOa,pNa*, pfl, are also shown in Figures 7 and 8, respectively.In Fig-

q8 o alo -. 12

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Frc. 6. Isoplethic sections of the stability diagrams nepheline-kaolinite and nepheline-halloysite for marine (sea water) and non-marine (average river water) environments. FIc. 5. Stability diagram of nepheline-kaolinite and S and R are the compositions of sea water and average - nepheline-halloysiteat 25'C and 1 atm., in which pAl + river water. (A) Marine (pNa* 0.a8); (B) Non-marine pHnSiOr is plotted against pH and pNa*. (pNa. - 3.58). STABILITIES OF KAOLINITE AND HALLOYSITE 369

(eN.+ = 3 Sl

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Frc. 7. Isoplethic sections of the stability diagram (Na-montmorillonite and albite) for marine (sea water) and non-marine (averageriver water) environments.S and R are the compositions of sea water and average river water. (A) Marine (pNu* - 0.a8); (B) Non-marine (pNa* - 3.58). ure 7, Na-montmorillonite is expected to be stable with respectto albite in either non-marineor marine environment.Likewise, K-mica (or illite) will be 345674910 more stable than K-feldspar in either environment pH (Fig.8). Frc. 8. Stability diagram of K-mica and K-feldspar at Summary 25"C and I atm., in which pAl is plotted against pH. S and R are the composition of sea water and average river water. In averageriver water, both halloysiteand kaolinite are stable with respect to K-feldspar, albite, (OH)n. The values of aGto for halloysite and ka- and nepheline.In presentsea water kaolinite be- olinite with AlzSizOs(OH)4 compositionsrespec- comesstable with respectto K-feldsparand albite, tively, are -898.4 kcal/mole and -902.9 kcal/ whereasboth halloysite and kaolinite are stablewith mole. respectto nepheline. (a) BecauseAGto for halloysiteis largerthan that (Fig. Discussions for kaolinite 9), a supersaturatedsolu- tion of Al-silicate with respectto both halloy- Geologicfield observationsreveal that in the mod- site and kaolinite, in which its molar Gibbs ern weathering products of non-layer aluminum free energ5rexceeds those of both halloysite silicates,both halloysite and kaolinite are formed and kaolinite, will precipitateeither halloysite togetheror independently,depending on (l\ geo- or kaolinite,whereas a saturatedsolution with logic conditionrr-climate, topography,the degreeof respectto kaolinite will equilibrate only with leaching,erc (Sand, 1956; Parham, 1969; Keller, kaolinite.A similarsolution phenomenon was 197O); (2) geochemical conditions-activities of Al, H4SiO4,Na*, K*, andH*. In the ancient weathered SOLUTIONS products,however, kaolinite is more abundantthan ------suFrsatutated with re3Pect to hallovsite and kaolinite I I halloysite.The questionthat remainsto be answered ------l---i- - - -- iaturatdwithrBpecr to halloysite, suF iaturared wrh is why the ratio of kaoliniteover halloysiteincreases r6Fct to kaoliniie with an increaseof geologictime. This may be ex- plainedfrom the following points: z

- -l- - - - - satu.ared with respect to (l) kaolinite, undersatu.ared wi$ From a uiewpointof Gibbsfree energy(AG') aGi=-902 , I kcal/mole sFct lo halloysite. To simplify discussionshere, assumeboth halloy- Frc. 9. Relative thermodynamic stabilities of kaolinite and site and kaoliniteto have the composition,A12Si2O5 hallovsite. 370 WEN H. HUANG

pointedout by Ernst (1969) for crystalliza- rate of transformation of halloysite to kaolinite will tion of aragoniteand calcite.Therefore, from dependon ( 1) activationenergy and (2) tempera- a viewpoint of solution chemistry,halloysite ture. Geologically, the transformation may result will crystallize from a solution supersa- from diageneticprocesses. No data are availableat turatedwith respectto halloysite,but kaolin- this time for possiblequantitative evaluation.If dis- ite will crystallizeunder these same condi- solution of halloysite and accompanyingprecipita- tions or also from a solution saturatedwith tion of kaolinite is the transformationmechanism, respectto kaolinite. Furthermore,any hal- the rate of such ransformationcould be determined loysite formed will ultimately be transformed by the rate of such dissolution and precipitation. to kaolinite. This may be explainedin sec- Further kinetic studies of such transformationis tion (b). necessarybefore any definite interpretationsmay (b) Consider the equilibrium reaction between be made. halloysiteand kaolinite as follows: Al,Si,O5(OH)n: AlzSi,Ou(OH)n Acknowledgment halloysite kaolinite This work was supported by the Earth SciencesSection, AG"o(Gibbs free energyof reaction) National Science Foundation NSF GA-33558. I wish to : AGro (Gibbs free energyof formation thank ProfessorsW. D. Keller and A. L. Reesmanfor their for kaolinite) critical reviews of this manuscript. - AG'o (Gibbs free energyof formation References for halloysite) : - 902.9- (- 898.4) Berns, T. F. (1962) Halloysite and gibbsite formation in : - 4.5kcal (<0) Hawaii. Proc. Ninth Nat. Conf. Clays Clay Minerals, p. 315-328. Sincethe AG,.ois negative,the reactionis sponta- Cunus, C. D., mlo K. Knrvsnv (1965) The detection of neous;that is, the reactionwill go from left to right. minor diagenetic alteration in shell material. Geochim. Therefore halloysite is a metastablephase, and will Cosmochim. Acta, 29, 7 l-84. be ultimately transformed into kaolinite. This is Enxsr, W. G. (1969) Earth Materials. Prentice-Hall Inc., p. analogousto the transformationof aragonite to EnglewoodCliffs, N.J., 149 E. RonrnsoN, rNo W. L. Porzen (1964) calcite. Fern, J. H., C. Sources of mineral constituents in water from granitic and Nevada. U.S. GeoI. (2) From a uiewpoint (a,H) rocks, Sierra Nevada, California ol actiuationenergy Suru. Water Supply Pap.1535-I. Although the transformation of halloysite to ka- Grnnrrs, R. M., rNo C. L. Cnnrsr (1965) Solutions, oliniteis a spontaneousprocess, the questionremains Minerals, and Equilibria. Harper and Row, New York, p. as to how long this transformationprocess will take. 450 E. Rosrnsox, C. J. LrNo, eNo W. L. Porzen This is a kinetic problem, Heu, J. D., C. which involves activation (1973) Chemical interactionsof aluminum with aqueous energy (aFl) rather than Gibbs free energy (AG), sifica at 25"C. U.S. Geol. Suru. Water Supply Pap., as shown in Figure 10. The magnitudeof AFl will lE27-E, 57 p. dependon reactionmechanism, and the temperature Hess, P. C. (1966) Phase equilibrium of some minerals in of the reactionis a rate-decisionfactor for any given the K,O-NaO-SiO-H,O system at 25"C and I atmo- 264,289-309. mechanism(Curtis and Krinsley, 1965). Thus the sphere.Am. J. Sci. HulNc, W. H. (1973) New stability diagrams of some clay minerals in aqueoussolution. Nature, Phys. Sci. 243, 35-37. phase(?l Intermediate rNo W. D. Kr.lrrn (1972a) Geochemical me- chanics for the dissolution, transport, and deposition of pathway aluminum in the zone of weathering. Clays Clay Min- erals, 20, 69-7 4. E z AND - (lgj}b) Standard free energies of u formation calculated from dissolution data using specific (-4 5 kcal/molel mineral analyses.Am. Mineral. 57, 1152-1162. KAOLINITE IKI AND _ (l97ja) New stability diagrams of Ftc. 10. Relationshipbetween activation energy (Allrs-xr) some phyllosilicates in the SiO-AIO-K,O-H"O system. and free energy of reaction (AG"ru xrn) for the kaolinite- Clays Clay Minerals, 21, 5. halloysite system. AND - (1973b) Standard free energies of STABILITIES OF KAOLINITE AND HALLOYSITE 371

formation calculated from dissolution data using specific of halloysite from Otaki, Nagamo Prefecture, central mineral analyses. III. Clay Minerals. Am. Mineral. 5E, lapan. 1972 Int. CIay Conf., Kaolin Symp., p. 3l-40. 1023-1028. Plnnlu, W. E. (1969) Halloysite-rich tropical weathering eNo W. C. KnNc (1973) Standardfree energiesof product of Hong Kong. 1969 Int. Clay Conf. p.403-416. formation calculated from dissolution data using specific Ronrc, R. A., eNo D. R. Werpseuu (1968) Thermody- mineral analyses. II. Plagioclase feldspars. Am. Mineral. namic properties of minerals and related substances at 58. l016-1022. 298.15'K (25"C) and one atmosphere (1.013 bars) Knlrnn, W, D. (1970) Environmental aspects of clay pressure and at higher temperatures. U.S. Geol. Suru. Bull. minerals.I. Sediment.Petrol. 40r 788-813. 1259,256 p. E. F. HeNsox, W. H. HulNc, AND A. CenveNrss SeNo, L. B. (1956) On the genesis of residual kaolins. (1971) Sequential active alteration of rhyolitic volcanic Am. Mineral 41r28-40. rock to endellite and a precursor phase of it.at a spring in Michoacan, Mexico. Clays Cla1,Minerals, 19, l2l-127. Manuscript receiued,luly 5, 1973; accepted MrN,lro, H., AND M. Ureoe (1972\ A mode of occurrence f or publication,Nooember l, 1973.