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Of Kaolinite and Halloysite in Relation to Weathering Feldspars And

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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 trans- formed 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 ob- tained 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 param- eters,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 greaterthan 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
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