SOLUBILITY DIAGRAMS FOR EXPLAINING ZONE SEQUENCES IN BAUXITE, KAOLIN AND PYROPHYLLITE-DIASPORE DEPOSITS

Clays and Clay Mnerals, Vol. 24, pp. 297-302. Pergamon Press 1976. Prnted n Great Brtan SOLUBILITY DIAGRAMS FOR EXPLAINING ZONE SEQUENCES IN BAUXITE, KAOLIN AND PYROPHYLLITE-DIASPORE DEPOSITS YOSHIRO TSUZUKI Department of Earth Scences~ Faculty of Scence, Nagoya Unversty, Chkusa-ku, Nagoya, Japan (Receved 26 August 1974; and n fnal form 25 July 1976) Abstract--Solublty dagrams defned by log [A13 +] and log [H,SO,] are gven for hydrous alumna or alumnum slcate mnerals whch appear n bauxte, kaoln and pyrophyllte<lapore deposts. They are constructed based on thermodynamc data of relevant reactons both at the room temperature and at elevated temperatures. An aqueous soluton reacts to a mneral, n ths case K-feldspar, and, by dssolvng t, becomes saturated wth respect to a certan mneral. Ths mneral begns to be precptated and the soluton changes ts composton as a result of the precptaton as well as further dssoluton of the orgnal mneral. Then, t attans saturaton wth respect to another mneral, whch s precptated thereafter. Thus, dfferent mnerals are precptated n turn. The sequences of precptaton of mnerals can be shown on the dagrams under dfferent condtons. A sequence, alumnum hydroxde ~ kaolnte or pyrophyllte---} slca mneral plus kaolnte or pyrophyllte s expected n a weakly acd soluton. In contrast, a sequence, slca mneral----} slca mneral plus kaolnte or pyrophyllte s expected n a strongly acd soluton. The possblty of applcaton of the sequence of precptaton thus expected to alteraton zonng s also dscussed. INTRODUCTION Bauxte, kaoln, and pyrophyllte-daspore deposts are formed by weatherng or hydrothermal alteraton, both of whch are caused by aqueous solutons permeatng rocks; and zonng of alteraton products s a common feature of these deposts. Solublty dagrams have been proved useful n nterpretng natural mneral assemblages or chemcal reactons occurrng naturally. Log [A13 or log [A102]-pH-log [H,SO,] dagram by Garrels and Chrst (1965) and by Curts and Spears (1971); (ph-~ pa13+}-ph,so4 dagram by Kttrck (1969), and log (total dssolved A1)-log (total dssolved S) dagram by Gardner (1970) are examples. The wrter constructed smlar solublty dagrams both at room temperature and at elevated temperatures. In ths paper, these dagrams are gven and zone sequences expected from them are shown. Constructon SOLUBILITY DIAGRAMS Solublty dagrams wth coordnates of log [A13+] and log [H4SO4] were constructed at fxed actvtes of H + and K + and at fxed temperatures, and are shown n Fgure 1 and Fgures 4-9. The followng reactons were consdered n ther constructon: KA1S30 8 + 4H + + 4H20 = K + + A13+ K-feldspar(mcroclne) + 3H,SO4, (1) A12S4Olo(OH)2 + 6H + + 4H20 = 2A13+ pyrophyllte AlzS2Os(OH)4 + 6H + = 2A13+ + H20 kaolnte + 4H4SIO4, (3) + 2H,SO4, (4) A10(OH) + 3H + = A13+ + 2H2 O, (5) daspore A10(OH) + 3H + = A13+ + 2H20, (6) boehmte AI(OH)3 + 3H + = A13 + 3H2 O, (7) (crystallne) gbbste SO2 + 2H20 = H4SO4. (8) c~-quartz The equlbrum constants of these equatons at varous temperatures were gven by Helgeson (1969) except for (3), (5) and (6), for whch they were computed by employng the equatons proposed by Helgeson (1969) and the thermodynamc data for pyrophyllte (Haas and Holdaway, 1973), daspore (Haas, 1972) and boehmte (Barn and Knacke, 1973). Usng the equlbrum constants of the above reactons, the solublty lnes on whch a sold mneral coexsts wth soluton were drawn for varous mnerals on each dagram. For reacton (1), as an example, equlbrum constant K 1 s expressed by KAlaS3Olo(OH)2 + 10H + = K + + 3A1 a muscovte + 3H4SIO4, (2) 297 IK + ] I-A13+ ] IH4SO4] 3 K 1 = [H+] 4, (9)

9 298 YOSHIRO TSUZUKI or logk1 = log [K +] + log [A13+] + 3 log [Hr - 4log [H+]. (10) If log [K +] and log [H +] have fxed values, ths equaton s represented by a straght lne on a log [A13+] vs. log [H4SO4] dagram. Lnes F1 and F2 n Fgure 1 are such lnes. The solublty lne of amorphous slca was drawn after Kennedy (1950), when the solublty of amorphous slca was mportant. Although many workers (e.g. Frnk and Sawhney, 1967; Parks, 1972; Huang and Keller, 1972) have dscussed the state of A1 n aqueous soluton, domnant speces n ths study are consdered to be A13+, AI(OH) z+ and AI(OH)2, followng Helgeson (1968) and Helgeson et al. (1969). Assumng the actvty coeffcents of the ons to he unty, the total dssolved A1 can be gven from the actvty of A13+ and the ph usng the equlbrum constants of the followng equatons, and s ndcated as the ordnate on the rghthand sde of the dagrams. AI(OH)2 = A13+ + 4OH-, (11) AI(OH) 2+ = A13+ + OH-, (12) H20 = H + + OH-. (13) Although H4SO 4 dssocates nto H + and H3SO4, the rato of HaSO 2 to H4SO 4 s neglgbly small n an acd or nearly neutral soluton. Interpretaton Here, the chemcal reactons whch follow the congruent dssoluton of K-feldspar nto an aqueous soluton are assumed. These reactons at ph = 5, [K +] = 0.001, and temperature = 25~ may be predcted based on the solublty dagram, Fgure 1. The 25"C ph=5 0 0 ~ ~ ~.~ ' Sol. Feld Fgure 1. An example of solublty dagrams for K-feldspar alteraton. G--gbbste, K--kaolnte, S--amorphous slca, F1--K-feldspar (mcroclne) at [K+] = 0.001, F2--K-feldspar (mcroclne) at [-K +] = 0.01. I--ntal path for K-feldspar dssoluton. dssoluton of K-feldspar causes the total concentraton of slca, alumnum and potassum n the soluton to ncrease, keepng the relatve proportons of 3 : 1 : 1. Therefore, the concentraton of alumnum and slca produced by the dssoluton of K-feldspar should fall on the straght lne, I, whch passes the pont log (total dssolved A1) = 0 (mal = 1) and log [HgSO4] = 0.477 (mh,so, = 3) and nclnes 45 ~ to the ordnate. As the dssoluton of K-feldspar proceeds, the concentraton of alumnum and slca change from lower left to upper rght along ths lne, whch s called the ntal path. When the ntal path reaches the gbbste lne at pont a, gbbste begns to be precptated. The contnung dssoluton of K-feldspar brngs about the ncrease of dssolved slca along the gbbste lne, because the precptaton of gbbste keeps the A13 + concentraton constant. When the path reaches pont b, the ntersecton of the gbbste and kaolnte lnes, gbbste begns to be dssolved and kaolnte begns to be precptated. The materal balance at ths pont s expressed by 2KA1S3Os + 4Al(OH)3 + 2H + = 3A12SzOs(OH)4 + 2K + + H20. (14) After gbbste s totally consumed, the path turns to the lower rght along the kaolnte lne, on whch the Hr concentraton ncreases wth the precptaton of kaolnte. When the path reaches pont c, the ntersecton of the kaolnte and amorphous slca lnes, amorphous slca begns to be precptated together wth kaolnte. The materal balance at ths pont s expressed by 2KA1S3Os + H20 + 2H + = 4SIO2 + AIzS2Os(OH)r + 2K+. ' (15) The path does not proceed further along, the slca lne. If the path proceeded downward along the slca lne, the dssoluton of K-feldspar would not take place. Proceedng n ths drecton means a decrease of the dssolved A13 wthout precptatng Al-bearng mneral, whch s contradctory to the dssoluton of K-feldspar. The reactons occurrng n ths system are llustrated n Fgure 2. Evaluaton of basc assumptons Before lookng at the applcaton of solublty dagrams to zonng, the valdty of basc assumptons nvolved wll be examned. I Dssoluton of K-feldspar I I Precptaton of gbbste I I Dssoluton of gbbste, l Precptaton of kaolnte I I 'l Precptaton of amorphous slca I I L~ I 1 I I [ I a b c Tme Fgure 2. Schematc dagram showng the sequence of chemcal reactons whch follow the congruent dssoluton of K-feldspar at ph = 5, [K +] = 0.001, temperature = 25~

Zone sequences n bauxte, kaoln and pyrophyllte-daspore deposts 299 (1) Thermodynamc data employed. As mentoned above, the thermodynamc data used for constructng solublty dagrams were derved manly from Helgeson (1969). For room temperature, more recent data were gven by Curts and Spears (1971), Huang and Keller (1973) and others. These data produce slghtly dfferent dagrams, but essental features of the dagrams are not affected. (2) Dssoluton and precptaton mechansm. In ths study, the dssoluton of K-feldspar followed by the precptaton of other mnerals s consdered nstead of the replacement of K-feldspar by other mnerals. O'Nel and Taylor (1967), Wlson et al. (1971), Keller et al. (1971), Nakagawa et al. (1973), and Tsuzuk et al. (1973, 1974) suggested that the alteraton process of a mneral n a soluton can be regarded as composed of the dssoluton of ths mneral, followed by the precptaton of another mneral. (3) Congruent dssoluton. The ntal stage of the dssoluton of K-feldspar may be the exchange reacton of K + for H + (Wollast, 1967). Experments by Correns (1961) and Morey and Chen (1955), however, show that K-feldspar s dssolved almost congruently n the man stage. (4) Dssoluton of K-feldspar. Natural rocks have, of course, more complex composton than monomneral K-feldspar. If the most soluble mneral n a rock s Na-feldspar nstead of K-feldspar, the sequence of reactons shown n Fgure 2 remans unchanged, because the postons of the ntal path and the solublty lnes are the same as n Fgure 1. Because these two mnerals are wdely dstrbuted and lable to be dssolved, the present nterpretaton of solublty dagrams may be appled to natural zonng n varous rocks. (5) Partal equlbrum. In the nterpretaton of solublty dagrams, the partal equlbrum was assumed followng Helgeson (1968) and Helgeson et al. (1969). That s, t was assumed that the precptaton of mnerals occurs, mantanng equlbrum, whle K-feldspar s beng dssolved contnuously nto a soluton,.e. equlbrum s not yet attaned. Ths assumpton may be allowed, because the dssoluton of K-feldspar s a slow reacton. The ncompatble assocaton of K-feldspar and kaolnte s commonly observed n altered grantc rocks (Meyer and Hemley, 1959). (6) Change n concentraton of K + and H +. Although the solublty dagrams were constructed for fxed K + and H + concentraton, these concentratons vary wth the dssoluton and precptaton reactons n queston. Ths change n K + and H + concentraton can be estmated quanttatvely by the method of Helgeson (1968) and Helgeson et al. (1969), but, for the purpose of the present study, a smpler method of estmaton s suffcent. The change n ph durng the reacton has no mportant effects on sequences of precptaton, because the mutual relatons between the solublty lnes of pyrophyllte, kaolnte, daspore, boehmte and gbbste are not affected by the change, n ph. The effect of changng K + concentraton can be estmated by drawng solublty fnes of K-feldspar and sercte at dfferent K + concentratons, lke F1 and F2 n Fgure 1. At pont c n Fgure 1, K + concentraton of the soluton ncreases wth the precptaton of amorphous slca and kaolnte and the dssoluton of K-feldspar, resultng n a leftward shft of the solublty lne of K-feldspar. Ths lne becomes passng pont c at the K + concentraton defned by equaton (9); ths means that the soluton becomes saturated wth K-feldspar and the seres of reactons descrbed n the prevous secton termnates leavng an amorphous slca-kaolnte-k-feldspar assemblage behnd. Applcaton to zonng Natural weatherng and hydrothermal alteraton take place by the acton of mgratng aqueous solutons. Therefore, the successve reactons shown n Fgure 2 are expected to occur as a functon of mgraton dstance as well as that of tme. The only dfference from Fgure 2 s the absence of stage b n whch the dssoluton of gbbste and the precptaton of kaolnte occur smultaneously. When the soluton becomes saturated wth respect to kaolnte, t moves away from where t precptated gbbste; therefore, no gbbste s avalable for dssoluton. Thus, a zonng, dssoluton zone ~ gbbste zone---* kaolnte zone--~ kaolnte-slca zone--* unaltered zone, may be formed. Ths state s llustrated n the left part of Fgure 3. Contnuous flow of the soluton wll brng about a shft of zone boundary by the followng reason. When a new soluton reaches the boundary of the dssoluton zone and the gbbste zone, the A13 + concentraton s not yet hgh enough for precptatng gbbste, because the amount of K-feldspar has decreased by dssoluton n the dssoluton zone. Therefore, the boundary of these two zones shfts further from the source wth tme. Other zone boundares shft lkewse, resultng n the mgraton of zones as llustrated n Fgure 3. Accordngly, the mode of Tme Fgure 3. Schematc dagram showng development of zonng pattern under condtons shown n Fgure 1. F--feldspar, G gbbste, ~kaolnte, S--amorphous slca, V--vacant space. V G F

- 300 YOSHIRO TSUZUKI o.,o/~ ' 25"C ph: 5.5 / G b\\ c \. Sol. Feld r 9... I Fgure 4. A solublty dagram under weatherng condtons. G--gbbste, K--kaolnte, S--amorphous slca, I--ntal path for K-feldspar dssoluton. geness of a mneral s not unform; gbbste, for example, can be formed n ether of the followng two ways, (1) K-feldspar --* soluton ---* gbbste, (2) K-feldspar ~ soluton ~ kaolnte ~ soluton ---, gbbste. 0 ~ ~0 DIAGRAMS FOR VARIOUS CONDITIONS Dagrams for weatherng condtons For applyng zone formaton, under weatherng condtons, the dagrams at 25~ and at a weakly acd ph soluton, were prepared. A dagram at 25~ and ph 5.5 s shown n Fgure 4. The sequence of precptaton s gbbste ~ kaolnte ~ amorphous slca plus kaolnte. A dagram at 25~ and ph 4 s shown n Fgure 5. The sequence s kaolnte---* amorphous slca plus kaolnte. A zonal sequence g wthout a gbbste zone s expected under such low ph condtons. Dagrams for kaoln and pyrophyllte-daspore deposts Interestng zonal sequences smlar to hydrothermal deposts, such as kaoln deposts and pyrophylltedaspore, were prepared at 200 and 300~ The mnerals concerned are boehmte, kaolnte and amorphous slca at 200~ and daspore, pyrophyllte and a-quartz at 300~ Accordng to Kennedy (1959), boehmte s formed metastably around 200~ whle daspore s formed at hgher temperatures. Tsuzuk and Mzutan (1969, 1971) showed that kaolnte was transformed nto pyrophyllte at 270~ Mzutan (1966) reported that amorphous slca was formed at a fumarole at 200~ Examnaton of solublty dagrams at dfferent ph revealed that there are two dfferent types of sequence of precptaton: (A) sequence from weakly acd solutons and (B) that from strongly acd solutons. Examples of type A are shown on the dagram at 200~ and ph 4 n Fgure 6 and that at 300~ and ph 3 n Fgure 7. In these dagrams, the ntal paths meet the boehmte lne or the daspore lne, and the paths turn clockwse smlar to the example above. The sequence of precptaton s boehmte--* kaolnte--, amorphous slca plus kaolnte, or dasp0re--* pyrophyllte--~ e-quartz plus pyrophyllte. If [K = 1.0 on the dagram n Fgure 6, muscovte s precptated nstead of kaolnte. A sercte zone, therefore, may be formed by K rch soluton. If the ntal soluton s a lttle more acd, say ph = 2.3, the ntal path shfts to the rght and frst meets the kaolnte lne (Fgure 8). Boehmte or daspore zone does not occur n alteraton by such solutons. An example of the type B sequence s shown n the dagram at 200~ and ph 1.8 n Fgure 9. In 25"C ph=4 / 200*C ph=4 ' I xx 5 \ G,--,,---1 "~ \ 0 w "~x \ o \\ " Sol. Feld :n, I,, Fgure 5. A solublty dagram under weatherng condtons. The soluton s more acd than n Fgure 4. G--gbbste, K--kaolnte, S--amorphous slca, I--ntal path for K-feldspar dssoluton. log [ H4S04] Fgure 6. A solublty dagram under hydrothermal condtons. B--boehmte, K--kaolnte, S--amorphous slca, M--muscovte at [K +] = 1,0, F--K-feldspar at [K = 1.0, I--ntal path for K-feldspar dssoluton.

Zone sequences n bauxte, kaoln and pyrophyllte-daspore deposts 301 300*(3 ph=3 ths dagram, the ntal path frst meets the amorphous slca lne at pont d, and then the soluton changes ts composton upward along ths lne, precptatng amorphous slca. Thus the drecton s antclockwse. When the path reaches pont c, kaolnte begns to be precptated together wth amorphous slca. Consequently, the sequence expected s slca --~ slca plus kaolnte. _o -15 a Sol. Feld b Jog [ H4S04] Fgure 7. A solublty dagram under hydrothermal condtons. D~aspore, P--pyrophyllte, Q~a-quartz, I--ntal path for K-feldspar dssoluton. ~ o ~ 200"C ph= 2.3 o,,. 9 I Fgure 8. A solublty dagram under hydrothermal condtons. B boehmte. K kaolnte. S amorphous slca. I--ntal path for K-feldspar dssoluton.,7-.9 % B 200"C ph= 1.8 c ~,, I,, r -o 0 $ _g /0 3 log I H~04] Fgure 9. A solublty dagram under hydrothermal condtons. The soluton s more acd than n Fgure 6. B--boehmte. K--kaolnte. S--amorphous slca. I--ntal path for K-feldspar dssoluton. x~ Acknowledgements--The author s grateful to Dr. W. H. Huang of Unversty of South Florda for readng the manuscrpt and makng many helpful comments. He s ndebted to Dr. K. Nagasawa of Nagoya Unversty for encouragng support, helpful suggestons and crtcal comments. He also wshes to express hs thanks to Dr. N. Naka and Dr. S. Mzutan of Nagoya Unversty for ther constructve suggestons. REFERENCES Barn, I. and Knacke, O. (1973) Thermochemcal Propertes of Inorganc Substances: Sprnger, Berln. Correns, C. W. (1961) The expermental chemcal weatherng of slcates: Clay Mn. Bull. 4, 249-265. Curts, C. D. and Spears, D. A. (1971) Dagenetc development of kaolnte: Clays & Clay Mnerals 19, 219-227. Frnk, C. R. and Sawhney, B. L (1967) Neutralzaton of dlute aqueous alumnum salt solutons: Sol Sc. 103, 144-148. Gardner, L. R. (1970) A chemcal model for the orgn of gbbste from kaolnte: Am. Mneralogst 55, 1380-1389. Garrels, R. M. and Chrst, C. L. (1965) Solutons, Mnerals, and Equlbra: Harper & Row, New York. Haas, H. (1972) Daspore-corundum equlbrum determned by eptaxs of daspore on corundum: Am. Mneralogst 57, 1375-1385. Haas, H. and Holdaway, M. J. (1973) Equlbra n the system A1203-SO2-H20 nvolvng the stablty lmts of pyrophyllte, and thermodynamc data of pyrophyllte: Am. J. Sc. 273, 449-464. Helgeson, H. C. (1968) Evaluaton of rreversble reactons n geochemcal processes nvolvng mnerals and aqueous soluton~i. Thermodynamc reactons: Geochm. Cosmochm. Acta 32, 853-877. Helgeson, H. C.. (1969) Thermodynamcs of hydrothermal systems at elevated temperatures and pressures: Am. J. Sc. 267, 729 804. Helgeson, H. C., Garrels, R. M. and Mackenze, F. T. (1969) Evaluaton of rreversble reactons n geochemcal processes nvolvng mnerals and aqueous solutons II. Appfcatons: Geochm. Cosmochm. Acta 33, 455-481. Huang, W. H. and Keller, W. D. (1972) Geochemcal mechancs for the dssoluton, transport, and deposton of alumnum n the zone of weatherng: Clays& Clay Mnerals 20, 69-74. Huang, W. H. and Keller, W. D. (1973) Gbbs free energes of formaton calculated from dssoluton data usng specfc mneral analyses. III. Clay Mnerals: Am. Mneralogst 58, 1023-1028. Keller, W. D., Hanson, R. F., Huang, W. H. and Cervantes, A. (1971) Sequental actve alteraton of rhyoltc volcanc rock to endellte and precursor phase of t at a sprng n Mchoacan, Mexco: Clays & Clay Mnerals 19, 121-127. Kennedy, G. C. (1950) A porton of the system slcawater: Econ. Geol. 45, 629~553. Kennedy, G. C. (1959) Phase relatons n the system A1203-H20 at hgh temperatures and pressures: Am. J. Sc. 257, 563 573. C.C.M. 24/6~

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