Stability Fields of Clays and Aluminum Phosphates: Parageneses In

American Mineralogist, Volume64, pages626434, 1979

Stabilityfields of claysand aluminum phosphates: parageneses in lateritic weatheringof argillaceousphosphatic sediments

PutLpprVrnrLleno, Yvns Tenny Laboratoirede Pedologieet de Geochimie,38 ruedes Trente-Six Ponts 31078Toulouse Cedex, France

nNnDnrmr- NnsoN Laboratoirede Petrologiede la Surface,40auenue du recteurPineau E6000Poiliers, France

Abstract

Severalmethods of estimatingthermodynamic data of naturalphosphates were unable to predictwith enoughaccuracy stability fields of suchminerals and the paragenesesobserved in natural conditions.Detailed observationsmade on lateritic weatheringprofiles, from the bottom to the top, yield the following mineralassociations: Fluor-carbonate-apatite*montmorillonite4calcite-fluor-carbonate-apatite+montmorillonite-fluor-carbonate-apatite*kaolinite-crandallite*kaolinite-wavellite * kaolinite* augelite- gibbsite. A refinedmethod for estimatingGibbs freeenergies of formation of thesephosphates is proposed.A first stepof estimationis basedon the summationof the Gibbs freeenergies of formation of simplecomponents and an additionalterm which dependson the nature of cations in each compound.A secondcalibration step is basedon the correctionof the additionalterm so that the Gibbs freeenergies of formationand the fieldsof stabilityof the phosphatesand clay mineralswould be in good agreementwith the natural observedse- quence.These estimations yield the followingvalues (kcal/mole):

Fluor-carbonate-apatite,Caro(POr )rCOsFs: LGot= -2983.3 C randallite,CaAlr(PO.)r(OH )iHzO: LGi : -1336.7 Wavellite,AL(PO{ ),(OH )g(HzO )iH,O: AGl = -1330.0 Augelite,AlrPor(oH)b AGi : -660.9 Variscite,AtPOi2HrO: AGl : -498.8 Ca-millisite,Ca,.oAl.(PO.)r(OH )i3HrO: AGot = -2614.0

Introduction of formation of phosphateshas beenapplied to the determinationof the stabilityfields of theseminerals, The phosphateplateau of Thies in Senegalyields but hasshown a largediscrepancy between the calcu- an almost completesequence of clays and calcium- lated and the observeddata. For example,Nriagu's aluminum-hydroxy-phosphates,commonly found in valuesdetermine a fieldof stabilityof crandalliteand lateritic weatheringprofiles: fluor-carbonate-apatite, wavellitetoo largewith regardto the stabilityfields of calcite,montmorillonite, kaolinite, millisite, crandal- montmorillonite and kaolinite;in theseconditions, lite, and wavellite. Some profiles in other regions clay mineralsshould not existin naturalparageneses. yield also millisite, variscite,augelite and gibbsite An attempt to refinesuch estimated data by examin- (Flicoteauxet al., 1977).A recentmethod proposed ing their coherencewith field observationsis now by Nriagu (1976)for estimatingGibbs free energies presented. 0n/O340/-X / 79 /0506-0626$02.00 626 VIEILLARD ET AL.: ARGILLACEOUS PHOSPHATIC SEDIMENTS

Roughestimation of Gibbs free energiesof formation energiesof formation of the oxide and the aqueous of apatites and calcium-aluminum-hydroxy-phosphatescation). This concepthas been extended by Vieillard(1978) to more complex compoundssuch as monetite Vieillard (1978)compiled the differentvalues of (CaHPO.).For example: Gibbs free energy,enthalpy, and entropy of forma- _tl tion of phosphatesavailable in theliterature. Most of AGiCaHPOT= AGiCa,(POn),+ AGTHgPO{ the thermodynamicdata arefrom measurements,but 5 i someare simplyestimated. Different methods of esti- 84 mating enthalpiesand Gibbs freeenergies of forma- TXT AO2-Caa(POn), tion, basedon the corresponding-staterelationship [mean principle, have been presentedfor relatively simple - mineralssuch as meta-, ortho-, and pyrophosphates. meanAO'- HsPOil Q) Recently,Nriagu (1976)and Nriagu and Dell (1974) Coefficients{andi are,respectively, the numberof proposeda method,roughly similar to thatof Tardy oxygenbounded with eachof the two compounds and Garrels(1974), for naturally-occuring hydroxy- t-3ioi-icar(po.)r,t for*H,Po.l; a'is a specificcoeffi- phosphates.This method is basedupon the summa- cient for neutraland acidphosphates formed by three tion of the freeenergies of formationof simplecom- components;mean AO'z-Car(POa)2 and meanAO2- ponents.The Gibbs freeenergy for a givenhydroxy- HsPOl refer respectivelyto the weightedaverage of phosphateis equalto the sum of threeterms: AGi the AO'- cationconstituting the simplecompounds : - hydroxy-phosphate \ A,G",1r,1+ xAG.l HrO e, Cag(POr)zand HsPO.,as for example: in which 2, LG",1r,1is the sum of the Gibbs free gqq#-sAo,-! energiesof the hydroxides or simple phosphates meanAO2-Cag(POr)z = formingthe hydroxy-phosphates,x is the numberof moles of water involved in the complexphosphate, l.5ao,-H + 2.5AOr-P andQ:2.303Rf 2n1logn,, where nl is thestoichio- mean AO'-HrPOo metric coefficientof the rth hydroxideor the simple For all complex orthophosphateswith phosphate.This method yields a differencebetween three cat- ionsa' : 0.335.The differencebetween the measured the predictedand experimentalresults generally bet- and the calculatedfree energyis statisticallyequal ter than 15 percent;however, it appearsthat this to t2.5 kcal/mole. differenceis too large for interpretationsof natural parageneses. For the estimation of the Gibbs free energyof formationof basicphosphates such as AlrPOr(OHL, Vieillard(1978) used Nriagu's method of estima- an equationsimilar to equation(2) wasused: tion but modifiedthe expressionof the term Q ac- cordingto the newdata obtained by Tardy andGar- AG?AI,PO.(OH)b: AG?AIPO{+ AG?A(OH)s rels (1976, 1977),Tardy and Gartner (1977), and Tardy and Vieillard(1977). Then, incorporating this - p x !.ff 1r"unaO,-A(oH)s modification,the Gibbs freeenergy of formationof a - given compound [Car(POr)zas an example]formed meanAO'- AlPOll (3) of two oxidesis equalto B is a coefficientanalogous to c'. From experimental data for augelite : -671.5 kcal/ AGi Ca,(POn)z:3AGlCaO * AG?p,Ou [AG?AlrPOi(OH), mole (Wiseand Loh, 1976)lone obtains0 = 0.77. -" The estimated value given by Nriagu (1976) - ao,-p) (r) -665.3 +it(Ao,-ca IAG?A|,PO1(OH),: kcallmolel yieldsB = 0.45. ln this equation a is a coefficientcharacterizing the By usingalternatively one of thesetwo valuesfor B, samefamily of compounds(phosphates, carbonates, Vieillard(1978) estimated the freeenergies of forma- etc.); n, andn" are,respectively, the numberof oxy- tion of severalphosphates (Table I ) suchas wavellite gen boundedwith each of the cations (n, : 3 for and crandallite.These minerals can be consideredas : CaO, n" 5 for PrOu);AO,- Ca and AO2- are hydroxy-phosphatesafter removal of HrO. The parameterscharacterizing the cations(aOr- cationis Gibbs freeenergy contribution of HrO wasestimated equal to the difference between the Gibbs free from the valuesof hydration of AIPO..2HzO or 628 VIEILLARD ET AL.: ARGILLACEOUS PHOSPHATIC SEDIMENTS

CaSOr.2H2Oand fixed at -56.7 kcal/mole for zeo- Table l. Gibbs free energies of formation of several phosphates litic waterand -57.0 kcal/molefor ligandwater. from different sources Table I showsa considerablediscrepancy among Mineral nanes Ac'- AG: data from different sources.Further investigations JT J and fomulae |leasured EstiMted calculaEed i basedupon naturalobservations necessitate a drastic (Nrlagu, (Vieillard, I 978) t976) B=O.77 B=0.45 B=0.30 revisionof the valuesof the coefficientB. The value -671,50 -665.30 -662.11 Augelite -61 | 50(l) -665.30 whichcorresponds to the observationsmust be close A12poq (oE)

-1343.20 -1335.00 -1331.15 to 0.30instead of 0.45calculated from Nriagu(1976) Vave11 i le -1339. and0.77 from Wiseand Loh (1976). A1: (POq) z (oH) r (HzO)q.Hzo -r364.02 -1345,50 -1336.51 Crandall i re -1342. Paragenesesof lateritic weatheringon CaAl 3 (Poa) 2(oH) 5 (H2o) phosphaticsediments Ca1.5A15 (POa)q (OH)9. 3H2O

Lateritic weathering of alternate layers of cal- -503 (2) -498 8 (3) careous phosphatesand argillaceoussediments, AIPOq (HzO) z transformedinto aluminousphosphates, are found in - Relercnces : (l) - Wise and Loh (1976) ; (2) Lindsay, Peech and Clark Senegal(Capdecomme, 1952, 1953; Slansky et al., - (1959) ; (3) This study. 1964);in Nigeria(Russ and Andrews, 1924); in Flor- (kcaI/nole) ida (Espenshadeand Spencer,1963, and others);in ^c: or minerals based on : -53.00 AGf ArPoq = -)42 70 ac"îl,o (1is.) - -57.00 Ao2- H- = J' Siberia (Zanin, 1968);and -12-07 on ChristmasIsland ^c; A1(oH)3 = -273.35 AGi H20 (zeo].) = -55.59 Loz caz = -71.00 (Trueman,1965). Recent studies of Flicoteauxet al. AG: ca(oH)2 = -214.76 ôz ^1r = -48.71 ôz Pr = (1977) on the Eocene-Oligoceneargillaceous and phosphaticdeposits of Lam-Lam (ThiesPlateau in Brewer,1964, p.317). All thesetransformations are Senegal)have shown vertical and lateralrelationships achievedwith conservationof originalvolumes and during mineral parageneses.Weathering profiles sedimentaryfeatures. yield a successionof horizonsor layers,which from (c) Gray and och rous calcium-aluminum-pho sphat e bottomto top are(Fig. l): horizon.In the argillaceousor argillaceoussilty lay- (a) Parentrocks composedof alternating(1) argil- ers,the quantity ofkaolinite decreases and is replaced laceous or argillaceoussilty layers with mont- by a millisite* crandallitecrystalliplasma. Crandal- morillonite(80 percent) * illite(10 percent) or rarely lite is not veryabundant at the baseofthis zone,but attapulgite(70 percent)* montmorillonite(30 per- increasesprogressively and becomesthe dominant cent),with a smallamount of phosphate;(2) calcium mineralat the top. In the phosphatebeds the same phosphatelayers with carbonate-fluorapatiteand a developmentof crandalliteis observed:(l) radiating dispersedclay mineralfraction composed of mont- needleson theorder of l-3mm in thecrystalliplasma morillonite+ illite. sensustricto; (2) short perpendicularneedles at the (b) A transitionhorizon which showsa progressive edgesof voids developingupwards. Detrital quartz, upwardtransformation of parent-rockminerals. (l) so far preserved,is marked by corrosionvugs or In theargillaceous or argillaceoussilty layers, kaolin- cracks cutting grains into several pieces. Under ite rapidly forms from the montmorillonite and crossedpolarizers simultaneousextinction can be reaches90 to 100percent of the claymineral fraction. seenin all piecesof an original quartz grain, which Illite is presentin the amountof 0 to l0 percent.The indicatesdissolution in situ.The dissolutionvugs are carbonate-fluorapatitefraction is transformedinto filled with crandallite crystalliplasma.Here again, millisitewhich forms small sphericalcrystal aggre- originalvolumes are preserved in the horizon. gates.This horizonis initiallygray, but becomesred (d) White aluminophosphatehorizon. Wavellite or ochrousowing to the formationof goethite.(2) In crystalsfirst form on the edge of voids closeto the the calciumphosphate layers the porosity increases. needlesof crandallite.Then thesecrystals begin to The clay mineralfraction rapidly becomeskaolinitic. appearand become more numerous in the crandallite In the lower transition horizon the layers are still crystalliplasmaitself. Progressively, needles of wavel- composedof carbonate-fluorapatitewith a few milli- lite replaceall the crandalliteand spreadout into the site spheres.Upwards, the millisitespheres become plasma.Quartz is completelydissolved, and kaolinite morenumerous, and crandalliteappears and spreads is presentin very small amounts.In this horizon out. The millisite-crandalliteassociation constitutes originalsedimentary features and volumesare still a widely-developedcrystalliplasma (in the senseof conserved. VIEILLARD ET AL.: ARGILLACEOUS PHOSPHATIC SEDIMENTS 629

goethite segregation CR crandallite Ml+GO+K cristalliplasma

void crandallite with cristalliplasma of cl

millisite spheres kaolinitic plasma K+ Ml+ G0 gcethite segregation CD .9 relict ot pellet o = MI+GR millisite + crandallite o cristalliplasma o] AP+K o '-ar, o o o kaolinitic plasma o o g@thite segregation miltisite spheres

millisite spheres carbonate - f luoraoatite

rnontmorillonitic plasma tz

carbonate tluorapatite cristalliplasma secondary calcate granular pellet of carbonate - f luorapatite

Fig. l. Mineralparagenesis sequences in the Lam-Lamlateritic profiles (Thies plateau, Senegal). Column A refersto phosphaticbeds; column B refersto argillaceousbeds. Size of lettersis roughlyproportional to the abundanceof minerals.MO : montmorillonite;I : illite;CA=calcite;AP:fluor-carbonate-apatite; K:kaolinite;MI:millisite; 69:goethire; CR=crandaltite;W=wavellite.

(e) Secondaryminerals filling uoids and uqcks. A served;above, but alwaysin the lower part of pro- systemof voids and cracksof varioussize marks the files,secondary apatite, quartz and chalcedonyare different horizons. These voids. which cross sedi- present;in the upper horizons,fillings or coatings, mentary featuresand structures,are sometimessec- often red in color, are composedof kaolinite and ondarily coatedor filled by mineral accumulations. goethite.Later, kaolinite is sometimesreplaced partly The nature of these secondaryminerals changes or completelyby millisite;kaolinite coatings are lat- within the profile: at the bottom, in the almost fresh erally transformedinto millisitecoatings. apatite bedrock,sparry calcitecrystal coats are ob- All these secondaryminerals (calcite, quartz + 630 VIEILLARD ET AL.: ARGILLACEOUS PHOSPHATIC SEDIMENTS

Table2. Chemicalformulae, Gibbs freeenergies of formation,and solubilityproducts of mineralsused in the diagrams

Minetal AG.- (kca1/ Solubility product at 25'C name mole, t Cafcite CaCO3 roq lcu2+lt l!r*l * Loq fCO2 = 9.16 Montmorillo 2 3 orzolo(on) r279.24 roe t t 14 1os t 22 roq'-4 T-H-sio-l3?.10 nite-Ca lsi:. etoro- :: I zc"o. ru, [ca2+] ftt'l [A13+J l"tl

Kaol-inite A12Si2o5 (oH) 90'7 -64 2 roq z roq = 7.63 4 ll\13+l / Lts+13 lHasio..l 3 Gibbsite A1 (OH) 274 -BB roq = a -24 3 [nr3*l / 1."*] calo (PO4), (co,) F3 2943.3 8,sroqlca2+l / ft'12 * 1,s 1oq[ca2+] I roq [r-] + s rog lirrnoo] + ros f co2 = 4t.06 2 'Arl'] l. cd13 (Po4) (oH) (H2o) 1336.7 roq t 3roq / z ros 21.95 2 5 [c.2*r,/F+] lu*] fHreo*l

, 1+, ,-+,1 Ar3 (PO4) (OH) (H2O) H2o 1330. O : roq / + 2 Ioe t2.95 2 3 4. lrr"l La-]' lHlPo4' .. 1+, +.1 Al2PO4 (OH) 660 -9 z roe r.Loq = 1.O.4'7 3 lrr"] Z Ln']r fureonl 'nrPo.J A1PO4, (H2O) 498. B roq + ros = 2.6'1 2 far3*l z lu*]3

ca1.5At6 (Po4) (oH) 3sro 2614-o 1,s ros / | 6 4 9. lca- I lH l' + 4 foq lH^PO-l

,. )L- Fluorite CaF 2'79-0 roq + 2 ro9 F-l = 2 lc.'-l

References : (1) - HelSeson (1969) 12) - ftLtz and Tardy(1973) ; (3) - Vieillard (1978) ; (4) - This study. * Solubillty products are baseal on A""f H4sio4 (aq) = - 312.56 kcal/mole AG"t co2 (g) = - 94.254 kcal/mole

Ac"- A13+ (aq) = - 116.0 kcar/mole A"'f F (ag) =-66.64 kcaf/mole

Ao", ca- (aq) = - 132.3 kcal/mole o"., H2O (1iq)= - 56.69 kcal/mo1e AG"f H3 po4 (aq) = - 2i3-ro kcaf/nole chalcedony* apatite,kaolinite) are orientedper- the ultimateterm of the evolutioncan be reached. pendicularlyto the edgesof the voids.Thus, they are Furthermore,in theweathering of Eocenephosphatic authigenic.These sequences of secondaryminerals sedimentsof the Eboida area (Ivory Coast, near show:(l) the leachingof materialfrom the horizons Ghana),millisite is missingand varisciteoccurs be- above,and (2) the orderof accumulationin thepro- tweencrandallite and wavellitezones (Charpy and file: the most calcitic,the most solubleminerals are Nahon,1978). locatedlower and the mostaluminous, i.e. the least The sequenceof mineral weatheringpresented in soluble,are locatedhigher in the profiles. Figure 2 and Table 2 is: calcite,montmorillonite, The lateriticprofiles of Lam-Lam show that two apatite,kaolinite, crandallite, wavellite, augelite, and sequencesof weatheringare present: gibbsite.Calcium is the elementmost easily leached, first from carbonates,then from silicates,and finally montmorillonite- kaolinite from phosphates.Silica is removedrapidly as com- carbonate-fluorapatite- millisite - crandallite- paredto phosphorus,which remainsa long time in wavellite the profiles,thus delayingthe formationof gibbsite. Before the completetransformation of carbonate- Iron, presentin montmorillonite,appears as goethite fluorapatite,kaolinite is formed and secondarycal- and is not includedin phosphates.This observed cite is dissolved.In other lateriticprofiles of Lam- sequencecan be usedto refine the thermodynamic Lam, millisiteis occasionallyabsent, and augelite data and stabilityfields of calcium-aluminumphos- appearslocally near the surfacegrading to wavellite phatesand silicates. (Capdecomme,1952; Flicoteaux et al., 1977).The successionis as follows: Stability fieldsof apatite,millisite, crandallite, wavellite,variscite, and augelite carbonate-fluorapatite+ crandallite - From theinitial estimated values of AGi a firstset wavellite- augelite of stabilityfields was drawn and then modified until it Visse(1952) described gibbsite in theThies Plateau wasin good accordancewith sequencesin the field.A profiles.This indicatesthat underparticular condi- three-dimensionaldiagram, using as axes log tions,following kaolinite and aluminous phosphates, [H1SiO1],loe lCa'+)/[H+]'zand log [H'PO.], wasfi- VIEILLARD ET AL.,' ARGILLACEOUS PHOSPHATIC SEDIMENTS 631

ROCHEMERE ( A-B ) MONTMO-CO+APATITE+ CALCITE

B CALCITE ( B-C ) MONTMO-Co+APATITE MONTMO- Co KAOLINITE ( c-D ) KAOLINITE+ APATITE D KAOLINITE ( D_E ) KAOLINITEr CRANDALLITEr APATITE .-CRANDALLITE

E i FLUORCARBONATE APATITE tE-r, KAOLINITE+ CRANDALLITE KAOLINITECRANDALLITE F (F_G) KAOLINITE+CRANDALLITE KAOLINITE-CRANDALLITE KAOLINITE.CRANDALLITE - WAVELLITE ( G-H ) KAOLINITE+ WAVELLITE

t- KAOLINITE- WAVELLITE (H-r) KAOLINITE+WAVELLITE

- KAOLINITE- WAVELLI TE -AUGELITE ( I_J ) AUGELITE

.- AUGELITE- GIBBSITE ( J-K )

ALTERATIONCROISSANTE

Fig. 2. Schematicchemical path of solutionsduring weatheringof argillaceousphosphates. The senseof €volutionis that of a progressivedilution of solutionsfrom the baseto the top of the profiles. nally obtained(Fig. 3). A simulatedchemical-weath- francolite[Ca1o(POn)u(COs)Fs], and montmorillonite ering path from A to K is presentedin Figure2 and is consideredas a pure calciumaluminum silicate. detailedin thetwo-dimensional diagrams of Figure4. The solubility constantsused for drawingthe dia: The pathA-K illustratesthe general chemical change gramsare given in Table2. of a solutioninitially in equilibriumwith primary The AGi valuescalculated for the alumino-hy- rnineralsin a parent rock, and then progressively droxy-phosphatesfrom known solubilityconstants dilutedand continuouslyat saturationwith respectto and from approximatedconstants based upon min- the successivesecondary mineral associations. This eral parageneseswere usedto calculatea new value evolutionsimulates a lateriticweathering profile, pre- for the parameterp. This valueis found to be closeto sentedin a reversesense to that of circulatingsolu- 0.30for the four mineralsconsidered (calcic millisite, tions. crandallite,wavellite, and augelite). The associationsencountered along the path are Severalproblems were encountered in calculating the onesdescribed in the previoussection and illus- the stabilityfields of mineralsin a two-dimensional tratedin Figure l; the chemicalcompositions of the diagram:log lCa'+l/[H+]', log [H'PO.]. For the mineralsrepresented by the stability fieldsare those fluor-carbonate-apatite,the activity of [F-] was as- of natural minerals, except apatite and mont- sumedto be limited by the fluoriteprecipitation [2log morillonite, for which the formulas have beensim- (F- ) + log (Ca'z+) : - l0l, and the fugacityof CO, is plified.Natural fluor-carbonate-apatiteis replaced by presumedto be such that log KO, : -2.5. The 632 VIEILLARD ET AL,: ARGILLACEOUS PHOSPHATIC SEDIMENTS AUGELITE \ CRANDALLITE \ GIBBSITE h^2+l laa|-vv J KAOLIN IT E :"v[rl'

MONTMO-Co ,'{r'lYt CALCITE

FRANCOLITE

WAVELITE los[u.SoJ Fig. 3. Three-dimensionalstability field diagram,for calcite,francolite, calcium aluminum silicates, and phosphates. stabilityfield of varisciteis not representedin Figure porationof sodium,and (2) pure calcicmillisite is 4 for simplificationonly. It appearsbetween crandal- unstablerelative to crandallite. lite and wavellite.The stabilityfield of Ca-millisiteis not representedin Figure 4 becausethe value ob- Discussion tained in Table 2 does not fit this mineral stability The methodfollowed in this work utilizestwo dif- betweenapatite and crandalliteas observedin the ferentapproaches to determinethe valuesfor Gibbs field. freeenergy, i.e. estimationof thermodynamicdata on It shouldbe rememberedthat, althoughin nature onehand and feedbackcontrol from naturalobserva- millisiteappears between apatite and crandallite,this tions on the other.The secondmethod is in fact based mineralis not entirelycalcic but containsa significant on the assumptionsof internal consistencyamong quantity of sodium [Car.zoNao.uAl6(PO1)1(OH)r.stability fields of silicatessuch as montmorillonite, 3HrOl, according to published analyses.In con- kaolinite, and gibbsite and phosphatessuch as clusion,it can be supposedthat, in naturalcondi- francolite, millisite, crandallite,variscite, wavellite, tions,(l ) millisiteis formedonly becauseof the incor- and augelite. VIEILLARD ET AL,: ARGILLACEOUS PHOSPHATIC SEDIMENTS

t2 MOR.-Co 12

t0 tos r0 [x.siod-3)s. =-3,10 ro9 [{.sio1] D 8 I UOR-CARBONATE APATITE ; KAOLINITE 6 - ros[x.s'O.]=-r,Zs 6 --- tos !.siol.-z,zo I t, WAVELLITE t,

2 I ----.-J... tosiIHreo.] toe[Hreo,] 0 -9 -8 -7 -L -3 -2 -9 -8 .-",^. [co2*] ,"nff;+ lslz 12 CRANDALLITE 12 CRANDALLITE

los t0 tog =-3,s0 [H1siol]=-3,{,0 t0 E tos[H.so,]=-:,so [Hlsio(] APATITE ros[Hlsiol]=- 3,so lo9 [H1sio1]=-3ss I I tos [H1si01]=-3,80 FLUOR-CARBONATE KAOLINITE APATITE KAOLINITE 6 6

WAVELLITE 1 1 - WAVELLITE !o9[H,s'o.]=-9ao --- @E tos[u.s'o.]=-gss 2 2 tos[Hreo.] tos[rreo.] 0 0 -9 -8 -9 -8 tos. lcoz*l Frf ,"sffi 12 CRANDALLITE t2

FLUOR- CARBONATE-APATITE 10 l0

toe[qsro.]=-4,1s l-{ I 8 I tog [u.siod=-tte tog I [H1Sio.]=-1,18

6 KAOLINITE 6

L WAVELLITE t --- tog[ns'o.]=-a,19 - 1on 2 [H.5'6.]=-r,ts 2 ros [Hreo.] toe[Hreo.] 0 0 -9 -8 -7 -6 -5 -t -3 -2 -9-8-7-6-54-3-2

Fig. 4. Two-dimensional activity diagrams (log [Ca'z+],/[H+]2,log [HgPO.]) for different values of log [HrSiO.]. 634 VIEILLARD ET AL,: ARGILLACEOUS PHOSPHATIC SEDIMENTS

Clearly,in lateriticprofiles, the presenceof phos- Fritz, B. and Y. Tardy (1973)Etude thermodynamique du systEme phate in the bedrock alters the usual geochemical gibbsite,quartz, kaolinite, gaz carbonique.Application i la behaviorof calcium,silicon, aluminum, and iron ob- gendsedes podzols et desbauxites. Sci. Ghol. Bull., 26, 339-367. (1969) servedin the weatheringof pure Helgeson,H. C. Thermodynamicsof hydrothermalsystems silicaterocks under at elevatedtemperatures and pressures.Am J. 5ci.,267,729- same climatic settings.Calcium is quickly leached 804. from silicate-derivedprofiles but is retainedlonger in Lindsay,W. L., M. Peechand J. S.Clark (1959)Solubility criteria phosphateformations. Kaolinite is dominantin pro- for the existenceof variscitein soils.Soi/ ,Sci. Soc. Am. Proc..23. fileswith silicateparent rocks but becomesephemeral 357-360. (1976) in a phosphaticweathering sequence. gibb- Nriagu,J. O. Phosphate-claymineral relations in soilsand Finally, sediments.Can. J. Earth Sci.,13,717-736. site,a frequentweathering product on crystallinesili- - and c. I. Dell (1974)Diagenetic formation of iron phos- caterocks under humid tropical conditions, is rarein phatesin recentlake sediments.Am. Mineral., 59,934-946. weatheringof phosphaticdeposits. The presenceof Russ,W. and C. W. Andrews(1924) The phosphatedeposits of low-solubilityaluminum phosphates keeps the mole AbsokutaProvince. Nigeria Geol. Suru. Bull.,7,9-38. (1964) fraction of diasporewithin Slansky,M., A. Lallemandand G. Millot La s6dimentation the aluminum-richgoe- et l'alt6rationlat6ritique des formationsphosphat6es du gise- thite extremelylow until the aluminumphosphates mentde Taiba. Bull. Seru.Carte Gbol. Alsace-Lorraine. 17.3ll- are destroyed.Only then does goethite reach the 324. value of 15-25 percentmole fraction of diaspore, Tardy, Y. and R. M. Garrels(1974) A methodof estimatingthe usuallypresent in iron crust(ferricretes). Such obser- Gibbs free energiesof formation of layer silicates.Geochim. vationsillustrate the geochemicalrole phosphate Cosmochim.Acta, 38, ll0l-ll 16. of - and - (1976) prediction of Gibbs free energiesof in lateritic weathering. formation,I. Relationshipsamong Gibbs free energies of formation of hydroxides,oxides and aqueousions. Geochim. Cosmo- Acknowledgments chim.Acra. 40. 105l-1056. We are deeplyindebted to R. Flicoteaux(University of Mar- - and - (1977) Predictionof Gibbs free energiesof seille), J. Lucas (University of Strasbourg),and M. Slansky formationof compoundsfrom theelements, IL Monovalentand (B.R.G.M.) for their help. We also wish to acknowledgeVickie divafentmetal silicates. Geochim. Cosmochim. Acta, 41, 87-92. Bennett(University of California,Los Angeles) for herfinal review - 6nd L. Gartner (1977)Relationships among Gibbs free of the manuscript.This work wascarried out with the supportof energiesof formation of sulfates,nitrates, carbonates, oxides the grant 75-7-1005of D6l6gationG6n6rale d la RechercheScien- and aqueousions. Contrib. Mineral. Petrol., 63, 89-102. tifiqueet Technique(D.G.R.S.T., Paris). - and P. Vieillard (1977) Relationshipsamong Gibbs free energiesand enthalpiesof formationofphosphates, oxides and References aqueousions. C ont ri b. M ineral. Pet rol., 63,'l 5-88. Trueman,N. A. (1965)The phosphate,volcanic and carbonate Brewer,R. (1964)Fabric and Mineral Analysisofsorls. Wiley, New rock of ChristmasIsland (Indian Ocean). J. Geol.Soc. Aust., 12, York. 26t-283. Capdecomme,L. (1952)Sur lesphosphates alumineux de la r6gion Vieiflard, P. (1978) Gbochimiedes phosphates. Etude thermodyna- de Thids(S6n6gal). C. R. Acad. Sci.Pais, 235, 187-189. mique.Application d la genbseet d I'altbrationdes apatites. Sci., - (1953) Etude min6ralogiquedes gites phosphat6sde la Geol..Mem..5l. r6gionde Thids(S6n6gal). Proc. l9th Int. Cong.Gbol., Alger,I l, Visse,L. D. (1952)Pseudowavellite et millisitedans les minerals 103-l r8. phosphat6sdits lat€ritoidesblancs de la r6gionde Thi0s(S6n6- Charpy,N. andD. Nahon(1978) Contribution e l'6tude lithostra- gal). C. R. Acad. Sci.Paris,234, 1377-1378. tigraphiquedu tertiaire du Bassinde C6te d'lvoire. Rapp. Wise,W. S.and S. E. Loh (1976)Equilibria and origin of minerals Depart.Sci. Terre,Abidjan, Ser. Docum. 18. in the systemAlrOs-AlPO.-HzO. Am. Mineral.,61,M413. Espenshade,G. H. and C. Spencer(1963) Geology of phosphate Zanin, Yu. N. (1967)Zones of lateriteweathering of secondary depositsof Northern PeninsularFlorida. U. S. Geol.Suru. Bull. phosphoritesof Altay-Sayan region.Geologia i Geofizika,9, 56- II18. 65 (transl.Int. Ceol.Reo.,10, ll19-1127, 1968). Flicoteaux,R., D. Nahon and H. Paquet(1977) Gen€se des phos- phatesalumineux d partir dess6diments argilo-phosphat6s du Tertiaire de Lam-Lam (S6n6gal):suite min6ralogique.Per- manenceset changementsde structures.Sci. Gbol. Bull.,30,153- Manuscript receiued,July 3, 1978; t74. acceptedfor publication,Nouember 8, 1978.