Palygorskite Genesis Through Silicate Transformation in Tunisian

ClayMinerals (2003)38, 187–199

Palygorskitegenesisthroughsilicate transformationinTunisiancontinental Eocenedeposits

1 2 2, F. JA M O USSI ,A.BENABOUD A ND A . LO´ PEZ-GALINDO *

1 Laboratoire‘ Ge´oressources’, INRST BP95,2050 Hamam-Lif, Tunisia, and 2 InstitutoAndaluz de Ciencias de la Tierra,CSIC-Universidad de Granada, Facultad de Ciencias, Fuentenueva s/ n,18002 Granada, Spain

(Received 30 April 2002; revised 21 October 2002 )

ABSTRACT:The mineralogical and geochemical characteristics of Eocene continental sediments in south central Tunisia (Chebket Bouloufa and Jebel Hamri) and in north central Tunisia (Jebel Lessouda and Jebel Rhe´ouis), which contain considerable amounts of palygorskite, were studied. The clay fraction of the sediments also comprises illite, kaolinite, Mg smectite and Al smectite, together with carbonates (calcite and/or dolomite), quartz, gypsum and feldspars, all of which are present in extremely variable proportions. The textural characteristics of the samples containing most palygorskite, as well as the chemical composition of the fibres and the contents of certain trace and rare earth elements suggest that the genesis of this fibrous clay is intimately linked to the diagenetic transformation of illite, mixed- layered minerals and/or Al smectite, as has also been observed in contemporaneous deposits in Morocco.

KEYWORDS:palygorskite,illite, Tunisia, Eocene.

Withthe aim of drawinga mapof potentiallyuseful ContinentalEocene facies, studied by numerous claysin Tunisia, a systematicsurveyof the authors,are deposited in two main areas: in central Palaeozoicto Quaterna rysediment shasbeen Tunisia,around the so-called ‘ Kasserineisland’ performedover the last few years by one of the (Burollet,1956), and to the south of the Gulf of authors(F.J.).Thesurvey showed numero us Gabe`sand the northern part of Jeffara. In central palygorskite-richEocenelevels that might be of Tunisia,Truc (1981) and Zouari (1984) found economicinterest. Eocenesediments at Jebel (J.) Chaambi and Sassi InTunisia, Eocene sediments present important et al.(1984)found them on the south-eastern flank faciesvariations from South to North, changing ofthe J. Kebar.In the J. Lessoudaand Koumine, fromcontinental to deep marine deposits (Bishop, Kadri et al.(1986)and Kadri (1988) detected the 1988;Jamoussi et al.,2001a;Fig. 1). These samefacies, but Jamoussi (2001) pointed out that changesin facies and thickness are mainly due to theywere not immediately above the Ypresian Alpineco mpressivetec tonicev entsand th e carbonatebar. In the northern area of the Chotts halokineticmov ementso fTriassicmateri als belt,Abdeljaoued (1983, 1991, 1997) defined the affectingthe Central Atlas domain during the continentalformation of Bouloufa, dated by bulimes Eocene(Be ´dir,1995; Boukadi & Be´dir,1996). (continentalgastropods) as Lower Eocene; in J. Rhe´ouisand Boudinar, around the Kasserine island, continentaldeposits were also discovered and in J. *E-mail: [email protected] Chamsisuch sedimen tshave been suspec ted DOI: 10.1180/0009855033820088 (Jamoussi,2001; Jamoussi et al.,2001a).Finally,

# 2003The Mineralogical Society 188 F. Jamoussi et al.

Tunis IA R E G L A

Mahdia

J. Rhéouis * J. Lessouda Sfax Kerkennah * Island *J. Boudinar Gafsa

J. Hamri J. Bouloufa ** Jerba Gabès

N Medenine

0 50 100 km

Emerged area

Phosphate deposits A Y B Tidal zone (Sebkha) LI Supratidal to tidal zone Nummulitic limestone Marine deposits * Sectionsinvestigated

FIG.1. Lithofacies map of the Lower Eocene in Tunisia (modified from Jamoussi et al.,2001) and the locations of the sections studied. thefirst oil drilling carried out in the southern part samplesin Tunisia. The better sequences are ofthe Gulf of Gabe `s,as well as some drilling in the locatedin some of the above-mention edareas northernpart of Jeffara, found continental Eocene (ChebketBouloufa, J. Hamri,J. Lessoudaand J. depositsknown as the Tanit Formation (Pochitaloff, Rheóuis).Some samples from J. Boudinarwere also 1968).The mineralogy of someof these deposits has studied. beendescribed by, among others, Abdeljaoued (1997),Srasra et al. (1995),Ben Aboud (1998), GEOLOGICALSETTING Hachi(1998), Ben Aboud et al.(1999),Fakhfakh (1999)and Jamoussi (2001). Theend of the Cretaceous is marked in central and Inthis paper we study the mineralogy, geochem- southernTunisia by important palaeogeograph ical istryand microtexture of palygorskite-ric hEocene andsedimentological modifications. Marine sedi- Palygorskite in Tunisian sediments 189 mentation,represented by white limestones with synsedimentaryinfluences on the distribution of inoceramidsand echinoderms, known as the Abiod faciesand their thickness from Upper Cretaceous Formation(Burollet, 1956), changes towards a more untilMiddle Miocene (Kadri, 1988). The sequence, detrital,clayey sediment during the Palaeocene. ~20m thick,is made up of alternating white, red Aftercompressive tectonic movements of transverse andgreen marls, with fine intercalations of gypsum. faultsduring the Upper Cretaceous (Zargouni, Althoughthe presence of bulimes in these 1985;Boukadi, 1994; Be ´dir,1995) some emerged continentalEocene deposits clearly indicates a areasappeare d,such as the above-me ntioned LowerEocene age, recurrenc esof continent al KasserineIsland. faciesseem to be diachronic and closely influenced Tertiarysequences are particularly well repre- bylocal tectonic conditions of the geological sentedin the North Chain of theChotts, and include substratum.The mineralogy of the clay fraction, severallithostratigraph icunits, such as Beglia and andparticularly the presence of palygorskite, could SeguiFormations, consisting of sand, clay and bean element of identification and inter-relation- conglomerates.One particular lithological member shipbetween these series, which have been marked wasdefined by Abdeljaoued (1983) as theBouloufa bycompressive movements and halokinetics of Formation,extending over a widearea on the Triassicrocks. southernedge of the Gafsa-Metlaoui phosphate basin.This formation appears, in most cases, as METHODOLOGY calcretesand dolocretes, with vacuolar appearance. At J.Chambi,Sassi et al.(1984)found one faunal Themineral phases were determined by X-ray assemblageconsisting of continental gastropods diffraction(XRD), usingPhilips PW 1710and (Romanellahopii , Vidaliella)andhelicids SiemensKristalloflex 810 diffractometers, Cu- Ka (Paleocyclotus )thatallowed this formation to be radiationat a scanspeed of 2 to6º 2 y min –1. datedas Lutecian-Bartonia n. Analysiswas performed on both the whole sample Themost repres entativesequenceso fthe andthe clay fraction. Oriented aggregates were BouloufaFormation crop out 25 km northwest of preparedfor clay mineral analysis and were then El Hammatown, at J. Bouloufaand J. Hamri.They treatedwith ethylene glycol, dimethyl-sulphoxi de are~150 m thickand are made up of alternating andheating to 550º C for1 h.The reflecting powers conglomerates,marls and limestones, white and red ofSchultz (1964), Biscaye (1965) and Barahona incolour, with some gypsum and silex nodules. (1974)were used to quantify the different mineral TheEocene deposit located at J. Rheóuis,close phasesdetected, as well as the chemical composi- toa Triassicdiapir (Soussi et al.,1996),is situated tionsof samples (Lo ´pez-Galindo et al., 1996). incentral Tunisia, 10 km to the south of Fa ˆõ d, on Majorele mentswe rea nalysedb yatomic theboundary between the Sidi Bouzid and J. absorptionspectrometry, using Perkin Elmer equip- Goubrar1:50,000 topographic maps. It appears in mentwith acetylene or protoxide acetylene flame t. theintersection of NE –SW structuresof the North– Traceand rare earth elements ( REE)weremeasured SouthAxis, known as J. Goubrarand Boudinar, and usingan ICP-MS PerkinElmer SCIEX Elan-5000 the NW –SE foldof J. Ksa ¨õ ra(Boukadi and Be ´dir, device.The detection limits of the elements were 1996).Triass ichaloki neticmoveme ntsalo ng 10 ppb for REE andTh, 100 ppb for transition transversefaults led to cropping out of these elementsand Cs, Rb, Sr, Ba and Pb, and 1000 ppb structures,particularly during the Eocene (Boukadi for Li and B. andBe ´dir,1996). The sequence is ~40 m thick,and Thescanning electron microscope (SEM) obser- ismade up of white and grey limestones and marly vationswere performed using a ZeissDSM 950 limestones,marls and clayey marls. Some fine (equippedwith LINK microanalysissystem). The gypsumintercalations appear occasionally. transmissionelectron microscope (TEM) observa- Finally,J. Lessoudais situated ~10 km to the tionsand microanalyses were carried out on the Northof Sidi Bouzid town, at the western part of clayfraction of selected samples, using a Philips theNorth– South Axis. It constitu tesa N40E CM 20(equipped with an EDAX microanalysis anticlinesuperimposed on an Upper Cretaceous system). diapiricdome (Creuzot and Ouali, 1989), emerging Factoranalysis (Principal Components Analysis, inthe centre of a vastplain. It is fractured by km- PCA) wasused to establish the relation between the long N –S, NW –SE and E –Wfaults,which have differentminerals and major and trace element 190 F. Jamoussi et al. contents.The factors (principal components, Pcs) Two clearlydistinguished groups of palygorskite- weresele ctedfo reigenvalues>1(Swan & richoutcrops can be established: those located in Sandilands,1995) applying a Varimaxrotation. southcentral Tunisia (J. Bouloufaand J. Hamri), whichcontain considerable amounts of quartz, calciteand kaolinite, and those located in north R E S U L T S centralTunisia (J. Rheóuisand J. Lessouda),with Mineralogy dolomiteas the most distinctive mineral. Thus,at J. Bouloufathe samples comprise InFig. 2 thebulk and clay mineralogy of the four phyllosilicates,carbonate s,quartz, gypsum and mostrepresentative sequences mentioned above are tracesof feldspar. With the exception of the given.Table 1 containsthe average geochemical bottomof the sequence, where quartz and gypsum characteristicsof these. arethe main phases, carbona tes(calcite and

TABLE 1. Average geochemical data for the sequences studied.

J. Bouloufa J. Rhe´ouis J. Bouloufa J. Rhe´ouis J. Hamri J. Lessouda J. Hamri J. Lessouda

Major components in the clay fraction REE contents in the clay fraction SiO2 53.88 53.17 58.44 56.72 La 24.45 29.64 21.58 20.54 Al2O3 16.59 21.34 10.68 14.28 Ce 38.42 59.62 32.31 32.43 Fe2O3 3.52 3.51 3.35 3.47 Pr 3.92 4.60 3.35 3.36 MgO 7.24 5.21 9.41 7.77 Nd 12.25 14.50 10.77 10.80 CaO 0.29 0.47 0.33 0.33 Sm 1.83 2.25 1.62 1.71 Na2O0.20 0.40 0.17 0.21 Eu 0.35 0.41 0.33 0.37 K2O 2.25 3.13 1.41 2.29 Gd 1.26 1.58 1.16 1.21 TiO2 0.18 0.28 0.14 0.19 Tb 0.19 0.25 0.18 0.20 Mn2O3 0.04 0.04 0.04 0.04 Dy 1.14 1.45 1.06 1.23 LOI15.39 12.28 15.58 14.32 Ho 0.23 0.30 0.21 0.25 Er 0.65 0.87 0.58 0.71 Trace element contents in the clay fraction Tm 0.11 0.14 0.10 0.12 Ni 28.7 38.2 13.1 13.3 Yb 0.71 0.92 0.64 0.72 Co 7.4 9.7 5.2 6.9 Lu 0.11 0.14 0.10 0.12 Sr 98 219 50 106 Ba 142 194 82 76 Structural formula of palygorskites (AEMdata) V 107 119 69 95 Si 7.65 7.70 7.85 7.64 Cr 101 107 80 92 Al IV 0.35 0.30 0.15 0.36 Zn 118 165 78 108 Mg 1.91 1.54 1.76 2.04 Cu 14.9 26.6 7.2 10.4 Fe3+ 0.51 0.53 0.45 0.58 Li 40 74 78 86 Al VI1.48 1.73 1.64 1.35 Y 5.3 7.7 5.0 9.2 Ca 0.09 0.07 0.06 0.08 Rb 115 124 84 93 Mn 0.03 0.03 0.01 0.02 Zr 119 136 154 192 Ti 0.04 0.02 0.02 0.05 Nb 21.9 24.1 29.4 36.2 K 0.11 0.17 0.13 0.11 Cs 5.8 5.9 4.0 4.7 Hf 3.1 3.6 3.7 4.8 Granulometry of samples (%) Th 6.6 7.8 4.5 8.6 <1 mm78.26 76.14 71.83 74.58 U 1.2 1.4 1.0 3.6 1 –2 mm15.84 16.61 19.07 17.35 Be 2.5 2.8 1.7 1.8 2 –3 mm2.79 2.86 4.09 3.73 Sc 11.7 13.8 8.0 10.5 3 –4 mm1.59 2.14 2.12 2.06 Ga 25.1 26.7 18.0 22.2 4 –5 mm0.81 1.06 1.56 1.06 Ta 1.6 1.8 2.0 5.1 5 –7 mm0.60 0.87 0.93 0.92 Mo 2.0 2.0 0.8 1.5 7 –10 mm0.11 0.23 0.26 0.20 Sn 2.1 2.3 1.7 1.8 10 –20 mm0.00 0.10 0.14 0.11 Pb 9.3 13.4 0.3 0.4 >20 mm0.00 0.00 0.01 0.01 Tl 0.6 0.9 6.4 9.4 Mean 1.02 1.07 1.13 1.09 Palygorskite in Tunisian sediments 191

FIG.2. Bulk and clay mineralogy of the Tunisian Eocene sediments studied. 192 F. Jamoussi et al.

Calcite + Dolomite Illite + Kaolinite

J. Bouloufa J. Hamri J. Rheouis J. Lessouda 50 J. Boudinar 50 50

50 50 Phyllosilicates Quartz + Feldspar Al smectite Palygorskite + Mg smectite

FIG.3. Mineralogical composition of the samples studied. dolomite)are generally the most abundant phases, regardingboth the sediment supplies and their sometimesreachi ngup t o77%,and qua rtz chemicalcharacteristics . represents~1 0%(Fig. 2). The clay minera ls detectedin the <2 mfractionare palygorskite, m Geochemistry illite,Al smectiteand kaolinite. Palygorskite, with contentsof ~50 –70%,is the most importan t Theaverage major component, trace element and phyllosilicate;illitecontents range from 11 to REE contentsof the clay fraction of selected samples 36%,smectite and kaolinite do not represent more arepresented in Table 1. Southern sequences (J. than10%, although at the bottom, there is up to Boloufaand J. Hamri)are richer in Al 2O3 (J. Hamri 27%kaolinite. also in K2O),corresponding to their higher illite Themineral phases found at J. Hamriare the content.MgO content ranges from 5.21 to 9.41%, same,but in different proportions. Quartz (up to whilethe central sequences are richest in Mg. 54%),phyllosilicat esand calcite are the main Weobtained the correlation matrices of all the components.In the clay fraction, illite is the most mineralogicaland geochemical variables (major abundantphase (38 –59%),followed by palygors- elementsand the complete set of trace and rare kite (14 –40%).Kaolinite normally appears in small earthelements). The results obtained show that amounts (5 –10%)and Al smectiteis systematically Al2O3 isclosely linked to Fe 2O3 (r = 0.97), K2O (r present,reaching 28% at times. = 0.98), TiO2 (r =0.98)and Mn 2O3 (r = 0.70), as Onthe other hand, J. Rheóuisand J. Lessouda wellas to the phyllosilicates of clearly detrital showrather similar compos itions,which are origin,with r >0.8(illite, interstratifi edillite- differentfrom the southe rnsequen ces.They smectite,Al smectiteand kaolinite). The REE consistalmost exclusively of phyllosilicates and and,among others, transition elements (TRTE) Ni, dolomitein varying proportions, with quartz and Co,V, Cr,Cu, Sc behavein a similarfashion, feldsparsas trace minerals. In addition, in J. presenting r >0.8among themselves and with the Rheóuissome samples contain small quantities of aforementionedvariables. clinoptilolite(<3%). The clay fraction is more Inview of the good correlations and considering variable,containing palygorskite (38 –96%) and thecalculation established by Lo ´pezGalindo et al. illite (3 –23%)as major components. Mg-smectite (1996),we determined from the theoretical REE (notpresent in the southern sequences), Al smectite, andTRTE contentsthat pure palygorskite would be kaolinite(at J. Lessouda)and opal are also present, presentin these deposits (Fig. 4). Thus, the REE butnever in significant quantities (up to 10%). contentswould be ~40 ppm (Bou-Loufa and J. Figure3 showsthe mineralogical composition of Hamri),50 ppm (J. Rheóuis)and 70 ppm (J. allthe samples studied. Even in the same sequence, Lessouda).Likewise, the amount of TRTE inthis thiscomposition is very diverse, as shown by the purepalygorskite would vary from 125 to 175 ppm. breadthof the fields, indicating highly variable When the REE contentsof the different Tunisian conditionsofthede positionale nvironment, depositsare normalized to NASC, wefind an Palygorskite in Tunisian sediments 193

FIG. 4. REE and TRTEcontent of the studied samples as afunction of the percentage of detrital phyllosilicates. essentiallyhorizontal pattern, containing between 0.05atoms per structural formula), and there is a 25and 100% of the NASC contentsand that there systematicpresence of K andCa in the interlayer isa slightenrichment in light rare earth elements withpredominance of theformer (at times up to 0.4 (LREE)(2La/ Lu4.5). No significant anomalies Katomsare found per unit cell). weredetected, except for one case in the J. Hamri Itis interesting to note that the J. Hamrideposit, deposit,where the richest palygorskite sample had a whichcontains the most clearly detrital mineral clearpositive anomaly in Ce. assemblage,also has the palygorskites richest in Al Principalcomponent analysis was applied to andwith the largest amount of K inthe interlayer. mineralogicaland c hemicalvariables.Thre e factorsexplaining the 80% of the total variance Micromorphology wereobtained (Fig. 5). Factor 1 (40%)and factor 2 (24%)are clearly interpretable as genetic factors, Themicrotexture observed by SEM variesfrom andseparate the clearly detrital minerals and onesequence to another, or even within the same associatedchemical elements from those compo- sequence.The most frequent manner in which nentsof presumably different origin (palygorskite, palygorskiteappears is as chaotic arrangements of amorphoussilica and Mg smectite). Factor 3 (16%) shortfibres (<2 mm,Fig. 7a), or in more or less discriminatesbetween silicates and carbonates.

Chemical composition Table1 summarizesthe mean compositions of thepalygorskite analysed by AEM inthe different sequences.Figure 6, representing the composition ofthe octahedral layer, shows the wide variability existingeven between samples from the same deposit.Practicallyall the particl esanalyse d presentcompositions intermediate between diocta- hedraland trioctahedral (this is particularly clear whencompared with the compositional field of the differenttypes of smectites, Fig. 6c). We should alsopoint out that substitution of Si byAl inthe tetrahedralsheet is relatively moderate (0.15 –0.35 atomsper unit cell), there is a weakpresence of Ti FIG.5. R-mode factor analysis showing the contribution and/orMn in the octahedral sheet (never more than of statistically dominant variables (Factors 1and 2). 194 F. Jamoussi et al.

FIG.6. Composition of the palygorskite octahedral sheet: (a) south central sequences; (b) north central sequences; (c) comparison with smectite composition; (d) REE distribution of apure palygorskite normalized to NASC. planarstructures with filamentous edges (Fig. 7b) D I S C U S S I O N consistingof particles which, on a smallscale, presentsome preferen tialorientati on(Fig. 7c). Thereare numerous studies of the genesis of Longerfibres (up to 5 mm)are also found arranged palygorskitein contine ntalenviron ments(e.g. inbundles or sheaves and which, taken as a whole, Millot,1964; Singer, 1984; Velde, 1985; Jones & havethe appearance of a mator rug (Fig. 7d). Gala´n,1988, among others). As is known, this Thepresence of idiomorphic dolomite crystals fibrousclay is found especially in soils, calcretes coveredby palygorskite fibres (Fig. 7e) or growing andlacust rinedeposi tsand there is genera l onsuch fibres (Fig. 7f) illustrates the different agreementthat its formation requires alkaline growthgenerations of this fibrous clay, depending conditions,high Si andMg activity and an arid or onthe physical-chemic alvariations of the environ- semi-aridclimate,withmarked dry and wet ment. seasons. Theaverage size-range of the clayey particles, Bearingi nmindt hen atureoftheclay measuredboth by laser granulometer and by image paragenesis(cf. Chamley, 1989), most of the clays analysisof photomicrographs obtained by TEM, is foundin theTertiary continental deposits of Tunisia summarizedin Table 1, where we can see that arebasically detrital and their accumulation is >90%of the fibres are <2 mm. undoubtedlylinked to water and/ orwind supply in

FIG. 7 (facing page).Microtextures of the samples studied. (a) Chaotic arrangements of short fibres (J. Rheóuis). (b) Planar structures with filamentous edges (J. Lessouda). (c) Detail from the edge of aplanar aggregate (J. Rheóuis). (d) Imbricated fibres of palygorskite with amat aspect (J. Bouloufa). (e) Idiomorphic dolomite crystals covered by palygorskite fibres (J. Bouloufa). (f) Dolomite growing on palygorskite fibres (J. Bouloufa). Palygorskite in Tunisian sediments 195 196 F. Jamoussi et al. closed,lacustrine-t ypebasins or in playa-lakes Thesevalues coincide on the whole with those accumulatedduring flooding episodes (Jamoussi et determinedin other Spanish and Moroccan deposits al.,2001a,b).The source area for the Tertiary (BenAboud, 1998) and which are intermediate depositsmust have been the surrounding formations betweenthose of the two mineral groups mentioned (Jamoussi,2001), because they contain considerable above. amountsof clays, particularly Al smectite,illite, The REE distributionmodel of a threoreticalpure kaoliniteand, as is the case for the Triassic diapirs, palygorskite(Fig. 6d) shows a slightimpoverish- chlorite,the altera tionof which could have mentin heavy REE (Gd –Lu, HREE),thatcan be providedadditionalMg(Jamoussi,2001). attributedto the pH of the interstitial solutions, Moreover,the gene sisof b othprimar yand whichaids accumulation of light REE (La –Eu, diageneticcarbonates, as well as the precipitation LREE)inalkaline conditions (Nesbitt, 1979), as ofgypsum (present in all the sequences studied) wellas to the mobility of the HREE in natural increasesthe Mg/ Caratio of the interstitial water, solutionsbecause of their ability to form bicarbo- guaranteeingan alkaline pH. Finally, in a recent natedand organic solutions more soluble than the studyon climatic evolution in the Tethys during the LREE (Fleet,1984; McLennan, 1989), or also Palaeocene-Eocene,Bolle & Adatte(2001) showed becauseof differential adsorption of lanthanides theprogres sivedevelop mentof arid climati c ontohydrogenous particle surfaces due to their conditionsin southern Tunisia during this period. differentpolarizing power (Turner & Whitfield, The REE andtrace elements data, particularly 1979). thosefor the first transition series (TRTE) provide Asregards chemical composition, palygorskite is interestingc luestoexplainth egenesiso f aclaythat accepts quite considerable substitutions palygorskitein these continental Tertiary deposits. inthe octahedral sheet, its composition typically Infact, most of the natural water has extremely low beingintermediate between the dioctahedral and REE concentrationsof ~10 –7 –10 – 2,incomparison trioctahedralend-member s(Weaver& Pollard, withthe values found in the rocks (McLennan, 1973).Later studies by Paquet et al. (1987), 1989).This is mainly due to the fact that mobility Duplay(1988) and Gala ń&Carretero(1999) andfractionation of the REE arevery unlikely at haveshown that the variation interval in the thefluid/ rocklevel (Taylor & McLennan,1985), compositionof palygorskite is even larger than i.e. the REE movefrom the alteration profile to the firstthought, with many analyses clearly falling sedimentarybasins without being affected by any withinthe dioctahedral domain, as is frequently the chemicalprocess during transport (Fleet, 1984). casewith the Tunisian samples, especially from J. Thebehaviour of the REE duringthe different Hamriand J. Rheóuis.Regarding the average alterationproces sesis mainly contro lledby compositionssummarized in Table 1, we should inheritanceand their pattern in clayey sediments pointout the high Fe andAl contentspresent in the isclearly related to that of the mother rock (Cullers octahedralsheet, as well as the high K andCa et al., 1975;Bonnot-Courtois, 1981; Fleet, 1984). valuesin the interlayer, the proportions of which The REE areessentially adsorbed by the clays and aresimilar to that detected in other aluminosili- arefound in considerable quantities in clearly cates,such as illite and/ orAl smectite. detritalclays and in very small amounts in clearly Asin the cases of other palygorskites from Spain neoformedclays, such as sepiolite and/ orstevensite andMorocco described in previous studies (Ben (~250ppm in the former and <20 ppm in the latter, Aboud et al., 1997;Ben Aboud, 1998), the Torres Ruiz et al.,1994).In low-tempera ture geochemicaland textural data suggest that the aqueoussolutions, transition elements, such as Ni, genesisof this fibrous clay is intimately linked to Co,V, Cr,Cu and Sc, behave in similar fashion to thetransformatio nofa detritalaluminosilica te the REE andusually appear in equally low precursor,especially smectite and interstratifie d concentrations(McLennan,1989). In Spanish illite-smectite.On this point we should mention deposits,this quantity has been determined at that,when studyin gthemineral assembla ges ~450ppm in detrital minerals and <80 ppm in throughoutt hegeolog icalrecord in Tunisi a neoformedphases (Torres-Ru õ´z et al., 1994). (Jamoussi et al.,2001b),it can be seen that, The REE andTRTE contentsfound in the duringthe Eocene, the proportion of smectite falls palygorskitefrom Tunisian deposits vary from 40 dramaticallyin sequences where palygorskite is to70 ppm and from 125 to 175 ppm, respectively. present.This type of mechanism has often been Palygorskite in Tunisian sediments 197 suggestedas themost probable cause of the presence contentsof Tunisian pure palygorskite are also ofpalygorskite,e.g. the papers by Mart ´õ ndeVidales intermediatebetween clearly authigenic clays, such et al.(1988)on lacustrine -palustrineMiocene assepiolite (<50 ppm), and detrital clays, such as sedimentsin the Madrid basin, who suggest illite,mixed-layer I-S andAl smectite(>400 ppm). alterationof the structure of pre-existing mont- Wesuggest that this fibrous clay originated morillonitein a mechanismof dissolution-precipita- duringthe first stages of diagenesis, due to the tiondue to the presence of magnesian waters during destabilizationof the existing aluminosilicates ,ina thecarbona tationprocesse sinvolvingcalcret e mechanismof dissolution-prec ipitationthat could formation;by Sua ´rez et al.(1994)o nthe bedue to the presence of magnesian waters during Bercimueldeposit (Segovia, Spain), who suggested post-depositionalcarbonationprocesses. mechanismsof dissolution of detrital mica with K andAl lossand Si andMg gain, giving rise to ACKNOWLEDGMENTS structuralformulae very similar to those described here;or byDePablo (1996) on Eocenepalygorskite- This research is part of the projects ‘Identification, richmudstone in the Yucatan Peninsula, where cartographie et valorisation des mate´riaux utiles en palygorskiteformed in a tranquilmarine back-reef Tunisie‘(SERST, Tunisia), and CSIC1999TN002, lagoonenvironment of high Mg, Si andAl activity, DGI-BTE2000-0777 and research group RNM-0179 of bydiagenesis of precursor montmorillonite, from the Junta de Andaluc ´õ a(Spain). The authors are reactionbetween montmorillonite, dolomitic lime- grateful for the helpful comments by Prof. Herve´ stoneand silicic acid. On the other hand, in his study Chamley (Universite´Strasbourg) and Prof. M.D. Ru õ´z- ofthe manner of genesis of an Al andMg Cruz (Universidad de Ma´laga) which helped to improve our paper. Wealso thank Prof. Ian palygorskiteintheBouloufaFormation, MacCandless (DepartmentFilolog õá Inglesa, Abdeljaoued(1997) showed that this mineral can Universidad de Granada) for assisting us with the appearboth as directprecipitation from solution, and English grammar. asrecrystallization of smectites, with no particular predominanceof either of these mechanisms. 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