ANALYSES OF PALYGORSKITES AND ASSOCIATED ...clays.org/journal/archive/volume 41/41-4-401.pdfFigure 3. X-Ray diffraction pattern from oriented clays showing illite, palygorskite, and

$Page 1: ANALYSES OF PALYGORSKITES AND ASSOCIATED ...clays.org/journal/archive/volume 41/41-4-401.pdfFigure 3. X-Ray diffraction pattern from oriented clays showing illite, palygorskite, and$
Clays and Clay Minerals, Vol. 41, No. 4, 401--411, 1993.

ANALYSES OF PALYGORSKITES A N D ASSOCIATED CLAYS FROM THE JBEL RHASSOUL (MOROCCO): CHEMICAL CHARACTERISTICS A N D

ORIGIN OF FORMATION

AZZEDINE CHAHI, JOELLE DUPLAY, AND JACQUES LUCAS

Centre de Grochimie de la Surface, Institut de Grologie, 1 rue Blessig, 67084 Strasbourg Cedex, France

Abstract--The "Formation Rouge" from the Jbel Rhassoul in Morocco is composed ofdetrital sediments which have a lacustrine origin. The clays contained in the less than 2 ~tm fraction of the sediments are detrital phengites and illites, illite/smectites or smectites, and palygorskite. Due to the presence of well preserved long fibers, the palygorskite could not have been transported. They are authigenic and must have formed directly by precipitation from solutions rich in Mg and A1. The detrital illites are impoverished in K and tetrahedral AI. The illite/smectites or smectites, on the contrary, are K-rich but have a low tetrahedral charge. They are also richer in Mg and Fe and have a different crystal size, composition, and crystallinity from the illites. They most probably formed by crystallization, similar to the palygorskites, directly from the solution. The A1 could have been provided by the detrital illite, which may have been unstable in an alkaline environment and released K and A1 to the solutions. Key Words--AEM, Jbel Rhassoul, Neoformation, Palygorskite.

I N T R O D U C T I O N

The Tertiary lacustrine formation of the Jbel Rhas- soul (Morocco) has great commercial interest because it is exploited for magnesian clays which are used as fuller's earth. In this work we are particularly interested in the fibrous clay, palygorskite, which is associated with other clays in the sediments of the "Formation Rouge" at the bottom of the Jbel Rhassoul mountain. This palygorskite has been previously identified (Trauth, 1977; Lucas and Prrvr t , 1976), but its origin has not been clarified. According to the literature, there are two mechanisms to generate palygorskites in surface conditions: 1) by neoformation (Weaver, 1984; Isphord- ing, 1984; Esteoule-Choux, 1984; Singer, 1984); or 2) by transformation of another clay, such as illite (Galan and Castillo, 1982) or smectite (Singer, 1984). It is proposed in this work, first, to characterize structurally and chemically the palygorskites and associated clays in the Rhassoul formation by means of X-ray diffraction and AEM analyses; and second, to suggest a mech- anism for their genesis.

FIELD OCCURRENCE AND SAMPLE LOCATION

The Jbel Rhassoul is a small mounta in in the Mou- louya Valley, located at the East side of the Middle Atlas in Morocco (Figure 1). The Mio-Pliocene sediments of the Jbel Rhassoul have a lacustrine origin (Raynal, 1952; Trauth, 1977; Lucas and Prrvrt , 1976). An angular unconformity separates these sediments from the underlying Cretaceous formations.

The "Formation Rouge" is composed of detrital material from the erosion of Jurassic to Cretaceous marine

Copyright �9 1993, The Clay Minerals Society

deposits. The sediments of the "Formation Rouge" interbedded with a chalky dolomite incrustation (Fig- ure 2) are essentially marls and mudstones showing cross bedding, trough-cross bedding, and rill erosion features typical of a lacustrine delta (Lucas and Prrv6t, 1976).

Five representative specimens (1, 2, 3, 4, 5) chosen for this study were obtained from different levels of the "Formation Rouge" (Figure 2). They contain days; dolomite; traces of celestite and quartz; and, in one case, gypsum (Table 1). The clay is quantitatively dominant. There is also a tendency for increasing dolo- mitization from bottom to top in the "Formation Rouge."

EXPERIMENTAL METHODS

Sample preparation

The samples contain large proportions of sulfates and carbonates and minor amounts of hematite. Acid treatment of the <2 um fraction was effective in re- moving them but was destructive to the clay. There- fore, a method of treatment using cation exchange resins was chosen to remove carbonates and sulfates without damage to the clay minerals (Chahi, 1988; Chahi et al., 1989; Chahi and Weber, 1991). The resin, used at 60~ was amberlite IRC-50H eluted with HC1 (1.5N). This treatment purifies and also gives a good dispersion of the mineral suspension. Some samples were exchanged with Ca and K after purification.

X-ray powder diffraction analysis

XRD analysis was performed on oriented clays at atmospheric conditions (N), on specimens saturated

401

402 Chahi, Duplay, and Lucas Clays and Clay Minerals

North MOROE[O ~

A

Jurassic-Cretaceous ~ Mio-Pliocene ~ Quaternary formations formations formations

Figure 1. Location of the Jbel Rhassoul in Morocco.

with ethylene glycol (EG), and on samples heated at 490~ for 4 hours (H). Diffraction patterns were obtained for the less than 2 ~m fraction in all samples and for a smaller fraction (< 0.2 #m) in Sample 2.

Scanning electron microscopy

Rock samples were treated with diluted HC1 prior to observation by SEM (Jeol JSM 840) to emphasize the habits o f the fibrous clays and their textural rela- tions to other minerals. The treatment consists of heat- ing the sample on a hot plate and then adding a drop of IN HC1. When effervescence stops, the rock is then gently washed with deionized water (pH = 5.5).

Table 1. Semiquantitative estimation, based on X-ray diffraction, of the mineralogical composition in the sediments of the "Formation Rouge" from top (5) to bottom (1).

Sample Clay Dolomite Quartz Celestite Gypsum

Top 5 + + + + + -- tr -- 4 + + + ++ + - - - -

3 + + + + ++ tr -- - -

2 + + + + + tr -- Bottom 1 + + + + tr -- + +

+ + + + (75-100%), + + + (50-75%), ++ (25-50%), + (<25%), tr: trace.

O

(IJ I....

t .J

Figure 2.

2m

0

chatky dotomite

pink mart

clay pink gypsite UNCONFORIql T Y

Sample locations in the "Formation Rouge."

Transmission electron microscopy and analytical electron microscopy

Electron microscopy is an indispensable tool for observation and chemical analysis of the clays, particularly palygorskites. It has been successfully used for palygorskites and sepiolite from the Mormoiron Basin, France (Paquet et aL, 1987; Duplay, 1988). For observation by TEM, clays were diluted in water and treated in an ultrasonic device; a drop of this suspension was deposited on a carbon-coated Cu grid. In this way, after drying, the platy clay particles are oriented with the c-axis perpendicular to the carbon film. Ob- servations were made at 120 kV in a Philips CM12 scanning transmission electron microscope (STEM).

The chemical analyses were performed with an X-ray spectrometer fitted with a solid state detector for energy dispersive analysis (Edax 9900). Analyses were carried

Vol. 41, No. 4, 1993 Palygorskite from Morocco 403

Table 2. Semiquantitative estimation, based on X-ray diffraction, of the clay species in the samples of the "Formation Rouge."

I n t e r - s t r a t - i f i e d

S a m - i l l i t e / p i e F r a c t i o n l l l i t e P a l y g o r s k i t e C h l o r i t e s m e c t i t e

5 <2 #m + + + + + tr -- 4 <2 #m + + + + + tr -- 3 <2 um + + + + + tr tr 2 <2 #m + + + + + ++ 2 <0.2 um ++ ++ + ++ 1 < 2 ~ m + + + + + tr - -

+ + + + (75-100%), + + + (50-75%), ++ (25-50%), + (<25%), tr: trace.

out in STEM mode with the beam rastered over a region of the particles to minimize element diffusion (Peacor, 1992). The specimen was tilted 30 ~ to the incident electron beam, and X-ray data were collected for 100 s. For accurate quantitative analysis, correc- tions were made following the Cliff and Lorimer (1975) procedure and using experimental K factors obtained with a standard (phlogopite) for thin films. The estimated analytical errors in the concentrations are of the order of 2% for major dements, and 5 to 10% for minor elements.

The clay mineral suspension must be considerably diluted to obtain a good dispersion of the particles in which no particle overlap occurs. Special care was taken to analyse individual particles. Nevertheless, for palygorskite it was necessary to analyse bundles of fibers to increase the X-ray counts because individual fibers were too thin. In each sample, about 25 iUite particles as well as a few bundles of palygorskite fibers were analysed. The structural formulae were calculated for half unit cells on the basis of 11 oxygens for illites, 14 for chlorites, and 10.5 for palygorskites. In AEM analyses it is not possible to distinguish trivalent from di- valent cations; therefore, Fe in illites and palygorskites was assumed to be trivalent and attributed to the octahedral sheet.

RESULTS

Mineralogy

The diffraction diagrams show that palygorskite is the dominant clay, except in Sample 2 where illite is dominant (Table 2). Palygorskites are always associated with illites and sometimes with chlorite (Figure 3). In Sample 2, interstratified illite/smectite (Figure 4) has been found in the <2 #m fraction and in the < 0.2 #m fraction. The percentage of expandable layers in the interstratified illite/smectite, estimated by use of NEWMOD (Reynolds, 1980, 1985), is about 50%.

The palygorskite is monoclinic owing to the well separated 121 and 121 reflexions (Figure 5). The chlo-

/ palygorskite illite !,! t

!t

}I .:1

~t H : Heated :t

.q

E : Ethylene-Glycol ~!t N : Normol ,~ .~

,~ ', ? �9 li ' "Y

polygorsklte l-: icNontel

,,,,,o 1 I/ I2,A: I ", \ ch lor i te / I I / t, H ~ / :YI I l i / ...... : J

E ~,: ', .............. ;~:'~, ......... : I I /

5 6 7 8 9 10 15 20 30t,O /A ~ I ~ I I 1 ~ i i t i i I I

Figure 3. X-Ray diffraction pattern from oriented clays showing illite, palygorskite, and chlorite (Sample 1).

rites are well crystallized, and the illites are of 1Md type.

Texture

SEM pictures ofde-dolomitized rock fragments show that long palygorskite fibers (I0/~m) form matted films (Figure 6a) and cover other minerals such as authigenic dolomite (Figure 6b). They also form bridges between minerals, thus suggesting their authigenic formation (Figures 6c and 6d).

With TEM, one can distinguish the palygorskite fibers and platy clay particles (Figure 6e). There are two major populations of platy particles according to their size: 1 ) Large and thick, platy and rigid particles (1/~m and more) with a polyhedral shape and common Moir6 fringes (Figure 60 are well crystallized based upon their diffraction pattern. The (060) reflection from the electron diffraction pattern is 1.503/~, which is characteristic of dioctahedral phyllosilicateso These mica-type particles which are important in volume and in num- ber, are illites. 2) Thin and tiny platy particles of 0.2 to 0.7 um diameter, with a more or less polyhedral shape. Their electron diffraction pattern corresponds to well crystallized clays and is identical to that of micas (Figures 6g and 6h). They have an 060 dimension of 1.515/~, which is on the border between dioctahedral and trioctahedral phyllosilicates. These clay particles, which appear ubiquitously with the palygorskite fibers,


i l l i te

H : H e a t e d E : E t h y l e n e - G l y c o l

N : N o r m a l

i l l i le chlorite

A A !\'AL, H. .............

ii F!

f i chlorite

iil L,e

5 6 7 8 91o 15 20 i i i i i i l

Figure 4. X-Ray diffraction pattern from oriented clays showing illite, chlorite, interstratified illite/smectite and palygorskite (Sample 2).

have the platy and polyhedral form of illite or interstratified illite/smectite with few smectite layers.

Chlorite is rare and no turbostratic textures typical of smectites are observed.

The palygorskite fibers are 3 to 5 #m long and often form bundles of a few fibers. It is also remarkable that the palygorskite fibers and the illite particles are clearly separated. There are no transitional forms between illite and palygorskite. This suggests that the palygorskite is authigenic and that it did not grow from a clay but precipitated directly from solution.

Chemistry

The triangular diagram of Figure 7 (Yoder and Eugs- ter, 1955) shows the charge distribution, i.e., the tel-

P = P a l y g o r s k i t e

I = I l l i t e

Q = Q u a r t z

P

P I �84

1,5 2 2.5 ] I i , i i ~ t /

P a l y g o r s k i t e

1

Figure 5. X-Ray diffraction pattern of non-oriented material showing 121 and 121 peaks of palygorskite characteristic of a monoclinic structure.

rahedral, octahedral, and layer charges in half-cells of the platy polyhedral particles. In this way, the different species can be identified by comparison to the charge distribution domains defined by Weaver and Pollard (1975) or KSster (1982). Figures 8, 9 and 10 give the percent distribution of A1, Fe, and Mg in the octahedral sheet and AI+ Fe, Mg, and K in illites and palygorskites. One can recognize three populations (Figure 7) according to the charge distribution. Two of the analysed large mica-type particles have a high layer charge (0.69 and 0.9 Eq.), and a high tetrahedral charge (stars on Figure 7) and are Al-rich in the octahedral sheet and K-rich in the interlayer space. These particles are phengites ofdetrital origin. The other large to medium- sized illite particles (filled circles on Figure 7) have much less layer charge compared with the phengites. They are Al-rich in the octahedral sheet and their tetrahedral and layer charges range from 0.1 to 0.6. These are illites, but for some of them the charges are low and correspond more to smectites or interstratified illite/smectites. It is noticeable that there were no smec- rites or interstratified illite/smectites detected by X-ray diffraction analysis in the coarse fraction (<2 #m), except in Sample 2. Interstratified illite/smectite could be clearly identified in the very finest fraction (<0.2 ~m). The low charge particles have the same morphology and structure as the other large size illites, but have lost a large part of their interlayer K compared with the others. That K- depletion could be interpreted as element diffusion during analysis. But in our analytical procedure, special care has been taken to min-

Figure 6. a) Matted film of very long palygorskite fibres (sometimes over 10 #m). b) Covering of fibres on dolomite rhombohedra, c) Bundles of fibres forming bridges between dolomite (D). d) Bundles of fibres surrounding phantoms of dolomite crystals removed by dedolomitization, e) View of the < 2 #m fraction showing palygorskite (P) and illlite particles


(I). There are big particles (0.2-2/,m) and fine particles (<0.2 tim). f) Big mica particle showing Moir6 fringes, g) View of fine platelets ofillite with rounded shapes, h) Electron difti'action pattern of a fine illite showing a well formed pseudo hexagonal structure,


domain of montmorillonites CELADONITES Coarse fraction (0.2-2#m) (K6ster, 1982) ~ phengites

K [ z = 5 ] Si4010(OH)2 �9 illites domain of random ~ / ~ o interstratified illite/chlorites iUite/smectites ~ / \ . . . .

�9 , ~" 0.sA k0.8 v ine t rac t ion ( < 0 . 2 # m ) (Srodon and Eber l , 1984) ..~;r / \ ~.z o interstrat if ied i l l i te/smecti tes

:--= domain o f illites , f f L ~ 0 . 6 / / - - ~ \ x~0.6 o~ or smecti tes . . . . (K6ster, 1982) o f f ~ ' / / / ~ \ o~,

. ? hO 0.4/~ , ' ' ~ " ~ " ~ 0 . 4 ~O

K[z = 6 ] Si3AIOI0(OID2,. . . . . o' " .v ~ o / [ z=6] Si4010(OH)2 o 0 2 ~ 013 0 [] ~Y02

" x 0 / / ,.-" O 0"4~X y0 .4

~ . r~, 0.6~ ~/0.6

VERMICULITES

[z=7] Si3AIO10(OH) 2

z = sum of octahedral charges for half-a-cell

Figure 7. Charge distribution (for halfa cell) in the phengites (stars), illites (filled circles), illite/smectite (hollow circles), and illite/ehlorite (hollow squares) in the different samples of the "Formation Rouge."

Al

/ / / ~ * Coarse fraction ( 0.2 - 2 #m)

~ c D ~ o Fine fraction (< 0.2 pm)

F e ~ 6o 4o 20 M g

Figure 8. Octahedral composition of coarse and fine clay particles.

imize the K-diffusion in every analysed particle (see experimental methods). The loss of K in illites cannot be wholly assigned to analytical errors. These K-poor illites may be transformed during transport from the source rock to the lacustrine basin. Their mean structural formula is given in Table 3.

Some rarer particles have an excess octahedral charge (hollow squares on Figure 7 and Table 3). They may be interstratified illite/chlorite with a high illite layer content.

The thin platy particles (hollow circles on Figure 7) also show variations in charge. The layer charge ranges from 0.25 to 0.5 and the tetrahedral charge from 0 to 0.4. These charge distributions correspond to illites for some and to montmorillonites for others, although the morphology and structure remains the same. Com- pared with the medium size illites (fraction 0.2-2 #m), the tiny particles are, on the average, enriched in Si,


AI + Fe

�9 C o a r s e f rac t ion ( 0 . 2 - 2 / ~m)

o Fine fraction (< 0.2 ~m)

80 60 40 20 Mg K

Figure 9. Differences in the octahedral (A1 + Fe 3+ ), and Mg, and interlayer K contents of the coarse and fine clay particles.

K, Fe, and Mg (Figure 8 and Table 3). They are also richer in K, Fe, and Mg compared with the average illite/smectite (40% to 60% expandable layers) given by Weaver and Pollard (1975, Table 3). Nevertheless the charge distribution is similar. According to Krster (1982) and Srodon and Eberl (1984), the charge distributions and the compositions correspond to interstratified illite/smectites or smectites, a species that did not appear by X-ray diffraction. These can be explained by the high proportions of palygorskite that may have overshadowed the diffraction effect of minor species. It is most probable, as could be pointed out in Sample 2 which contains less palygorskite, that the fine fractions contain minor amounts of interstratified illite/ smectites or smectites.

The palygorskites are Mg- and Al-rich with minor amounts of Fe (Figure 10). Their composition is nearly identical to that of palygorskites described in the literature (Table 4) but show a little more Mg and lower A1 and Fe in the octahedral sites.

R E E and trace element chemistry

Two samples (2 and 3) were selected for trace elements and REE analysis in illite and palygorskite. As it was impossible to separate the two species in one sample, the distribution in two samples was studied, one enriched and the other depleted in illite. REE and

AI

(3 bund les o f pa lygo r sk i t e f ibers

Fe 80 ~ ,0 20 Mg

Figure 10. Octahedral composition of palygorskites.

trace elements have been analysed by means of induc- tively plasma coupled emission spectrometry (ICP). The relative error in the concentrations is around 10% (Samuel et al., 1985).

The distribution of trace elements and rare earth elements (Figures 11 and 12) is identical in both samples (i.e., for illites and palygorskites). That means that the illites and palygorskites may have formed together in the same environment, or by the generation of one species from the other without loss of trace and REE elements.

K-Ar isotopic dating

Sample 2, containing the most K-rich clay (i.e., illite), was chosen for isotopic analysis. Different grain- size fractions (<0.2 urn, 0.2-2 #m, >2 ~m) corresponding to the different phyllosilicate sizes (phengite, illite, interstratified illite-smectite) have been analysed by mass spectrometry with a 120 ~ deflection electro- magnet (Bonhomme et al., 1975).

The analytical results give the following ages (Chahi, 1992):

<0.2 #m fraction: 114.2 + 4.9 Ma (Aptian) 0.2-2 #m fraction: 230.3 + 6.1 Ma (Paleozoic) >2 um fraction: 315.5 + 7.2 Ma (Paleozoic)

The illites and phengites from the intermediate and coarse fractions have ages corresponding to the Paleo- zoic and are probably inherited from clastic material

Table 3. Mean structural formula calculated for halfa cell of palygorskites and of the different species pointed out in Figure 8. Mean structural formula of illite/smectites (40 to 60% expandable layers) from Weaver and Pollard (1975).*

Species Sy mb o l Si AI w AI v~ Fe 3 ~ M g Ca K

Phengite 3.53 0.47 1.52 0.205 0.24 0.00 0.825 Illite 3.30 0.70 1.215 0.26 0.30 0.00 0.55 Illite/smectite 3.58 0.42 1.14 0.44 0.51 0.00 0.655 Illite/chlorite 3.54 0.46 1.74 0.15 0.24 0.00 0.31 Palygorskite 3.965 0.035 0.67 0.14 1.24 0.03 0.065

Illite/smectite* 3.63 0.37 1.585 0.14 0.275 0.135 0.35


Table 4. Structural formula of palygorskite from the Jbel Rhassoul compared to palygorskite formulae from the literature.

O r i g i n S i A I Iv A V ~ F e 3 + M g C a K N a

Jbel Rhassoul (1) 3.965 0.035 0.67 0.t4 1.24 0.03 0.065 0.00 Georgia (2) 3.895 0.105 0.785 0.19 0.94 0.16 0.02 0.045 Australia (3) 4.00 0.00 0.555 0.24 1.425 0.00 0.00 0.00 Yucatan (4) 4.00 0.00 0.765 0.21 1.06 0.015 0.045 0.005 Japan (5) 3.91 0.09 0.785 0.12 1.02 0.18 0.002 0.00 India (6) 3.90 0.10 0.565 0.435 0.915 0.07 0.115 0.015 U.S.A. (7) 3.905 0.095 0.83 0.18 0.915 0.005 0.00 0.225

1) this work; 2) Robertson, 1961; 3) Rogers et al., 1954; 4) Isphording, 1984; 5) Minato et al., 1969; 6) Hasnuddin Siddiqui, 1984; 7) Weaver, 1984.

coming from the far hinterland. The clays from the fine fraction show ages probably inherited from the Juras- sic-Cretaceous formations where they formed.

DISCUSSION

Solving the problem of the origin of the palygorskite from the "Formation Rouge" in the Jbel Rhassoul leads to two possibilities:

1) They are detrital and have the same origin as the detrital illites coming from the weathering of Ju- rassic to Cretaceous sediments before and during transport.

2) They are formed from solutions after deposition of the Jbel Rhassoul sediments.

The second assumption seems more plausible because morphological and textural evidence pleads for an authigenic origin. As a matter of fact, scanning electron micrographs show clearly that the palygorskite fibers are long and could be broken easily and destroyed by transportation. Pictures of palygorskite bridges between dolomite minerals suggest strongly that the palygorskite fibers were grown in place. Considering the

chemical point of view, the problem is simply to eval- uate whether the solutions could have provided the elements for palygorskite formation (i.e., Si, A1, and Mg). According to Weaver and Beck (1977), palygorskite formation needs high pH values and high Si and Mg activities (Figure 13). Several points suggest that the chemical environment was favorable to palygorskite neoformation:

1) The sediments of the "Formation Rouge" are essentially dolomitic. The dolomite nodules as well as the illites, most probably come from the erosion of the dolomitic Jurassic-Cretaceous formations near the Jbel Rhassoul. These nodules, showing textures of alteration during transport, were deposited in a lacustrine environment (Lucas and Pr6v6t, 1976). In addition to these nodules, there are some well- crystallized anthigenic dolomite crystals. In other words, there were good conditions for growth or aggradation of magnesian minerals such as dolomite in the sediments of the Jbel Rhassoul. There was Mg available in the solutions, which was most probably released, at least in part, from the Jurassic formations.

2 :K c o v 100

o

i i i I I I I I I i I

2 : fraction <2/zm enriched in illite 3 : fraction <2/zm enriched in

palygorskite

1000

t l I I I I I l t t

10 V Vi Cr Zn Cu Zr Mn Co Ba Li

Figure 11. Trace element diStribution in illites (Sample 2) and palygorskites (Sample 3).

I ~ I I I i I ~ I I i i i i i I

2 : fraction enriched in illite 3 : fraction enriched in palygorskite

1

o~

o

0 . 1 - i

La Ce Eu Yb Lu

Figure 12. REE distribution in illites (Sample 2) and palygorskites (Sample 3).


10

pn L

8

7

6 -6 -5 -4 -3 -2

log[H4SiO 4]

Figure 13. Stability fields of palygorskite, montmorillonite and aqueous solution at 25~ log[Al(OH)4] = -5.5 (from Weaver and Beck, 1977).

2 f

2) Field observations point out that the Jbel Rhassoul was subjected to an arid climate (Duringer, personal communication). Such a climate leads to concentration o f solutions and consequently an increase in pH and salinity. In that concentrated solution and alkaline environment, conditions were favorable for the precipitat ion of dolomite. These con-

ditions are also favorable to palygorskite precipitation (Singer, 1984). In the "Format ion Rouge," SEM observations show that palygorskites fibers cover authigenic dolomite crystals. They formed after dolomite when the solution was slightly impoverished in Mg and enriched in other elements.

3) Authigenic dolomite crystals show that there was precipitat ion of dolomite in the closed environment and, as a consequence, the solutions were enriched in other elements, particularly Si. That is corrob- orated by simulated evaporat ion tests made on waters sampled around Jbel Rhassoul. It is assumed that the present day waters are similar to those that filled the lake when the sediments were deposited because the paleogeography of the region was similar. The simulation was carried out with the help of calculation codes using thermodynamic prop- erties of minerals (Fritz, 1981). The test shows that dolomite precipitates first. After precipitation, the waters are impoverished in Mg and Ca and become Si-rieh (Chahi et aL, 1992).

4) Si in solution may have been provided in part by the degradation of minerals from the weathered Ju- rassic sediments and Paleozoic formations. The same is true for A1 and Mg. These elements also may have been provided by degradation of phyllosilicates. According to Paquet (1983), the phyllosilicates and clays are unstable in hyperalkaline conditions and tend to dissolve, releasing A1, Mg,

I HinterLand L Proximal zone Dis la t zone Lacustr ine zone l

J

PaIeozoic Jurassic r L CFetaceous I

I l

Ph~176

Figure 14. Schematic model of palygorskite formation in the Jbel Rhassoul.


and Fe. As a matter of fact, it is clear that the detrital illites coexisting with palygorskite show variable compositions. A large proportion of them are impoverished in interlayer K and tetrahedral A1. They may have been transformed from true illites, rich in Al, by stripping the K from the interlayer sites. This suggests strongly that a part of these phyllosilicates have been destabilized and have undergone a leaching of elements thus, providing A1 and K. In the lacustrine environment where the detrital illites were deposited, and which is strongly magnesian, the illites may have been chemically unstable and leached as previously observed by Srodon (1987) and Moberg (1989).

On the other hand, the alteration may have provided the solutions with K, which was not used for palygorskite formation but could have been used in the format ion of the small amount of neoformed illite/smec- rite that was observed. Their high Mg and Fe contents, compared with the illites (Figure 8), also suggest that they formed in a different environment than the detrital illites (which are Al-rich).

Considering field, chemical and mineralogical arguments, a model of formation can be proposed (Figure 14) where the different sources of detrital material, conditions of transport, and formation of the clays in the lacustrine basin are outlined. There are two important factors leading to the formation ofpalygorskite and illite/smectite in the basin:

1) Release of A1, Si, K, and Mg by altered dolomite and iUites from the hinterland.

2) Concentration of the solutions in the sediments with increase of pH and salinity, due to the climate.

SUMMARY AND CONCLUSIONS

Different analytical techniques (XRD, SEM, TEM and AEM, REE and trace elements, K-Ar isotopic dating) have been used to study the palygorskites and associated clays in the "Formation Rouge" of the Ter- tiary lacustrine basin at Jbel Rhassoul. The sediments in that basin have a detrital origin marked by phengites and illites. Associated with detrital minerals are palygorskites and tiny particles of illite/smectite. The chemical composition of the different clay species can be differentiated by use of AEM. The results show that the tiny illite/smectites have a particular composition, enriched in Fe, Mg, and K compared with the detrital illites. Morphological and textural arguments from SEM and TEM observations strongly suggest that palygorskite and the illite/smectite have an authigenic origin. Chemical arguments show that the conditions in the basin (high salinity, high pH, release of A1, Si, Mg by dolomite and detrital illites) were favourable for the formation of palygorskite and suggest that the palygorskites and the tiny illite/smectites in the "Forma- tion Rouge" are authigenic.

ACKNOWLEDGMENTS

We thank Jan Srodon for helpful discussions and Dennis Eberl for critical reading of the manuscript.

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(Received 28 July 1992; accepted 8 April 1993; Ms. 2251)

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ANALYSES OF PALYGORSKITES AND ASSOCIATED ...clays.org/journal/archive/volume 41/41-4-401.pdfFigure 3. X-Ray diffraction pattern from oriented clays showing illite, palygorskite, and