Clay Minerals (1988) 23, 309-321

CLAY M I N E R A L S T U D I E S OF THE TRIPOLI F O R M A T I O N (LOWER M E S S I N I A N ) , SICILY

E. A Z Z A R O , A. B E L L A N C A AND R. N E R I

lstituto di Mineralogia, Petrografia, Geochimica, Via Archirafi 36, 90123 Palermo, Italy

(Received 18 February 1988; revised 30 June 1988)

A B S T RA C T: The characterization of clay mineral assemblages in the diatomite-dominated Tripoli Formation (Lower Messinian, central Sicily) has resulted in delineation of suites dominated by generally well-crystallized dioctahedral smectite and illite of low crystallinity, with lesser amounts of kaolinite and chlorite. These minerals are thought to have a mainly detrital origin related to the calcareous and marly formations exposed in the margins of the depositional area. Vertical fluctuations of the montmorillonite/iUite ratio and changes of the crystallinity and chemical composition of these phyllosilicates support environmental interpretations from isotopic data of associated carbonates, and point to a wide variability of depositional conditions ranging from evaporating to brackish. Early diagenesis of the clay minerals was controlled by the primary composition of the sediment which, in turn, affected the porewater chemistry.

During the last few years, the Messinian sediments of evaporitic facies belonging to the "Gessoso-Solfifera" Series have become a matter of renewed interest. Stable isotope studies supported by mineralogical and sedimentological data enabled definition of the depositional conditions (Pierre & Fontes, 1978; Longinelli, 1979/80; McKenzie, 1985; Bellanca & Neri, 1986). In particular, mineralogy, textural features and oxygen-carbon isotopic compositions of carbonate components in the diatomite-dominated Tripoli Formation, stratigraphically underlying the evaporites sensu stricto, point to a wide variability of depositional environments, and suggest that the Tripoli sedimentation occurred in small basins characterized by periodic and marked restrictions towards the open sea (McKenzie et aL,

1979/80; Bellanca et al., 1986). Thus, on the basis of very positive 6180 values of carbonate minerals and the occurrence of evaporitic minerals and lack of fossils, the deposition of marl and carbonate beds, alternating with or overlying the diatomites, took place in an environment with highly evaporated waters. On the other hand, periodic influxes of meteoric waters during the Tripoli deposition are registered in relatively low 618 O and 613C values of some carbonate minerals.

The potential usefulness of clay mineral studies in the understanding of several depositional processes has already been amply proved. This work describes the chemical and mineralogical characteristics of clay mineral assemblages in the Tripoli Formation and lends further support to the conclusions of previous research concerning its deposition and diagenetic environment.

M A T E R I A L S

The Tripoli Formation (Lower Messinian) is located stratigraphically above the Tortonian pelagic marls and below carbonate beds, locally known as "Calcare di base", underlying the

�9 1988 The Mineralogical Society

310 E. Azzaro et al.

evaporite deposits of the Gessoso-Solfifera Series. It is ~5-50 m thick, and generally includes diatomites irregularly alternating with marl and carbonate layers, and argillites.

The diatomites are typically porous, white to cream-coloured rocks and usually exhibit fine, delicate lamination. They have variable contents of clay minerals, carbonates (calcite and/or dolomite) and diatoms still consisting of opal-A, and other biogenic fragments such as radiolaria and sponge spicules, calcitic foraminifera and skeletal phosphates are present. In some areas, soft marly diatomite beds alternate with harder laminated layers with abundant opal-CT (porcelanites).

Marls intercalated with diatomites and porcelanites show an abundant clay component associated with carbonates and small amounts of quartz and detrital feldspar. Fossil-rich marls contain calcite as the dominant carbonate phase, while fossil-poor marls contain mainly dolomite. Aragonitic and dolomitic limestone and marl beds, typically lacking fossils, occur in the upper part of the stratigraphic sequence and mark the contact between the Tripoli and the "Calcare di base". These carbonate materials are often referred to as "transition carbonates".

The clayey interbeddings (argillites) consist of phyllosilicates (75-87~), quartz (7-12~), feldspars (2-6~), and sometimes carbonates (1-10~o). Yellow efflorescences ofjarosite may also occur.

The samples studied came from two stratigraphic sections (Sutera: UB877546; Cozzo Campana: VB046562) outcropping in central Sicily (Fig. 1).

0 50 100 / I I

Km

PALERMO

N

�9 Sutera m Cozzo Campana

FIG. 1. Areal extent of the Gessoso-Solfifera Formation (central Sicilian Basin) according to Decima & Wezel (197l), and locations of the sections studied.

Clay minerals from Tripoli Formation 311

M E T H O D S

Mineralogical compositions were determined by X-ray diffractometry (XRD). The < 2 #m fraction was examined using oriented aggregates saturated with both 1 N MgC12 and KCI solutions, solvated with ethylene glycol, and heated at 350, 400, and 600~ for 1 h. The relative abundance of the clay minerals was determined according to the methods and data of Schultz (1964), Biscaye (1965), and Barahona et al. (1982). The < 5/~m and < 1 #m fractions of some samples were also analysed, but the results are not reported as the compositions were similar to those of the corresponding < 2 #m fraction.

The procedure described by Elverhoi & Romingsland (1978) for semiquantitative determination of kaolinite and chlorite was used when both were present in the same sample.

A measure of the smectite crystallinity was obtained from the "valley (v) to peak (p)" X-ray method of Biscaye (1965). XRD patterns of the clay fraction were obtained between 59 and 63~ at 1/4~ to measure the (060) peak position. Information about the chemical composition and crystallinity of illites was deduced from the Ioo2/I0ot ratio (Dunoyer De Segonzac, 1970) and Kubler's index (half-height width of the peak at 10/~, Cu-Kct; Kubler, 1968).

The <2/~m fractions were chemically analysed by X-ray fluorescence spectrometry (XRF).

R E S U L T S

Mineralogical characteristics

Smectite and illite are the most abundant clay minerals in the pelitic component of the Tripoli Formation. The former is generally predominant and in the Cozzo Campana section constitutes up to 80~o of the clay fraction. Kaolinite in minor amounts is present in all samples, and is more abundant in the Sutera samples. Chlorite occurs sporadically and in relatively low amounts. The XRD patterns of some samples (mostly located near the base of the Cozzo Campana section) treated with ethylene glycol show a 10/~ peak with an ancillary low-angle tail which, according to Srodon (1981), corresponds to ordered interstratified illite- smectite (I/S) mixtures with discrete illite.

The stratigraphic variations of the clay minerals with depth are shown in Fig. 2 and Fig. 3. Wide compositional changes characterize the Cozzo Campana profile in which fluctuations of the smectite/illite ratio reflect changes of lithology in the stratigraphic sequence. The argillites intercalated with diatomites, marls and limestones have lower contents of smectite. The clay fraction in the Sutera samples shows more restricted compositional changes not clearly related to the lithology.

The (060) spacing occurs between 1.499 and 1.504/~ for both smectites and illites, a shoulder on the high-angle side representing the kaolinite, although sometimes the 1-496/~ peak of this mineral is clearly resolved. As no peak was observed at greater d-values, the presence of trioctahedral minerals and pure dioctahedral nontronite is excluded. Some smectites from the top of the two sections show an unusual behaviour on heating as complete collapse does not occur at 400~ The application of Greene-Kelly's test (MacEwan & Wilson, 1980) indicates a composition intermediate between montmorillonite and beidellite, only one sample showing the presence of beidellite. On the basis of the same test, all other smectites are very similar to montmorillonite.

312 E. Azzaro et al.

I ~ smectite kaolinite v/p Kublers index 1(002)/1(001)

i l l i te

5 7i5 110 0.2 01.3 01.4

~ A ~ E = P 410 60 8[0

I 2 0 I I I I

~ - - e l .. ,ob_ N I : : : : : : : : : : : : : 4 9

, X ' ; ' : ' X ' : 4 4 _ "~?:T:'~',~2 - - 43 - -

4 z - -

::::::::::::: - 3 7 - - , ' , ' , ' , ' , ' , ' , 34

�9 ' ' " 3

I ; ' : ' : ' : ' i ' i ' - - 2 6 - -

i : i : i : i : i : : : : 17 - - 1 6 - -

:,:,:,:,:,:,:

I (%) 0"150 0"175

i

r

FIG. 2. Stratigraphic profile of the Cozzo Campana section with sample location, mineralogical composition of the clay fraction, Biscaye's index for the smectite, crystallinity index and (002)/(001) intensity ratio for the illite. A = argillites; C = carbonates and marls; D = diato-

mites; P = porcelanites (thickness: 10-50 cm).

D 20 4o

38 •

- 3 5 - - 3 4

_ 3 1 - - - a o - -

- 2 9 - -

- 2 7 - -

::~'~'~'~::~-:~'=;- 2 5 - - ~/'/'/'/'/'/'/'/'/'/'//'//'/'/'/'/'/'/'/'/////////~

~ 2

~ 2 o -I ,',',',',',',~ 19 �9 i...i...i...i...i,..i.q 1 7 ~12!}i=:!}i=:ili':il!~!ii='ili"ili 1 3

' / . . . . . 4 i i

5mectite ~ kaolinite v /p

chlorite

60 80 (%) 050 075 I I I I I 1 I I

.iiiiiiiiiiiiili

I I I

> ,,>

Kubler's index 1(oo2)/1(OOl )

,~ ,;, .~ op

FIO. 3. Stratigraphic profile of the Sutera section with sample location, mineralogical composition of the clay fraction, Biscaye's index for the smectite, crystallinity index and (002)/(001) intensity ratio for the illite. A=argi l l i tes ; C = c a r b o n a t e s and marls;

D = diatomites.

Clay minerals from Tripoli Formation 313

The crystallinity index values (v/p) of the smectites from Sutera (ff = 0.56) are intermediate between the very high values of authigenic smectites in Recent marine environments (Cole & Shaw, 1983) and the very low values reported by Griffin et al. (1968) for smectites from deep sea sediments. The v/p ratio shows considerable fluctuations throughout the section (Fig. 3). The smectites from Cozzo Campana tend to show higher v/p values characteristic of a good crystallinity (Fig. 2). At the top of the section, however, a very low value (v/p = 0.36) characterizes the smectite from an argillite, whereas the highest value (v/p = 0.8) is shown by montmorillonite from a limestone.

As to the measure of the smectite crystallinity, it must be considered that the depth of the valley (v) on the low-angle sid e of the peak at 17 A may be different for a pure mineral than for a mixture containing it (Srodon, 1981). However, physical mixtures of montmorillonite with illite in ratios similar to those measured in our samples should not involve remarkable changes in the XRD pattern from the low-angle region (Nadeau et al., 1984). On the other hand, if a fair amount of illite is interstratified with the smectite, the pattern shows a broad maximum at 17 A with a high background towards the low-angle side (Wilson, 1987). According to Reynolds (1980) and Wilson (1987), (001) illite/(002) (glycol)-smectite and (002) illite/(003) (glycol)-smectite were used to determine the presence and the proportions of illite interstratified with smectite. When the peak resolution was not satisfactory, suggestions by Srodon (1981) were also utilized. The Cozzo Campana samples contain pure smectite, but weak reflections indicative of randomly interstratified I/S (max. 40~o illite) are recognizable in some samples (mostly argillites). A similar smectite-rich mixed-layer mineral (up to 30~ illite) without discrete smectite was observed in many samples from Sutera (mostly diatomites and marls). A comparison of the XRD patterns from different samples of the two sequences is shown in Fig. 4. It is thus possible that the presence of mixed layers is responsible for the lower v/p values measured in samples from Sutera. However, no relationship was observed between the measured v/p values and the percentage of the illite component in the mixed layer, probably because of the low I/S layer ratio. Even conceding that the measure of v/p may reflect some aspects of interstratification, it is believed that these values, if analysed carefully, represent a valid parameter which yields information about the genetic history of the expandable mineral. For simplicity, pure smectite and smectite-rich I/S mixed layer are not distinguished in Figs. 2 and 3.

In the Sutera section, the illite crystallinity index (K) tends to fluctuate, with a trend towards values indicative of low crystallinity (2 = 6-3). Also, the chemical composition of the octahedral sheets in the illite changes with sharp variations in the height ratio (R) of the (002)/(001) peaks in the central part of the section. Wide variations in the illite crystallinity index (average value 7.2) occur along the Cozzo Campana section, with lower values prevailing in its central portion. The chemical composition of these illites is also variable (R = 0.21-0.46) and the R fluctuations are concomitant with, but opposite to, those of the K index. This is not surprising, as the chemical composition of the illite particles, in particular the AI/(Mg + Fe) ratio in the octahedral sheet, affects the ordering in the illite structure, and consequently its crystallinity (Dunoyer de Segonzac, 1970).

Chemical characteristics

The chemical analyses of the pelitic component of the Sutera samples are shown in Tables 1 and 2.

The striking variations in the contents of some elements are explained by the presence in

E. Azzaro et al. S

I Q S

s A ,

I

~ K . A /

K I

~ L T -

i

+l /S

314

ii

III

/ /

/

I / / !

I

~ I I 1 I

20 15 CuKa 10 5 Fit3. 4. XRD traces of clay assemblages (Mg-saturated, oriented and glycolated) from (i) a carbonate; (ii) an argillite (Cozzo Campana); (iii) a diatomite (Sutera). Variations of the v/p ratio (Biscaye's index) of the expandable mineral are shown. S = smectite, I = illite,

I/S = smectite-rieh mixed-layer illite/smectite, K = kaolinite, Q = quartz.

Clay minerals from Tripoli Formation

TABLE 1. Major element analyses (in wt~) of the clay fractions from the Sutera samples.

315

Sample SiO 2 T i O 2 A1203 Fe203 MnO MgO CaO Na20 K20 P2Os L.I.

$31 60-99 0-60 14.69 5-75 0.03 1-62 0.73 0.23 1.98 0,08 13-30 $27 53.43 0.94 22.37 7-13 0-01 2,46 1,06 0.27 2,99 0-08 9.49 $22 63.99 0.81 17.08 3,05 0.0l 1.85 0,12 0.28 2-56 0.03 t0.30 $20 63-21 0.84 13.72 6-76 0-06 2-01 1-43 0-27 1-60 0-16 10-7t SI9 48-80 0.80 I9,97 4-42 0-0I 2-65 4-86 0.25 2-79 0.II i5-33 S17 58.03 1-0l 15.64 6-59 0-07 1-86 5-01 0-24 2-01 0-28 9-39 S13 69-25 0.49 12.35 5,90 0.01 1.29 l - l l 0.19 1.47 0-10 8.16 S12 52-83 0.87 20-93 8-91 0.09 2,77 1.77 0,27 3.05 0.13 8-63 Sll 55-95 1.01 24-45 5.18 0-02 3-26 0.42 0.26 3.35 0.05 6-66 S 4 73-83 0.91 12.41 3.01 0-01 1,66 0.22 0,25 1-47 0.02 6-11

Total iron expressed as Fe203. L.I. - loss on ignition.

TABLE 2. Minor element analyses (in p.p.m.) of the clay fractions from the Sutera samples.

Sample Ba Cr Ni Cu Sr Rb Pb Zn Zr Y Nb La

$31 450 195 45 59 180 70 - - 396 420 - - - - 26 $27 465 272 63 69 110 102 74 363 365 - - - - 49 $22 450 205 35 18 - - 60 - - 58 - - - - - - 40 $20 300 148 104 67 - - 45 - - 162 - - - - - - 85 S19 620 224 67 55 225 108 - - 110 250 13 20 45 S17 335 i76 73 70 144 56 - - 120 180 - - 14 38 S13 350 128 82 85 - - - - - - 105 - - - - - - 30 S12 700 248 92 80 247 92 25 188 225 14 14 56 Sl l 460 218 42 18 100 115 32 81 190 14 25 43 S 4 300 145 33 55 - - 54 - - 63 - - - - - - 18

- - not detected.

the < 2 p m f r ac t i on o f m i n e r a l s o t h e r t h a n phyl los i l ica tes . For example , w i th the e x c e p t i o n o f

s ample S-19 in w h i c h t races of a n h y d r i t e were de tec ted , the s ign i f ican t r e l a t i onsh ip b e t w e e n

C a O a n d P205 (r = 0.92) suggests t h a t a C a p h o s p h a t e p h a s e is p r e sen t a n d indeed a

c ryp toc rys ta l l ine p h o s p h a t e was c o n f i r m e d by op t i ca l m i c r o s c o p e o b s e r v a t i o n s . In a d d i t i o n ,

the h i g h e s t c o n c e n t r a t i o n s o f Sr were m e a s u r e d in s a m p l e s d i sp l ay ing p r i s m a t i c c rys ta ls o f

celes t i te in t h i n sect ion.

T h e K, A1, Mg, R b a n d C r c o n t e n t s seem to b e assoc ia ted mos t ly w i t h t he clay mine ra l s ,

a n d t h r o u g h o u t the s t r a t i g r a p h i c profi le i t is poss ib le to obse rve c o n c o m i t a n t v a r i a t i o n s i n

the d i s t r i b u t i o n o f these e l e m e n t s (Fig. 5). T h e co r re l a t ion coeff icients are s ta t i s t ica l ly

s igni f icant , r a n g i n g f rom 0-72-0-98 ( m e a n = 0.88; std. dev. = 0-02). I n some samples , t he

a m o u n t of K 2 0 in the illite, ca l cu la ted o n the bas i s of 7 - 5 ~ K z O c o n t e n t in illite, is s l ight ly

lower t h a n the m e a s u r e d K 2 0 con t en t , a n d m i x e d layers m i g h t be r e spons ib l e for the

d i sc repancy . Also, Ti seems to be b o u n d to the clay mine ra l s , pa r t i cu la r ly to i l l i te a n d

kao l in i t e w i t h w h i c h it is pos i t ive ly cor re la ted . In some clay depos i t s , m o s t o f the TiO2 is

p r e sen t as pel le ts ( ana t a se ) w h i c h are b o n d e d to e a c h o t h e r a n d to kao l in i t e p la tes ( W e a v e r ,

316 E. Azzaro et al.

C

$ 3 1 - -

$ 2 7 - -

S 2 2 - -

S 2 0 _ _ S 1 9 - - S 1 7 - -

$ 1 3 S 1 2 ~ S l l

S 4 - -

[] MgO �9 K20 A I 2 0 3 o Rb �9 Cr

% 2 3 % I I I I I

15 2O I I I I I .

100 200 ppm I I i I I

~ x x x ~

/ / /

/ , r /

/ /

d

FIG. 5. Profiles illustrating the variations of MgO, KzO, A1203, Rb and Cr in the clay minerals in the stratigraphic sequence of the Tripoli Formation at Sutera. Sample numbers at the left

correspond to the measured section in Fig. 3.

1976). In addition to discrete mineral species, Ti may be present in the structure of kaolinites substituting for AI or Si (Rengasamy, 1976).

Fair amounts of Fe20 3 are present in the analysed < 2/~m fractions. Since neither Fe-rich smectite nor Fe-rich illite was detected by XRD, Fe is present as Fe oxides or amorphous coatings.

D I S C U S S I O N

The abundance of smectite in the Tripoli Formation rocks and in the so-called "transition carbonates" is consistent with the hypothesis according to which the pelitic component of these materials is genetically related to the formations exposed in the margins of the depositional area during the Messinian. These terrains were reef limestones developed on clastic, deltaic sediments deposited between the Torthonian and Lower Messinian. The limestones may have been a good source of smectite. In fact, soils from calcareous horizons offer geochemical conditions which favour the stability and/or formation of montmorillonite with respect to that of kaolinite for the soil clays (Weaver et al., 1976). The deltaic sediments, known as the Terravecchia Formation, consist of marls and marly clays intercalated with calcarenites, sands and sandstones (Catalano, 1979). The pelitic fraction of marly samples from these deposits contains abundant and rather well-crystallized smectite (57%; v/p = 0.6), minor amounts of illite (29%) and kaolinite (14%), and traces of chlorite (Bellanca et al., 1980, unpublished data).

The presence of kaolinite, although limited, is compatible with deposition of the diatomites and evaporitic carbonates occurring in the marginal zones of the Sicilian Basin as the kaolinite particles are generally larger than those of the other clay minerals, and are

Clay minerals jrom Tripofi Formation 317

therefore deposited closer to the source (Dunoyer de Segonzac, 1970). It is probably for this reason that higher amounts of kaolinite are found to be positively correlated with detrital quartz and feldspar in the Sutera samples.

The vertical fluctuations of the montmorillonite/illite ratio and the changes in the crystallinity and chemical composition of these phyllosilicates can be explained by different supply routes into the deposition basin e.g. due to changing sedimentation rate, shallowing (or deepening) of the water body, and/or periodic peaks of freshwater input from surface runoff. Diagenetic processes may also be reflected in the distribution patterns of the clay minerals.

The argillites intercalated with diatomites and carbonates, which are characterized by a montmorillonite/illite ratio (even if > 1) lower than that of the other samples, mark periods of considerable influxes of continental waters into the deposition basin. Indeed, these argillites contain carbonate with an isotopic composition typical of carbonates precipitated from meteoric or mixed marine/meteoric waters (Fig. 6; data from Bellanca et al., 1983). In the Sutera section, moreover, the carbonates underlying the argillites show extremely negative

13 C values suggesting for their formation the utilization of substantial amounts of C derived from 13C-depleted organic matter, which had been transformed into CO2 by bacterial activity (Bellanca et al., 1982-83). The carbonates overlying the argillites have 613C values

E -2 0 +2 +4 +6 +8 %0 I I I I I I I I I I I

c~ 180 carbonate minerals v/p

5 1 m

4 9 b _ _ 4 9 - - ~ 4 1

4 4 _ _ e - - - - - - - - - - _ ~ : ~ ~ ~

4 3 - -

4 2 - -

3 7 - - ~ " " " ~

333032 -- ~ _ f / 1

2 6

1 7

1 6 - -

5 m

2 m

�9 CaO0 3 minerals �9 dolomite

\

0.4 0.6 0.8 I I I I I I I

FIG. 6. Profiles illustrating the variations of oxygen isotopic composition (vs. PDB-1) of the carbonate minerals, and Biscaye's index for the smectite in the stratigraphic sequence of the Tripoli Formation at Cozzo Campana. Sample numbers at the left correspond to the measured

section in Fig. 2. Isotopic data from Bellanca et al. (1983).

318 E. A z z a r o et al.

~ 0 excluding substantial contributions of biogenic CO:. Thus, the argillites mark the passage from anoxic to more oxygenated environmental conditions.

Among the parameters measured, the smectite crystallinity index reflects well the variations of the depositional environment, and it is possible to observe a relationship between the fluctuations of this parameter throughout the sections and the oxygen isotopic composition of the carbonate minerals contained in the samples (Figs. 6 and 7). Higher 318 0 values, which are indicative of depositional environments with waters highly concentrated by evaporation, coincide with higher values of the v/p index. Paquet et al. (1969) suggest that the establishment of a sub-arid climate results, in continental areas, in the formation of soils containing abundant and well-crystallized smectites in regions of low relief and little rainfall. It is possible that similar environmental conditions were already sporadically present during the Tripoli deposition, prior to the arid conditions which favoured the great development of salt evaporites during the Upper Messinian. Such an environment was probably favourable to the formation of "evaporitic" carbonates in the Tripoli basin, and of smectite with high v/p in the coastal plains and exposed areas. In this context it should be noted that chemical analyses of the pelitic fraction indicate a rather low amount of MgO (ff = 2.14; a = 0.62) in samples which, on the basis of the isotopic compositions of the associated carbonates, were deposited in environments with highly concentrated waters. As the smectite formed in similar environments generally shows high Mg contents, the low concentrations of this element in the samples studied, and the negative relation between Mg and smectite

o 3 180 dolomite =E +2 +4 +6 +8 ~7~) = l I I I I. I I I I

38

35 34 32 31 30 29 27 26

25

22

2O

17

13 12 11

5 4

v/p

0.4 0,8 0.8

I., I I I I

FI6. 7. Profiles illustrating the variations of oxygen isotopic composition (vs. PDB-1) of the dolomite, and Biscaye's index for the smectite in the stratigraphic sequence of the Tripoli Formation at Sutera. Sample numbers at the lef! correspond to the measured section in Fig. 3.

Isotopic data from BeUanca et aL ([982-83).

Clay minerals from Tripoli Formation 319

(r = - 0-5) would exclude authigenesis and support the previous conclusion of a terrigenous source for this mineral. However, considering that in the evaporitic limestones from the Tripoli Formation the smectite is associated with abundant dolomite, it is possible that most Mg was linked to the carbonate mineral. Thus, both detrital and authigenic smectite may be present in these rocks. The occurrence of smectites close to beidellite in prevalently aragonitic limestones (e.g. S-31 and S-35) deposited in moderately concentrated waters suggests that the montmorillonitic smectite commonly found in dolomitic limestones results from an original beidellite re-equilibrated in porewaters concentrated by evaporation.

High amounts of a well-crystallized smectite in the diatomites from Cozzo Campana suggest a partly authigenic origin favoured by high Si(OH)4 and Ca 2+ activities depending on the dissolution of diatom frustules and foraminifera skeletons, respectively. Smectite formed in such a manner may also be present in the marls alternating with porcelanites, from the middle-low portion of the same section, which contain abundant siliceous remains.

Despite the potential availability of large amounts of silica in the early diagenetic fluids, in the Sutera diatomites the smectitic mineral was replaced by highly expandable randomly interstratified I/S. Reactions in which natural smectites are converted to illite via inter- stratification require K § addition, and are controlled by temperature, residence time at different temperature, contents of inhibitor ions such as Mg and Ca in porewaters, and local permeability (Ramseyer & Boles, 1986). For the aggradation of the smectite from the Sutera diatomites, the source of K § is the dissolution of detrital K-feldspar scattered in these sediments, the porosity of which favoured fluid migration. The presence of randomly interstratified clays also in some marls and argillites containing abundant quartz and feldspars confirms that the formation of the mixed layers is primarily dependent on the availability of K from detrital minerals releasing it. In the Tripoli Formation, diagenetic events connected with large amounts of organic matter are common (Bellanca et al., 1986). These processes involve reactions such as bacterial reduction of evaporitic sulphate as sulphide, resulting in the crystallization of diagenetic carbonates, and subsequent oxidation of sulphide. Both these reactions release large amounts of energy (Orr, 1978; Rouchy & Pierre, 1987) which might be expended to increase the local temperature and to initiate the conversion of the smectite to illite via I/S mixed layers. However, new data are required to define the influence of organic matter on the diagenesis of the clay minerals.

The presence of ordered interlayers (I/S) is restricted to the bottom of the Cozzo Campana section, and it seems that these I/S clays, frequently associated with burial diagenesis, are inherited from siliciclastic levels of the surrounding formations.

A particular example is represented by a marl characterized by beidellite and a zeolite of the heulandite-clinoptilolite group association (sample 5 from Cozzo Campana). Similar occurrences are often associated with episodes of intra-basin volcanic activity. As this association has never been reported in the Tripoli Formation, we do not exclude the possibility that this material could be derived from marls of the Lower Miocene outcropping in central-western Sicily, characterized by the same association (Barbieri et al., 1981).

C O N C L U S I O N S

The compositional characteristics of clay mineral assemblages in the Tripoli Formation indicate that a wide range of sedimentary environmental conditions occurred in the Mediterranean during the Lower Messinian. Vertical fluctuations of the montmorillonite/ illite ratio and changes in the crystallinity and chemical composition of these phyllosilicates

320 E. Azzaro et al.

occur which, combined with the isotopic compos i t ion of the associated carbonates , po in t to

deposi t ional condi t ions ranging from evapora t ing to brackish. O n the whole, the sediments studied appear to be smectite-rich. The smecti te is a d ioctahedral type, general ly very s imilar

to montmor i l loni te , wi th values of Biscaye's index mostly character is t ic of good crystall inity.

A detri tal or igin is proposed for this minera l , related to the calcareous and mar ly format ions exposed in the marg ins of the deposi t ional area. I t is also suggested that it re-equi l ibrated in waters concent ra ted by evaporat ion. Early diagenet ic processes affecting the clay minera ls

were controlled by porewater chemist ry dependen t on the ini t ia l sed iment composi t ion. Lower contents of smecti te are registered in the argillites which, in tercala ted with dia tomites and porcelanites, m a r k periods of considerable influxes of con t inen ta l waters into the

deposi t ion basin.

A C K N O W L E D G M E N T S

Thanks are due to Dr. M. J. Wilson and to an anonymous referee for criticism of the manuscript and for constructive reviews. We wish to recall the memory of the late Professor M. Carapezza who so kindly favoured the launching of this research project. This work was supported with a contribution from the Ministero della Pubblica Istruzione, Italy.

R E F E R E N C E S

BARAHONA E., HUERTAS F., POZZUOLI A. & LINARES J. (1982) Mineralogia e genesi dei sedimenti della provincia di Granada (Spagna). Miner. Petrogr. Acta 26, 61-90.

BARBIERI M., BELLANCA A. 8r NERI R. (1981) Origin of zeolites associated with montmorillonite and silica phases in Miocene deposits of Sicily. Miner. Petrogr. Acta 25, 41-55.

BELLANCA A., DI CACCAMO A. 86 NERI R. (1980) Mineralogia e geochimica di alcuni suoli della Sicilia centro- occidentale: studio delle variazioni composizionali in relazione ai litotipi d'origine. Miner. Petrogr. Acta 24, 1-15.

BELLANCA A., CALDERONE S. 8r NERI R. (1982-83) Evidenze geochimiche e mineralogiche di episodi evaporitici nella sequenza diatomitica (Messiniano "pre-evaporitico") di Sutera (Sicilia centrale). Rend. Soc. ItaL Miner. Petrol. 38, 1271-1280.

BELLANCA A., CALDERONE S. 8/, NERI R. (1983) Studio isotopico, mineralogico e tessiturale sul Tripoli di Cozzo Campana (Sicilia centrale): implicazioni ambientali. Miner. Petrogr. Acta 27, 91-103.

BELLANCA A., CALDERONE S. 8r NERI R. (1986) Isotope geochemistry, petrology and depositional environments of the diatomite-dominated Tripoli Formation (Lower Messinian), Sicily. Sedimentology 33, 729-743.

BELLANCA A. & NERI R. (1986) Evaporite carbonate cycles of the Messinian, Sicily: stable isotopes, mineralogy, textural features, and environmental implications. J. Sed. Pet. 56, 614-621.

BISCAYE P.E. (1965) Mineralogy and sedimentation of recent deep sea clays in the Atlantic Ocean and adjacent seas and oceans. Geol. Soc. Amer. Bull. 76, 803-832.

CATALANO R. (1979) Scogliere ed evaporiti messiniane in Sicilia. Modelli genetici ed implicazioni strutturali. Lavori Ist. Geologia Univ. Palermo 18, 1-21.

COLE T.G. & SHAW H.F. (1983) The nature and origin of authigenic smectites in some recent marine sediments. Clay Miner. 18, 239-252.

DECIMA A. & WEZEL F.C. (1971) Osservazioni sulle evaporiti messiniane della Sicilia centro-meridionale. Riv. Miner. Siciliana 130-132, 172-187.

DUNOYER DE SEGONZAC G. (1970) The transformation of clay minerals during diagenesis and low-grade metamorphism: a review. Sedimentology 15, 281-346.

ELVERnOI A. & ROIdlNGSLANO T.M. (1978) Semiquantitative calculations of the relative amounts of kaolinite and chlorite by X-ray diffraction. Marine Geology 27, 19-23.

GRIFFIN J.J., WINDOM H. & GOLDBERG E.D. (1968) The distribution of clay minerals in the World Oceans. Deep Sea Res. 15, 433-459.

Clay minerals from Tripoli Formation 321

KUBL/~R B. (1968) Evaluation quantitative du m~thamorphisme par la cristallinit6 de l'illite. Bull. Cent. Rech. Pau., S.N.P.A. 22, 258-307.

LONGINELLI A. (1979/1980) Isotope geochemistry of some Messinian evaporites: paleoenvironmental implications. Paleogeogr. Paleoclim. Paleoecol. 29, 95-123.

MACEWAN D.M.C. & WILSON M.J. (1980) Interlayer and intercalation complexes of clay minerals. Pp. 197- 248 in: Crystal Structures of Clay Minerals and their X-ray Identification (G. W. Brindley & G. Brown, editors). Mineralogical Society, London.

MCKENZIE J.A. (1985) Stable-isotope mapping in Messinian evaporative carbonates of central Sicily. Geology 13, 851-854.

MCKENzIE J.A., JENKYNS H.C. & BENNETT G.G. (1979/1980) Stable isotope study of the cyclic diatomite- claystones from the Tripoli Formation, Sicily: a prelude to the Messinian salinity crisis. Paleogeogr. Paleoclim. Paleoecol. 29, 125-149.

NADEAU P.H., TAIT J.M., McHARoY W.J. & WILSON M.J. (1984) Interstratified XRD characteristics of physical mixtures of elementary clay particles. Clay Miner. 19, 67-76.

ORR W. (1978) Biogeochemistry. Pp. 16Ll-16L19 in: Handbook of Geochemistry 11-2, (K. H. Wedepohl, editor). Springer-Verlag, Berlin.

PAQUET H. (1969) Quoted in Chamley H., Dunoyer de Segonzac G. & Melieres F. (1978) Clay minerals in Messinian sediments of the Mediterranean area. DSDP Initial Reports 42, 389-395.

PIERRE C. & FOYrES J.C. (1978) Isotope composition of Messinian sediments from the Mediterranean Sea as indicators of paleoenvironments and diagenesis. DSDP Initial Reports 42, 635-650.

R~d, lSEYER K. & BOLES J.R. (1986) Mixed layer illite/smectite minerals in Tertiary sandstones and shales, San Joaquin Basin, California. Clays Clay Miner. 34, 115-124.

RENGASAMY P. (1976) Substitution of iron and titanium in kaolinites. Clays Clay Miner. 24, 265-266. REYNOLDS R.C. (1980) Interstratified clay minerals. Pp. 249-303 in: Crystal Structures of Clay Minerals and

their X-ray Identification (G. W. Brindley & G. Brown, editors). Mineralogical Society, London. ROUCHY J.M. & PIERRE C. (1987) Authigenic natroalunite in middle Miocene evaporites from the Gulf of Suez

(Gemsa, Egypt). Sedimentology 34, 807-812. SCHULTZ L.G. (1964) Quantitative interpretation of mineralogical composition from X-ray and chemical data

for the Pierre Shale. U.S. Geol. Surv. Prof. Pap. 391-C. SRODON J. (1981) X-ray identification of randomly interstratified illite-smectite in mixtures with discrete illite.

Clay Miner. 16, 297-304. WEAVER C.E. (1976) The nature of TiO2 in kaolinite. Clays Clay Miner. 24, 215-218. WEAVER R.M., JACKSON M.L. & SYERS J.K. (1976) Clay mineral stability as related to activities of aluminum,

silicon, and magnesium in matrix solution of montmorillonite-containing soils. Clays Clay Miner. 24, 246- 252.

WILSON M.J. (1987) X-ray powder diffraction methods. Pp. 26-97 in: A Handbook of Determinative Methods in Clay Mineralogy (M. J. Wilson, editor). Blackie, Glasgow & London.