Clay Minerals (1979) 14, 47.

O C C U R R E N C E OF C H L O R I T I C I N T E R G R A D E S IN T H E R E C E N T S E D I M E N T S OF T H E A R N O R I V E R (ITALY) F. S A R T O R I , R. R I F F A L D I * A N D R. L E V I - M I N Z I * Istituto di M ineralogia and *Istituto di Chimica Agraria, Universitd di Pisa, Italy

(Received 30 June 1978)

ABSTRACT: Clay mineral assemblages from a Quaternary sequence, sampled near the estuary of the Arno River, commonly contain some intergrade minerals, associated with other 14/k phases and with illite and subordinate amounts of a kaolin mineral. The X-ray diffraction characteristics of such intergrade phases and their thermal modifications (100-600~ are analysed and described. The effectiveness of some chemical treatments for removal of interlayer material is investigated and the results discussed on the basis of X-ray diffraction pattern and C.E.C. changes. Finally the properties of these intergrades are compared with those of the 14 /k phases present in the sediments carried by the Arno River and its tributary the Era River as well as with those of a soil from the Era watershed. Hypotheses are expressed about the probable relationships between degree of chloritization and characteristics of the depositional environments.

In a study of 100 m section of Recent sediments sampled in a well near the estuary of the Arno River (in preparation), chloritic intergrades* present as important mineral components of the clay material were observed in all pelitic strata; the minerals were found to show somewhat variable characteristics with respect to stratigraphic position. In the fight of the particular interest developed in recent years on the properties of these intergrades and paucity of work on non-synthetic materials formed or deposited in alkaline environments it was considered worthwhile to undertake a detailed investigation of the minerals and of their variations within the stratigraphic sequence. It was considered particularly interesting to investigate the possibility of relationships between degree of chloritization and depositional environment, since the sediments of this sequence, according to paleontological and lithological criteria, appear to have accumulated in different environmental conditions. Moreover, since the sediments were certainly derived from the suspended load of the Arno River, it was worth attempting to identify the possible sources of the chloritic intergrades in its drainage basin, and thus compare such source area material with the deposits near to the estuary. Since the Arno River basin is relatively large (about 8-247 km z in area), with a great variety of mainly sedimentary rocks, the survey was restricted to the basin of the Era River, the last important tributary before the sea. Extensive outcrops of Lower Pliocene argillaceous formations are a feature of the basin. Chloritic intergrades were identified in the soils and the underlying argillaceous rocks of this area and in the sediments carried by the tributary. * According to Carstea et aL (1970c) the term 'chloritic intergrade' refers to a system intermediate between chlorite and vermiculite or smectite. 0009-8558/79/0300-0047502.00 9 1979 The Mineralogical Society

48

F. Sartori, R. Riffaldi and R. Levi-Minzi MATERIALS AND METHODS

Collection of samples Three groups of samples were studied (Table 1 and Fig. 1). The first were twelve samples (labelled with the letter 'i') from a water well drilled at the Tombolo Experimental Farm, on the coastal plain, about 4 km southwest of the present Arno River estuary; it has been drilled through Recent sediments, composed of interstratified sands, silts and clays

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to 100 m depth. The samples selected for the present study were only those from argillaceous or silty beds. The second group of samples were taken from the alluvia of the Arno River (two samples) and the tributary Era River (two samples). Finally the third group (samples 4-7) is of soil and its parent material. The site is located near the outer edge of a terrace cut by the Era River in the thick marine shale formation Lower Pliocene in age which acts as the parent material of the soil. Since the soil has no definite horizons, the sampling was simply carried out at increasing depths from the ground surface. Sample 7 was collected on the deposit of a mudflow on the slope of the terrace; its material represents the deepest levels of the parent rock.

Ana'lytical procedures The pH determinations (Table 1) were made on 1:2.5 sediment-water suspensions using the bulk samples.

Chloritic intergrades in Arno sediments

49

In all the other cases the < 2/~m fraction was used. It was obtained by sedimentation, after several washings of the bulk samples with deionized water. Any preliminary treatment to remove organic matter and free oxides or to favour dispersion was deliberately avoided. The muds were oven-dried at 60~ for 2 h. Cation exchange capacity (C.E.C.) was determined by Ba 2+ saturation and subsequent complete replacement of Ba2+ with Mg 2+, according to Bascomb (1964). Saturation with K and Mg cations was achieved according to the methods of Jackson (1956). Special care was taken in Mg-saturating procedures to prevent precipitation of Mg (OH)z, likely to occur when dealing with alkaline samples. For each sample three oriented aggregates: natural, K-saturated and Mg-saturated + glycerol-solvated, were prepared for X-ray analysis. Such aggregates were obtained by sedimenting and drying about 20 mg of clay suspended in water on glass slides. For Mg-saturated specimens a few drops of glycerol were added to the suspension. It is well known from literature (Gibbs, 1965) that this mounting technique results in a surface segregation of montmorillonite, which brings about a poor accuracy in quantitative evaluations of clay minerals. The technique is, however, characterized by a remarkable sensitivity in detecting also small amounts of poorly crystalline materials and by a good precision in comparing one sample with another. Since just these comparisons, rather than accurate quantitative informations, were considered essential to the research, the technique was always adopted. Intensity determinations, when required, were usually performed through peak area measurements (using a polar planimeter) on the chart record; in some cases, peak heights rather than peak areas were used. The K-saturated clay specimens were X-rayed after heating at 100, 200, 300, 400, 500 and 600~ in a muffle furnace for 2 h at each temperature. This complete procedure as well as the exchange capacity determination were repeated after each dissolution treatment. The samples were, in fact, tentatively subjected to some selected chemical treatment to remove hydroxy interlayers from the chloritic intergrades. Firstly the standard clean-up procedure with buffered sodium citrate-dithionite (Aguilera & Jackson, 1953) was applied; afterward a different aliquot of each sample was subjected to the HC1 dissolution treatment proposed by Carstea et al. (1970b).

RESULTS

Identification of clay mineral species All the investigated samples have a constant clay mineral suite, characterized by illite and 14 ~ minerals as the dominant components and by a kaolin mineral as subordinate component. Small amounts of a mixed-layer mineral are nearly always present. Sample 12i is anomalous in having as the only clay mineral a poorly crystalline smectite. Illite shares with the 14 A minerals the role of dominant component; in most of the samples they occur, in fact, in approximately equal abundance. In this respect the Arno River alluvia are unusual in showing a sharp decrease in illite amounts and a large dominance of the 14 A minerals. The X-ray diffraction patterns of illite frequently show asymmetry of the (001) peak toward the low-angle side, broadening of the (002) peak, and asymmetry of the (003) peak

F. Sartori, R. Riffaldi and R. Levi-Minzi

50

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