Clay Minerals (1984) 19, 59-66

'CORRENSITE-LIKE MINERALS' IN THE TARO AND CENO VALLEYS, ITALY M. F. B R I G A T T I

AND L. P O P P I *

lstituto di Mineralogia e Petrologia dell'Universitd, Via S. Eufemia 19, 41.100--Modena, and *Istituto di Mineralogia e Petrografia dell'Universitd, Porta S. Donato 1, 40.100 Bologna, Italy (Received 28 June 1983)

ABSTRACT: Mineralogical properties (XRD, DTG) and chemical compositions of some chlorite-smectite interlayer minerals in alteration products of ophiolitic rocks from the Northern Apennines are presented and discussed. The presence of iron hydroxides and the continuous variation in A1203 content suggest that the corrensites are intermediate stages in the process of alteration of chlorite to smectite in an environment characterized by high element mobility. A thermal test to characterizethe swelling componentof the interlayer species is proposed.

Lippman (1954)first used the term 'corrensite' to describe a regular 1:1 trioctahedral chlorite/swelling-chlorite interlayer mineral, but subsequently showed that the swelling component could be a vermiculite (Lippman, 1956) or a saponite (Lippman, 1959, 1976). At present, the concept of an interstratification of two different components (swelling and non-swelling) is generally accepted, as is the chloritic nature of the non-swelling component. However, the exact composition of the swelling component is still debatable, this uncertainty arising from the large variability in response to standard glycolation and heating treatments of the samples studied. Bailey (1982), in a report of the AIPEA Nomenclature Committee, defined corrensite as a 1:1 regular interstratification of trioctahedral chlorite with either trioctahedral smectite or trioctahedral vermiculite. The presence of 'corrensite-like minerals' in ophiolite rocks of the Northern Apennines has been recorded by many investigators (Gallitelli, 1956; Alietti, 1957a,b, 1959; Bocchi & Morandi, 1969; Mongiorgi & Morandi, 1970; Mejsner, 1977). The purpose of the present investigation was to survey the distribution of 'corrensite-like minerals' in the Taro and Ceno Valleys of the Northen Apennines, to examine variations in composition, and to study conditions of formation. METHODS All experimental work was carried out on 2-0.1 /~m fractions obtained by normal sedimentation methods from an ultrasonically-dispersed sample in double-distilled water. Monoionic clays obtained by shaking the fractions in teflon tubes filled with repeatedly changed solutions of superpure reagents, following standard procedures (Prost, 1976). X-ray analyses were performed on oriented films using a Philips diffractometer with a heating-stage attachment. Thermal studies were made on Du Pont equipment using a controlled gas flow. Chemical analyses were performed by X R F (Philips 1400) and atomic 9 1984 The Mineralogical Society

M. F. Brigatti and L. Poppi

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absorption spectrophotometry (Perkin Elmer 603). Electron microscopy was carried out on a Philips SEM 500 apparatus. X-RAY

RESULTS

X-ray patterns of the samples studied are shown schematically in Fig. 1. The main constituents of the clay fraction, normally a serpentine mineral (sample 48) in these basic rocks, are saponite (samples 146 and 147), chlorite (samples 41 and 52), and an interlayer mineral (samples 39, 40, 44, 57 and 59) with a typical reflection at 3 0 / k in the air-dried state which shifts to 32 A on glycerol solvation and contracts on heating. This contraction appears particularly marked if the reflection at 14 /k is considered (it shifts to ~12 A), whereas the reflection at 30/k tends to broaden on shifting to ~25 A. This feature is typical of a 'corrensite-like mineral'. This phase is present in many areas but it is often not purifiable and seems to show the same behaviour in all cases. It should be noted that the interlayer mineral is often associated with chlorite but seldom with saponite (sample 39). It

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'Corrensite'

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should also be noted that in this area an apparently more disordered saponite--as shown by the absence of all basal reflections except the first (sample 146)---is widespread. The response of these clay minerals to heating is also interesting: all the saponites collapse to ~ 10 ,~ at very low temperatures (100-200 ~ C) but in some cases (e.g. sample 147) they are able to rehydrate instantly even after heating to 6 0 0 ~ for up to 12 h. This behaviour, which has been well-documented (Poppi, 1971), must be considered because it can be confused with that of 'swelling chlorite'. For example, sample 57 shows a basal reflection at ~24 _A (chlorite + collapsed swelling mineral) if the X-ray trace is made on the sample immediately on heating, but the basal reflection shifts to ~ 2 8 / ~ if the X-ray trace is made at room temperature on the pre-heated sample. Another important question arises from the relative proportions of chlorite and interlayer minerals in these samples: the two phases are often associated but one always predominates. Samples 40 and 41 are found in two areas very close to each other: in the first chlorite is absent, in the second it is widespread. THERMAL

BEHAVIOUR

The thermal behaviour of the more significant samples is illustrated in Fig. 2. The following observations can be made.

r(~ FIG. 2. DTG traces of selected samples obtained using a heating rate of 20~ in controlled gas flow. Sap. = saponite, ser. = serpentine, ehl. = chlorite, int. = 'corrensite-like mineral'.

M. F. Brigatti and L. Poppi

62

1. The serpentine mineral is characterized by a slow dehydroxylation reaction and a high H 2 0 content.

2. Different types of smectite are present, these exhibiting variations in structural order as inferred from their dehydroxylation reactions in the range 550-750~ and also

[~100 200 300 400T(CI FIG. 3. Thermal behaviour of sample 41 after Ca- and Mg-exchange.

FIG. 4. Scanning electron micrographs of samples 41 (1, 4), 40 (2) and 57 (3). Magnifications: (1) and (2) • (3) and (4) x2500.

'Corrensite'

63

containing different exchangeable cation populations in interlayer positions as inferred from their low-temperature weight losses. 3. Iron oxyhydroxides occur in some samples as shown by a D T G peak at 250-300~ (especially marked for sample 45). 4. The sharp and symmetrical nature of the D T G peaks in the low-temperature region would suggest that the interlayer mineral is structurally well-ordered. 5. The chlorite of sample 52 exhibits typical thermal behaviour, whereas sample 41 shows an additional peak at ~250~ This peak also occurs in other samples (e.g. 59, 39) and can be correlated with the presence of Mg as shown in Fig. 3. The gycol solvation and heating treatments characterize this sample as chlorite (Fig. 1) but the thermal behaviour appears to be linked to an anomalous Mg content in interlayer positions of an incipient swelling structure. The morphology of this sample as observed by electron microscopy (Fig. 4(1, 4)) confirms that it is in the initial stages of alteration. This is particularly evident in comparison with samples 40 (Fig. 4(2)) and 57 (Fig. 4(3)) where the 29 A phase was completely formed. To test this hypothesis, two samples (146 and 40) and a vermiculite from Pike Lake, Perth, Canada, were exchanged with Ca and Mg and sub-

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FIG. 5. Effectof Ca and Mg in interlayersites on the thermal behaviourof'corrensite', saponite and vermiculite.

M. F. Brigatti and L. Poppi

64

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sequently analysed. The X-ray patterns were not significantly different but their thermal behaviour (Fig. 5) confirms our hypothesis. CHEMICAL

DATA

Chemical data for the samples are given in Table 1. Amounts of AI203, SiO 2 and MgO vary quite markedly. The Fe203 contents are fairly constant except for sample 45 which contains large amounts of iron oxyhydroxides. DISCUSSION

AND

CONCLUSIONS

Chlorite-smectite interlayer minerals are widely distributed in alteration products of ophiolitic rocks of the Northern Apennines. Often they are difficult to identify because they are present in small amounts, or they may be confused with chlorite and/or saponite or, perhaps, vermiculite: in fact, often the basal reflection at ~30 A is not seen and then only the glycerol solvation and/or thermal treatments indicate the presence of interlayer mineral. The occurrence in the area of discrete smectites and chlorites suggests that the interlayer mineral may be related genetically to these minerals. The materials studied show variations in A1/Mg ratio (Table 1) in accordance with a chlorite --} saponite evolution process with interlayer phases as intermediate stages. The original material is composed essentially of olivine, serpentine and pyroxenes (enstatite and diallage) (Beccaluva & Venturelli, 1973, 1976). During the alteration process these materials supply the solutions with the A1, Fe and Mg that are subsequently used to form the clay minerals. The question is whether this alteration can be linked to hydrothermal or weathering processes. According to Gallitelli (1956) and Alietti (1959), alteration was most likely hydrothermal, although weathering-like processes more sensitive to chemical factors than temperature could not be excluded. The alteration sequence chlorite --, chlorite-vermiculite -~ vermiculite has been well-described in the literature (Millot & Camez, 1963) and the environments of formation of vermiculite, as reviewed by Basset (1963), are very similar to those of chlorite-smectite interlayer minerals. It is interesting to note that vermiculite and the interlayer mineral can be found in carbonate and evaporitic sequences where large amounts of Mg are present. The role of Mg is clearly shown by the features of chlorite found near Borgotaro (sample 41). The presence of this phase leads us to believe that the genetic process can be due to meteoric alteration. Lastly, it may be noted that the response of the Ca- and Mg-exchanged samples to heating, as measured by DTG, enabled us to distinguish saponite from vermiculite. Using this technique it might also be possible to characterize the chlorite-smectite interlayer mineral on the basis of properties of the interlayer sites of the swelling component. Using this criterion, the interlayer mineral studied here (sample 40) was defined as a chlorite-saponite. ACKNOWLEDGMENTS Thanks are due to Prof, J. ,[. Fripiat for constructive comments on this paper. The interest of Prof. A. Alietti in this work is also appreciated. The authors are indebted to the Istituto de Mineralogiadell'Universitfidi Ferrara for the use of the X-ray fluorescence equipment and to Mr W. Lugli for drawing the figures. This work was supported by the ConsiglioNazionale delle ricerche of Italy, the Centro di Calcolo and the Centro Strumenti of Modena University.

66

M . F. B r i g a t t i a n d L . P o p p i REFERENCES

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