chromatic modification affecting cretaceous sandstones in...

DRAFT

12th International Congress on the Deterioration and Conservation of Stone

Columbia University, New York, 2012

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CHROMATIC MODIFICATION AFFECTING CRETACEOUS SANDSTONES

IN NORTHERN ITALY AND SOUTHERN SWITZERLAND

G. Cavallo,1* G. Corredig1 and G. Vola2

1 Institute of Materials and Constructions DACD-SUPSI, Switzerland

2 Geologist, Italy

*corresponding author [email protected]

Abstract

The Cretaceous sandstones investigated in this study belong to the Lombard Flysch

Group (Pontida Formation, Sarnico Formation, Piano di Sirone, Bergamo Flysch) and

are mainly exposed in Brianza and around Bergamo (Lombardy, Northern Italy),

subordinately in Southern Switzerland around Mendrisio. These rocks have been used in

the past and still in the present time as building materials.

Both ashlars in historical buildings and samples collected from quarries or outcrops

exhibit a characteristic chromatic modification of the bulk colour of the rock ranging

from red-orange to brown which is not limited to the surface.

Polarizing Light Microscopy (PLM) integrated with Scanning Electron Microscopy coupled with microanalysis and elemental X-Ray mapping (SEM/EDS)

allowed to infer that the Fe oxides and/or oxi-hydroxides are related to alteration

processes affecting Fe-rich micas such as biotite forming part of the detrital fraction; a

few samples exhibit alteration of K-rich micas such as glauconite. Furthermore, Fe-

based compounds are associated with Mg,Ca-carbonates of the detrital fraction. Pyrite is

also present as individual crystals sometimes associated with organic matter. The Fe-

oxides are also concentrated around the intergranular sparry calcite cement.

In situ observations both in quarries and monuments combined with analytical data

indicate that the chromatic modification is mainly related to the diagenetic environment

and can be considered intrinsic to the rocks mainly due to biotite oxidation. The reddish-

brownish colour is not an alteration process but the result of geochemical modifications

of the original detritus occurring during the eogenetic phase of diagenesis.

Keywords: Lombard Flysch Group, Cretaceous sandstone, Chromatic modification,

Biotite alteration, Diagenetic processes.

1. Introduction

A research developed in the frame of the Interreg Project between Italy and

Switzerland entitled The stone and the history. Safeguard of the landscape and

architecture between Lecco (Northern Italy) and Canton Ticino (Southern Switzerland)

(http://www.pietraestoria.eu), co-funded by the EC, was aimed at a detailed study of the

Cretaceous sandstones extensively used as building material in this area. Observations

carried out on historical building surfaces revealed a chromatic modification of the original greyish-greenish rock color into reddish-brownish, initially attributed to

chromatic alteration. Visits to the quarries, especially those close to the city of Bergamo

(Italy), showed the presence of grey sandstone with nodules and planar surfaces reddish

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in color. These preliminary observations suggested the causes of color change were

intrinsic to the rock.

The ICOMOS-ISCS illustrated glossary (Vergès-Belmin V., 2008) reports the term

iron-rich patina for describing the chromatic modification even if it refers to thin layers,

also as a result of longer exposure to open air. Discoloration (chromatic alteration) is

more general and is reported as a change of the rock color. Both the patterns are reported

as alteration phenomena.

The subject was not extensively treated until now; a few authors advocated

biological causes as responsible for superficial alteration (Krumbein, 1992; Valls del Barrio et al., 2002). Our research was intended as an attempt to clarify the role of Fe-

bearing minerals in causing the reddish-brownish coloration, the significance of the

diagenetic environment and the role of syn and post-depositional processes.

2. Geological and stratigraphic setting

The discontinuous outcrops of the so-called “Lombard Flysch Group”, which

includes Pontida Formation, Sarnico Formation, Sirone Conglomerate and Bergamo

Flysch, is approximately E-W oriented, 85 km wide, extending along the Southalpine

foothills between Brianza and Iseo Lake, northern Italy (Bersezio et al., 1990). The area

covers almost five different Italian provinces, Varese, Como, Lecco, Bergamo and

Brescia, including the southern part of Canton Ticino, southern Switzerland. The most important outcrops and quarries, most no longer active, are located in the Bergamo area,

between Mapello and Sarnico, and subordinately in Brianza, between Oggiono and

Viganò (Bigioggero et al., 2000; Bugini et al., 2004; Cavallo and Corredig, 2011).

Lombard Flysch, Upper Cretaceous in age, was described in details by Venzo (1954)

and reported in the geological map of Italy at the scale of 1:100.000. Subsequently, the

stratigraphic information was improved by various authors (Fernandez, 1963; Aubouin

et al., 1970; Bichsel and Häring, 1981; Gelati et al., 1982; Bersezio et al., 1990).

Recently, the same formations were illustrated on various Italian geological maps at

scale 1:50.000 (Bersezio et al., 1990; Jadoul et al., 2002; Bersezio et al., 2010; Bini et

al., 2010; Gaetani et al., 2010). The Pontida Formation (Middle Turonian - Turonian) is

composed of hybrid lithic sandstones, mostly light brown to brown in colour, in beds of

highly variable thickness organized in Bouma sequences, alternating with marls. The Sarnico Sandstone (Coniacian) presents alternating thin to medium layered grey

sandstones and shales, sometimes organized in coarsening-upward cycles, with

structures typical of the Bouma Sequence. The Sirone Conglomerate (Santonian)

presents massive conglomerates with clasts of centimetric to decimetric size and less

common conglomeratic sandstones in lenticular bodies. The Bergamo Flysch

(Campanian - Middle Maastrichtian) is mostly composed of alternating sandstones and

shales, thin to thick-bedded. These deposits of Late Cretaceous turbidite systems are

involved in the neo-Alpine south-vergent fold and thrust belt.

2.1 Quarries location, exploitation and use The formations of the Lombard Flysch Group were widely quarried all over the

Southalpine foothills for many centuries. The Sarnico Sandstone, also called “Sarnico

Stone” in the Province of Bergamo and “Oggiono Stone” in the Province of Lecco, was

already used as building material since the 11th-12th centuries (Figus, 2011); evidence of

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early uses dates back even to the Lombard period (Bigioggero et al., 2000). Today only

three quarries are active, two in Paratico (Brescia) and, a new one, in Gandosso,

Bergamo (Regione Lombardia, 2008). Almost the same lithological unit, called “Molera

Stone” in Brianza, and “Cote Stone” in the Seriana Valley, was quarried and used for

the production of millstones, which were traded for many centuries all over the world

(Valoti, 2010). The Sirone Conglomerate was used as millstone, as well (Bergamaschi et

al., 2003). The Bergamo Flysch, under the name of “Credaro Stone”, is exploited in

Credaro and Castelli Calepio (Bergamo) and used as building material, generally for

rustic ashlars (Regione Lombardia, 2008). Recently two different lithofacies are available on the market, called respectively “Medolo” and “Berrettino” types (Bettoni et

al., 1997). The sandstone called “Molera di Viganò” which is also part of Bergamo

Flysch was also quarried and used in the Brianza area near the villages of Missaglia,

Viganò and Montevecchia (Province of Lecco) as building material (Bugini et al., 2004).

In southern Switzerland the use of sandstones referable to the Pontida Flysch

Formation is limited in and around Mendrisio (Zala, 2010).

3. Materials and methods

Samples were collected from monuments and quarries; the locations are reported in

the Table 1. A large number of samples comes from monuments and historical buildings

located in Mendrisio (Canton Ticino, Switzerland) whilst the quarry samples come mainly from the Italian side as the outcrops in Canton Ticino are limited per se, hardly

accessible or completely hidden by the structures and infrastructures built in the last

centuries.

Table 1. List of the samples.

Samples Location

CSM 01-04-05-06-07 Historical buildings, Mendrisio (Switzerland)

SMB 03-07-08-10-12-14-17-18 12

th century bell tower, Santa Maria del Borgo

church, Mendrisio (Switzerland)

VS 07-10

Early 20th century building, Villa Sironi, Oggiono

(Italy)

FCL Artificial excavation (Switzerland)

FLY Outcrop (Switzerland)

CE 01 Molera quarry, Oggiono (Italy)

GAN 01-04 Gandosso quarry (Italy)

SA 01-04 Sarnico quarry (Italy)

ME 02 Merate quarry (Italy)

VIG 02-03 Viganò quarry (Italy)

Microscopic techniques were used, also according to the suggestions by Vazquez-

Calvo et al. (2007).

Polarizing light microscopy (PLM) was carried out on polished thin sections in

order to study the mineralogical paragenesis, the texture and the structure of the samples.

Petrographic examinations were combined with elemental analysis and X-ray mapping

by means of an electron microscope coupled with microanalysis (SEM/EDS). A Jeol

JSM-5910LV was used for BSE images and X-rays maps, operating at the following

conditions: 20 kV, vacuum mode HV, working distance 9 mm, spot size 43.

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4. Results

4.1 Petrography

All of the analyzed samples exhibit a similar mineralogical composition (Table 2)

including, in order of decreasing abundance, quartz and fragments of quartzs-feldspatic

rocks (magmatic and metamorphic rocks); variable amounts of chert fragments, lithic

fragments of micritic-biomicritic-microsparitic and sparitic-microsparitic limestone (the presence of dolostone is not excluded); intraclasts, rare peloids, ooids and fossil

fragments; K-feldspar, mica (mainly muscovite and chlorite, biotite is often altered to

chlorite and associated with Fe-oxides) and opaque minerals (Fe-oxides and traces of

Fe-sulfides recognizable by their cubic crystal habit). Traces of plagioclase, glauconite,

zircon and garnet are present in some samples.

Samples referable to Pontida Flysch Formation (Bernoulli and Winkler, 1990;

Bersezio et al., 2010; Gaetani et al., 2010) labelled CSM, SMB and FCL (Table 1)

exhibit a low amount of chert fragments, K-feldspar is orthoclase and the fossils content

is represented by foraminifera, sometimes gastropoda, and fragments of bivalvia and

brachiopoda. Micas exhibit both a preferred orientation sub-parallel to the original

stratification and a random distribution. Layers showing black material as in the sample CSM 01 are most probably rich in organic matter.

Samples from the Sarnico Sandstone Formation (Bersezio et al., 2010; Bini et al.,

2010; Gaetani et al., 2010) labelled CE, VS, GAN, SA (Table 1) are characterized by a

high presence of generally fibrous chert and chalcedony fragments, often opaque and are

rich in organic matter; in addition to orthoclase, microcline was detected in subordinate

amounts. The fossil content is represented by foraminifera and brachiopoda. Micas,

mainly chlorite, subordinate muscovite and altered biotite, are rare or occur only in

traces. Micas are slightly more abundant in the samples from Oggiono (VS, CE). Traces

of tourmaline, glauconite, zircon and garnet are also present, especially in the samples

from Gandosso (GAN) and Sarnico (SA).

Samples from the Bergamo Flysch Formation (Bersezio et al., 2010; Bini et al.,

2010, Bugini et al., 2004) labelled ME and VIG (Table 1) exhibit a low content of micritic limestone; in addition to orthoclase, microcline is abundant. The fossil content

is represented by fragments of brachiopoda. Micas are very abundant (mainly muscovite,

biotite often altered to chlorite and associated with Fe-oxides). Micas exhibit both a

preferred orientation sub-parallel to the stratification of the rock and a random

distribution.

The texture of all the samples is grain supported; the frame consists of cement and

granules; the cement is mainly sparitic calcite with variable amounts of micritic calcite;

a siliceous fraction cannot be excluded. Particle size distribution expressed in the Udden

grade scale modified by Wentworth (1922) ranges between silt and very coarse sand,

sometimes passing to granules. Fe-oxides are often arranged around the edges of the

sparry calcite crystals of the cement suggesting the presence of ferroan calcite. Crystals of biotite which exhibit their peculiar optical features are present in all of

the samples in low or trace amounts but are generally altered to chlorite or, in the form

of altered crystals, associated with Fe-oxides of black or dark-brownish color.

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Table 2. Mineralogical composition of the sandstones (Mineral abbreviations, where

applicable, as in Siilvola and Schmid, 2007).

Sample Paragenesis

Qtz Kfs Pl Ms Bt* Chl Glt Tur Zrn Grt QR CR OM

CSM 01 +++ ++ - + tr + - - - - ++ ++ +

CSM 04 +++ + - ++ + ++ - - - - ++ ++ +

CSM 05 +++ + - ++ + ++ - - - - ++ ++ +

CSM 06 +++ ++ - ++ ++ + - - - - ++ ++ +

CSM 07 +++ ++ tr ++ tr ++ tr - tr tr ++ ++ +

SMB 03 +++ ++ - ++ + ++ - - - - ++ ++ +

SMB 07 +++ ++ - ++ tr ++ - - tr - ++ ++ +

SMB 08 +++ ++ - ++ + ++ - - - - ++ ++ +

SMB 10 +++ ++ - ++ + ++ - - tr - ++ ++ +

SMB 12 +++ + - + + + - - - - ++ ++ +

SMB 14 +++ ++ - + tr + - - - - ++ ++ +

SMB 17 +++ + - ++ + ++ tr - - - ++ + +

SMB 18 +++ + - ++ + ++ - - - - ++ ++ +

FCL +++ ++ + ++ tr ++ - tr tr - ++ ++ +

FLY ++ + - ++ tr + tr tr - - ++ +++ +

CE 01 +++ ++ + + + + tr - - - ++ ++ ++

VS 07 +++ + - + + tr tr - - - ++ + +

VS 10 ++ tr - + + tr tr - - - + +++ ++

GAN 01 +++ ++ + tr tr tr tr tr tr - ++ + +

GAN 04 +++ ++ + tr tr tr tr tr tr tr ++ ++ ++

SA 01 +++ ++ + + tr tr - - tr - ++ ++ ++

SA 04 +++ ++ + tr + + tr tr tr - ++ ++ ++

ME 02 +++ ++ + ++ + ++ - - tr tr ++ + ++

VIG 02 +++ ++ + ++ + tr - - tr - ++ + +

VIG 03 +++ ++ + ++ + tr tr tr tr - ++ ++ +

Legend

QR: Qtz-Fs rock fragments of igneous, metamorphic or sedimentary origin (microcrystalline silica such

as chert), CR: carbonate rock fragments (micritic limestones, sparitic limestones, sometimes dolostones,

intraclasts, peloids and rare fossil fragments), OM: opaque minerals.

+++ main component; ++ minor component; + accessory component; tr traces; - absent or not observed.

* Generally altered.

4.2 Microanalysis

Microanalysis carried out on significant areas and micro-areas selected after

optical microscopy observations allowed to determine the chemical composition of

opaque phases otherwise undetectable, the composition of mineral defined on the basis

of crystal morphology and habit when the optical properties were masked by alteration

processes, the distribution of iron bearing compounds and heavy minerals.

Opaque phases can be classified as organic and inorganic compounds. The former

are C-rich compounds exhibiting variable morphologies ranging from rounded to

elongate (Fig. 1, 1a). The elongate morphologies often exhibit a layered micro-structure

as consequence of the depositional conditions. The C-rich compounds are in some cases

associated with Fe,S-bearing compounds, i.e. pyrite. Ti and Fe-oxides, Zircon occur in traces; REEs were also detected in a few samples.

Micas often exhibit a deformed morphology but their recognition was possible on

the basis of typical relict structure when the optical characteristics were masked by the

alteration of bivalent Fe (Fig. 2). The chemical analysis carried out on a selected area

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(Fig. 2a) indicates that such micas are Mg,Fe-rich micas referable to biotite (see also the

Mg and Fe X-Ray mapping in Fig. 2b).

Fig. 1. BSE image of the sample FCL. Squared area

shows an elongated opaque phase. The bright spots

inside are Fe-based oxides.

Fig. 1a. X-Ray mapping of Carbon.

Fig. 2. Altered micas (arrows) in a sample from

SMB (PPL, width 0.8 mm).

Fig. 2a. BSE image corresponding approximately to

Fig. 2.

Fig. 2b. Elemental chemical analysis and X-Ray mapping corresponding to Fig. 2a.

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Optical examination of a few samples (Fig. 3) shows along the contact between

quartz grains and altered mica a sort of brownish halo suggesting the presence of a

coating along the edge of the quartz crystals. Under SEM/EDS (Fig. 3a, 3b) the contact

between Al,Fe,Mg,K-micas with quartz grains is sharp and is marked by a very thin

fissure. The lack of correspondence between the two types of observations is unclear.

Fig. 3. Sample ME-02 (PPL, width 0.8 mm). Arrow

shows the halo between quartz and mica.

Fig. 3a. Sample ME-02, BSE image.

Fig. 3b. Elemental chemical analysis and X-Ray mapping corresponding to Fig. 3a.

Fe-oxides often occur as isolated individual crystals even if, in a few cases, as in

the sample FCL, clusters of particles are clearly visible (Fig. 4).

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Fig. 4. BSE picture of the sample FCL showing cluster of Fe oxide crystals.

5. Discussion and conclusions

Mineralogical and chemical elemental analysis allow to infer clearly that the

orange-brownish color of the Cretaceous sandstones cannot be related with alteration

processes after their use but is connected with modification affecting some Fe-bearing

minerals after the deposition of the sediment. Naturally these modifications can be

enhanced when the material is exposed to decay processes causing the migration of

Fe(II) towards the surface (Smith et al., 2010) and the precipitation in the form of

Fe(III).

The chromatic modifications of the studied materials coming both from quarries and historical buildings can be ascribed to the following causes listed in order of their

evidence: alteration of Fe-rich micas (biotite-type) into the oxidized form; precipitation

of calcite cement in the presence of solution containing Fe(II); formation of Fe-oxides

and/or Fe-oxy-hydroxides in individual crystals.

The mentioned modifications characterize the early stage of diagenesis (eogenesis)

and the steps during and immediately after burial, dominated by oxidation and reduction

reactions, called redoxomorphic stage where the principal reactants involved were iron,

oxygen, sulfur and carbon (Dapples, 1967), and where reactions may also be mediated

by bacteria (Worden and Burley, 2003). During the early diagenetic stages the

oxidation of organic matter led to a diagenetic environment rather reducing. During the

burial stage diagenesis was most probably controlled by reducing conditions leading to the precipitation of ferroan calcite suggesting a negative Eh (Bernoulli, 2012).

In situ observations of natural outcrops, quarry fronts, ashlars in historical buildings

supported with optical and electronic microscopy coupled with microanalysis allowed to

clarify that the orange-reddish-brownish colour modification of the greyish-greenish

bulk colour of the Cretaceous sandstones from Lombard Flysch Group is principally a

consequence of the oxidation of the detrital biotite during diagenesis.

In addition, individual crystals of Fe-oxides, sometimes clearly referable to

haematite under the optical microscope and originating from the detritus and/or

precipitating after oxidation processes, are also responsible for localized red coloration.

Ferroan calcite constituting the cement is also responsible of the color modification.

Considerations related with conservation aspects arise as sometimes the colour

modification could be interpreted an alteration process due to organic and inorganic phenomena whereas it is just intrinsic the rock being a geochemical modification of the

original detritus in the diagenetic environment.

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Acknowledgments

We are grateful to EC for supporting the research in the frame of the Interreg

Programme A, ID 7618878.

Particular thanks go to Prof. Daniel Bernoulli for fruitful discussion and suggestions

on some aspects concerning our research.

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