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0360-1323/$ - se

doi:10.1016/j.bu

�Correspondfax: +3902 503

E-mail add

alessandro.pave

(A. Riva), anal

(T. Cerulli).

Building and Environment 42 (2007) 68–77

www.elsevier.com/locate/buildenv

Chromatic weathering of black limestone quarried in Varenna(Lake Como, Italy)

Nicoletta Marinonia,�, Alessandro Pavesea,b, Angelo Rivac, Fiorenza Cellad,Tiziano Cerullid

aDipartimento di Scienze della Terra ‘‘Ardito Desio’’, Universita degli Studi di Milano, Via Botticelli 23, 20133, Milano, ItalybIstituto per la Dinamica dei Processi Ambientali, Sede di Milano, Via Mario Bianco 9, 20131 Milano, Italy

cEni S.p.A, Agip Division, Via Maritano 26, 20097 San Donato Milanese, Milano, ItalydMapei S.p.A., Ricerca e Sviluppo, Via Cafiero 22 20158 Milano, Italy

Received 16 August 2004; received in revised form 23 May 2005; accepted 25 July 2005

Abstract

The causes of chromatic weathering in black limestone have been investigated on samples from the main quarry of Varenna (Lake

Como, Italy), which provided building stones for Lombard architecture. Our studies have been carried out combing colorimetric,

mineralogical, chemical, microstructural and geochemical observations. The results suggest that the chromatic weathering affecting

the surfaces of black limestone is not due to oxidation of the original organic matter present in the bulk, as believed. Chemical and

mechanical weathering associated with the deposition of new organic matter are the most important phenomena occurring on the

surface, leading to a diffuse roughness of surface and de-cohesion.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Black limestone; Chromatic weathering; Inorganic fraction; Organic fraction; Chemical weathering; Mechanical weathering

1. Introduction

The use of local stones for architectural purposes hasbeen a common practice throughout the centuries inItaly. The remarkable availability of workable calcar-eous rocks in Northern Italy encouraged the use of thesematerials as structural and ornamental elements. Inparticular, black limestones were extensively used asbuilding stones, owing to their vivid colour. Forinstance, they were employed as profile of the bas-reliefson the Certosa church fac-ade (Pavia, Italy), as well asveneering on the fac-ade of the Colleoni Chapel

e front matter r 2005 Elsevier Ltd. All rights reserved.

ildenv.2005.07.028

ing author. Tel.: +3902 503 155 75;

155 97.

resses: nicoletta.marinoni@unimi.it (N. Marinoni),

se@unimi.it (A. Pavese), angelo.riva@agip.it

ysis.lab@mapei.it (F. Cella), analysis.lab@mapei.it

(Bergamo, Italy) together with white marble and redlimestone (Fig. 1) [1,2].

Black limestone are well bedded grey-dark micriticlimestone relatively rich in carbonaceous and bitumi-nous organic matter, which is supposed to be respon-sible for the dark colour of the stone [3,4]. Outcrops ofblack limestone are widespread in Mesozoic rocks ofSouthern Alps from Lake Como to Lake Garda(Northern Italy). These rocks display common macro-scopic features. Therefore, it is not easy to determine thequarry of provenance for each lithotype [5].

Black limestone often undergo chemical weatheringwhich is chiefly promoted by the action of air pollutantssuch as SO2, NOx, H2SO4, HNO3 and O3, and ofparticulates. Such pollutants are transferred to buildingsurfaces by wet and dry deposition leading to sulphationof Ca-bearing materials [6].

The black limestone displays a chromatic weathering,commonly observed on building fac-ades and quarries,

ARTICLE IN PRESSN. Marinoni et al. / Building and Environment 42 (2007) 68–77 69

which produces a change of the surface colourfrom a vivid black to a fading grey hue. Historicalrecords emphasised that the discoloration was attri-butable to oxidation of organic matter in stones [7,8].In particular, aromatic hydrocarbons were supposedto be the least stable carbon pigment which wereresponsible of the dark colour of sediments andproduced the expected colour change upon weathering[3,9].

Notwithstanding the importance of chromatic weath-ering in black limestone, the origin of this deteriorationis still an open debate. In the present research a detailedcharacterisation of black limestone surface, in termsof inorganic and organic constituents, was performedto shed light on the mechanisms responsible ofdiscoloration.

Fig. 1. (a) Photograph of the Certosa di Pavia (Italy) main fac-a

Fig. 2. (a) Ancient quarry in Varenna (Lake of Como, Italy), (b) photograp

core and (c) photograph of the black limestone outcrops in Varenna, showi

2. Experimental

2.1. Sampling

Six black limestone samples, known as Nero di

Varenna, were collected from the main quarry inVarenna (Fig. 2a) (Lake Como), which had a primaryrole in the past centuries in supplying decorativebuilding materials for Lombard Architecture [10].

The collected samples consisted of well–bedded grey-dark micritic limestone subdivided by thin sheets ofmarls and dark clays [11]. These rocks belonged to thePerledo Varenna Formation (Middle Triassic) andrepresented the platform/intra-platform basin facies ofpoorly oxic to anoxic environment, developed duringthe Ladinian in the West Southern Alps [11,12].

des and (b) detail of a black limestone decorative frame.

h of bulk black limestone displaying a deeply black colour in its inner

ng the chromatic weathering on sample surfaces.

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Each sample showed a vivid black colour in itsinner core whereas visual signs of weathering wereapparent on the exposed surface, in particular lighte-ning in colour from black to grey (Figs. 2b and c).Samples representative of the weathering layers wereobtained scraping the grey black limestone surfaces by ablade.

2.2. Analytical techniques

2.2.1. Inorganic fraction

The chromatic properties of the black limestonesamples and their weathering layers were measured bya MINOLTA CM 2002 colorimeter equipped with aXenon Lamp as a source, viewing angle of 101 andillumination area of 14mm diameter [13].

A full characterisation of the black limestone wasobtained by combining mineralogical, chemical andmicrostructural analyses.

A qualitative study of the black limestone surface wasperformed using Grazing X-ray Diffraction (GXRD),which is one of the most sensitive techniques toinvestigate surfaces [14]. A steady incidence X-ray beamstrikes the surface of a sample at an angle ac rangingtypically from 11 to 101; the X-ray penetration (l) in thesample is calculated by [15]:

l ¼l

4pac,

where l is the X-ray wavelength. Note that the smallerac, the deeper is the penetration of the radiation in thesample. In our case, we used an angle ac of 11 and 31,which allows to investigate the first 5 and 10 mm of thesample surface, respectively. Data collections werecarried out by an X’PERT PHILIPS PRO PW 3040Diffractometer, using a CuKa radiation in the angularrange 5–701, 2y step of 0.011 and 10 s/step, divergenceand receiving slits of 0.031. We adopted an experimentalset-up that is a compromise between high intensity,resolution, and minimisation of the geometrical aberra-tions.

Conventional X-ray powder diffraction patterns werecollected by a laboratory Bragg-Brentano parafocussingX’PERT PHILIPS Diffractometer (X-ray incident beamwith the wavelength of CuKa) to quantify the mineralphases occurring in the bulk limestone samples andweathering layers. The Rietveld method was used toanalyse the diffraction patterns, by means of thesoftware GSAS [16,17].

Morphological and elemental characterisation ofblack limestone were carried out by CAMBRIDGEStereoscan 360 scanning electron microscope equippedwith a Link Pentofet Energy Dispersive spectrometer(SEM-EDS).

2.2.2. Organic fraction

A detailed characterisation of the black limestoneorganic matter was performed.

The presence of a dark-coloured organic matter richlayers witness periods of time when conditions fororganic matter accumulation in sediments were particu-larly favourable. Therefore, the organic matter in theblack limestone can be considered the residue ofphysicochemical and biochemical degradation of abiogenic organic matter [18]. Note that organic matterin sedimentary rocks is commonly classified accordingto its solubility against organic solvents: (1) ‘‘bitumen’’,consisting of unsaturated, saturated, aromatic andheteroatomic hydrocarbons, soluble by organic solvents;and (2) ‘‘kerogen’’, referring to the insoluble organicfraction [9,19].

In the present research, the collected samples werefirstly scrubbed and cleaned with distilled water toremove any trace of superficial dirt and plant growth.

The determination of the organic matter weightpercentage, i.e. the total organic carbon (TOC) in theblack limestone samples was performed by a LECO CS225 carbon analyser. A 100mg ground rock sample wastreated with 20% HCl to dissolve carbonate minerals;the residue was then washed with distilled water toremove all traces of HCl and water-soluble chlorides.The TOC content was measured on the washed residueby a high-temperature combustion, at a heating rate of10 1C/min under an oxygen atmosphere, over thetemperature range 30–1400 1C.

A further characterisation of the organic matter wasperformed by a ROCK-EVAL II Instrument, equippedwith a mass analyser. Rocks (100mg) were pyrolised in aHe atmosphere for 3min at 300 1C, and then atincreasing temperatures (25 1C/min) up to 550 1C [20].This technique allows to measure the amount of light(Co40) and heavy (C440) hydrocarbons, and ofcarbon dioxide released during heating [20]. The ratioof the amount of hydrocarbons generated duringpyrolysis over TOC, yields the Hydrogen Index (HI)(mg(HC)/g TOC), providing information about theabundance of hydrogen rich organic matter. Theamount of pyrolytic CO2 normalised to TOC gives, inturn, the Oxygen Index (OI) (mg (HC)/g TOC), which isrelated to the content of oxygen rich organic matter. TheHI and OI indexes contribute to understanding thenature of the organic matter, i.e. its biological originand degree of microbiologic reworking or abioticoxidation [21].

The organic matter maturity was estimated by meansof Vitrinite Reflectance (RI), by a LEITZ ORTOPLANMPV III Microscope system (reflected light, oil immer-sion) following the standard procedure suggested byDurand [19].

Raman spectroscopy was employed to investigatethe presence of carbonaceous material in the black

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limestone samples, by REINISHAW-1000 LMR spec-trophotometer, equipped with a He–Ne laser (632.8 nm)and a resolution of 4/cm. The energy range from 1000 to3500/cm was explored, with particular care for thespectral intervals of the first- and second-order Ramanbands of carbonaceous materials, occurring at1200–1700 and 2350–3350/cm, respectively.

FTIR-spectroscopy was used by means of a NICO-LET NEXUS 670/870 spectrophotometer in the mid-infrared region (4000–400/cm). The spectra of thesamples on study were recorded at 4/cm resolution, intransmission mode, and compared with laboratoryreference spectra and search libraries provided by theOmic 6.0a software.

3. Results and discussion

3.1. Black limestone inorganic fraction

The appreciable colour changes between the weath-ering layers and the bulk black limestone, detected by

Table 1

Values of chromatic parameters (L*, a*, b* and C*) and total colour differe

Sample no. L* a* b* C*

BL1 26.33 0.23 0.09 0.25

WL1 40.44 �0.73 0.47 0.86

BL2 32.00 0.27 0.48 0.55

WL2 44.57 �0.85 �1.07 1.37

Assignment: BL ¼ bulk black limestone; WL ¼ weathering layer.

coun

tsco

unts

1600140012001000800600400200

010 20 30

10 20 30

2°

2°

2500

2000

1500

1000

500

0

Glancing angle 1°

Glancing angle 3°

d

d

c

c

Fig. 3. Thin slab black limestone diffraction pattern collected (a) at 11 inciden

d ¼ dolomite.

colour measurements, are reported in Table 1. Inparticular, the total colour change, DE*, is mainly dueto the lightness variation, DL*, since the differences ofthe chromatic co-ordinates, Da* and Db*, are almostnegligible. The positive DL* values show the tendencytowards brightening, leading to a chroma (DC*) increasewith less vivid and pure colours.

Fig. 3 displays the diffraction patterns collected ata ¼ 11 and a ¼ 31 by GXRD, on a 3� 2� 1 cm3 blacklimestone thin slab bearing signs of surface weathering.The intensity of the peaks due to dolomite increasessignificantly from the pattern collected at a ¼ 11 to theone at a ¼ 31, and reveals a larger content of dolomiteon approaching the surface.

A detailed characterisation of the mineralogicalphases constituting the bulk black limestone shows thatthey are chiefly composed of calcite, associated todolomite and quartz (Fig. 4). The X-ray diffraction ofthe HCl residue insoluble reveals the presence offeldspar, plagioclase, pyrite and sometime kaoliniteand mica as minor components. The latter is presumablya consequence of the anoxic environment where the

nces (DL*, Da*, Db*, DC* and DE*) in the collected samples

DL* Da* Db* DC* DE*

14.11 �0.96 0.38 0.61 14.15

12.57 �1.12 �1.55 0.82 12.71

40 50 60

40 50 60 70

Theta

Theta

ce angle and (b) at 31 incidence angle. Assignment refers to. c ¼ calcite,

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C

B

A

c

dq

g m f p

py

3500

3000

2500

2000

1500

1000

500

010 20 30 40 50

coun

ts/s

2° Theta

Fig. 4. X-ray diffraction profile of: (A) bulk black limestone, (B) the residue insoluble of bulk black limestone after HCl treatment and (C)

weathering layer. Assignment refers to. c ¼ calcite, d ¼ dolomite; q ¼ quartz; py ¼ pyrite; f ¼ feldspar; p ¼ plagioclase; g ¼ gypsum;

m ¼ mountkeithite.

Table 2

Results of the Quantitative X-ray Diffraction Analysis by Rietveld

method in the collected samples

Mineral phase weight

(wt.%, sigma)

BL1 WL1

Calcite 92.7 (0.43E�3) 63.2 (0.43E�3)

Dolomite 6.1 (0.39E�2) 24.6 (0.43E�3)

Quartz 1.2 (0.16E�2) 0.7 (0.01E�2)

Gypsum — 1.1 (0.13E�2)

Mountkeithite — 1 (0.07E�2)

Assignment: see Table 1.

N. Marinoni et al. / Building and Environment 42 (2007) 68–7772

black limestones were deposited. In the weatheringlayers, carbonates and quartz are associated to minorphases, such as gypsum and sometime clays. Claysmight be interpreted as the result of the hydrolysis ofsilicate minerals, in particular feldspars, whereas thepresence of authigenic gypsum is ascribable to asecondary crystallisation by wet or dry deposition [22].Occurrence of an unknown phase, with d spacings of11.44 and 5.66 A, and a high solubility in hydrochloricacid, was observed (Fig. 4). The identification of thismineralogical phase is still not clear; nevertheless, such acompound has an X-ray diffraction pattern similar tothat of mountkeithite, a Magnesium Iron CarbonateSulphate Hydroxide Hydrate [23].

A higher percentage of dolomite was measured in theweathering layers (average value 25wt%) as regards thebulk black limestone (average value 6wt%) (Table 2).The larger amount of dolomite close to the surface hasto be ascribed to the lower solubility of dolomite thancalcite, 14mg/l vs. 90mg/l at T ¼ 25 1C respectively [3].Therefore, dolomite remains as a residual phase in the

weathering layers when the dissolution attack by rain-water occurred on carbonate stone [22]. The occurrenceof dissolution phenomena on the bulk black limestonesurface was proved by microstructural investigationswhere calcite crystals with cleavage edges and dissolu-tion figures owing to rill-wash were observed (Fig. 5).

The carbonate phases of weathering layers and bulkblack limestone were compared with one another interms of crystallographic properties. In particular, thelattice parameters of calcite and dolomite were mea-sured by XRD, using Si-NBS as an internal standard.The results document homogeneity (Table 3) and EDXmicroanalyses prove that no important chemical sub-stitutions have occurred in the dolomite and calcitestructures. These observations suggest that the carbo-nates, both in the weathering layers and in the bulkblack limestone, have similar origin. Indeed, theyconfirm that the differences in terms of phase composi-tion between surface and bulk are mainly due todissolution phenomena.

The ESEM micrographs reveal considerable micro-structural changes between the grey surface and theinner core of the black limestone. The surfaces are roughwith a discontinuous geometry, characterised by asystem of intra- and inter-crystalline microfractures inthe first 20–30 mm, leading to a high porosity (Figs. 6aand b). The black limestone surface is chiefly composedof an inter-growth of micritic calcite and dolomite,whereas the bulk exhibits a higher percentage of micriticcalcite associated to dolomite crystals up to 10 mm inlength. Indeed, the black limestone inner core exhibits astructure with small pores (no larger than a fewmicrons), suggesting a complete re-crystallisation owingto packing during diagenesis (Fig. 6c). Pyrite crystals do

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Fig. 5. SEM micrographs: (a) black limestone surface deterioration

caused by rill-wash, and (b) dissolution figures on the twining and

cleavage planes in calcite crystal.

Table 3

X-ray structure refinements of calcite and dolomite in the collected

samples

Mineral phase a ¼ b(A) c(A) Sigma

Calcite* 4.9896 17.061

Calcite** 4.994 17.081

Calcite (BL1) 4.986 17.048 0.001

Calcite (WL1) 4.984 17.044 0.001

Dolomite* 4.803 15.98

Dolomite** 4.840 16.210

Dolomite (BL1) 4.828 16.116 0.002

Dolomite (WL1) 4.829 16.136 0.003

Calcite* ¼ Ca 0.09Mg0.01 CO3.

Calcite** ¼ Calcite sintetica.

Dolomite* ¼ Ca1.00 [Mg0.99Ca0.01] (CO3)2.

Dolomite** ¼ Ankerite ¼ Ca1.00 [Mg0.27Fe0.63Ca0.05 Mn0.05](CO3)2.

Assignment: see Table 1.

N. Marinoni et al. / Building and Environment 42 (2007) 68–77 73

not occur in the surface, whereas they reveal micritic andcubic habit in the bulk.

The reasons of the granular de-cohesion on the blacklimestone surface are not yet completely clear, and maybe ascribed to several causes. The mechanical weath-ering, in particular thermal cracking of calcite crystals,can play a key role in the development of the surfacemicrofractures. Thermal cracking is related to the highlyanisotropic thermal expansion of calcite [24,25]. It iswell known that the thermal expansion coefficients of a

and c lattice parameters range from �5.7� 10�6/1C to�3.7� 10�6/1C and from 25.6� 10�6/1C to 26.5� 10�6/1C, respectively [26]. Some authors [3,27] assert that thethermal anisotropy leads to a weakening of the rockcohesion increasing the porosity of its surface. More-

over, they confirm the thermal stress, resulting fromchanges in temperature of ambient air, could producemicrofractures among the mineral grains of a rock. Thismeans that the inter-granular de-cohesion starts atabout 40–60 1C [28] (a temperature range which is easilyreached on a stone surface during summer) leading to anirreversible increase in length of the sample, that is theonset of the thermal cracking [29]. At lower tempera-tures, the dilatation of a carbonate stone is mainlyaffected by the texture of the rock [28].

3.2. Black limestone organic fraction

We start discussing the results concerning the organicmatter in the bulk black limestone samples. The TOCcontents of the black limestone range from 0.24 to0.09wt.% (Table 4). Neither the light (Co40) and heavy(C440) hydrocarbons nor organic oxygen-bearingcompounds were revealed. Therefore, the organic matterdoes not contain bitumen, but it is mainly composed ofkerogen. This inert organic fraction is not able togenerate hydrocarbons, and can be considered the end-member of the hydrocarbon generative sequence [30].

Additional analyses have been carried out to char-acterise kerogen. In particular, a chemical method for itsisolation, commonly adopted in petroleum geochemistrywas used [19]. In particular, a selective dissolution byHCl and HF of the sedimentary rocks to eliminate theinorganic fraction (silicates, oxides, carbonates, etc) wasperformed. The obtained residue appears as an amor-phous material, with a colour ranging from dark brownto coal dust-like jetty black.

A study of the kerogen by optical microscope inreflected light attests that the vitrinite reflectance alwaysexceeds 0.45%, suggesting a high thermal maturity ofthe organic matter in the black limestone, consistentwith the results from pyrolysis.

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Fig. 6. SEM micrograph of black limestone transversal section: (a) highly fractured black limestone surface, (b) grain boundaries cracks in calcite

crystal (magnification of an area of the previous image (a)), and (c) inner part of the black limestone, highlighting a low porosity and a compact

structure due to packing.

Table 4

TOC and Rock Eval data measured in the collected samples

Sample No. TOC S1 (mg/g) S2 (mg/g) S3 (mg/g) HI OI

BL1 0.24 — — — — —

BL2 0.13 — — — — —

BL3 0.09 — — — — —

WL1 0.38 0.16 0.48 4.61 126 1213

WL2 0.59 0.90 2.61 4.12 442 698

WL3 0.72 0.94 2.51 5.26 349 731

Assignment: see Table 1.

N. Marinoni et al. / Building and Environment 42 (2007) 68–7774

Furthermore, the high maturity of the organic matterhas been confirmed by FTIR measurements, which donot exhibit absorption bands attributable to organicmoieties, i.e. aromatic CH and residual aliphatic groups[19]. Raman experiments evidence two peaks at1597–1289/cm due to the first-ordered band (G-band)and to the disordered-induced first-ordered band (D-band) respectively, attributable to a carbonaceousmatter with a structural model like disordered graphite

[31,32]. In order to attest the degree of crystallinity ofthe carbonaceous organic matter, the kerogen wassubjected to a graphitisation process, i.e., it was heatedin an inert atmosphere to increase the crystallinity gradein the graphite structure [32–34]. The X-ray diffractionanalysis was performed by X-ray diffractometerequipped with a high-temperature chamber where, inan inert argon atmosphere, the sample reached thetemperature of 9001. The X-ray diffraction patterns at

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250

200

150

100

50

025 26 27 28 29 30

T = 25° C

T = 900 °C

graphite(002)

2° Theta

coun

ts/s

250

200

150

100

50

0

coun

ts/s

Fig. 7. X-ray diffraction pattern of the kerogen collected (a) at room temperature (25 1C) and (b) after heat treatment at 900 1C for 10 h.

N. Marinoni et al. / Building and Environment 42 (2007) 68–77 75

room temperature have proven that kerogen is mainlycomposed of neo-formed fluoride minerals (as a result ofthe HF treatment), associated to a low crystallinitypyrite. In the patterns collected at 900 1C, the presenceof an ill-defined and asymmetrical peak in the range26–271 2y ascribable to the reflection 0 0 2 of graphite[32] suggests that the graphitisation process hasoccurred and a partially ordered graphite has formed(Fig. 7).

In the weathering layers the TOC contents is higher asregards the bulk black limestone, and ranges from 0.38to 0.72wt.% (Table 4). The HI and OI indexes havevalues ranging from 126 to 442 (mg HC/TOC), andfrom 698 to 1213 (mg OI/TOC) (Table 4). If HI and OIare plotted in a standard HI–OI discrimination diagram[35,36], the result is consistent with a lipid- and protein-rich organic matter, i.e. the common major constituentsof plant microorganisms. The presence of the bio-activity on the sample surface was proved by scanningelectron microscopy observations, which evidencedtraces of lichen activity and roots of plant microorgan-isms on the black limestone surface (Fig. 8).

4. Conclusions

The chromatic weathering of black limestone quarriedin Varenna (Lake Como, Italy) was investigated. Sixsamples were characterised combining colorimetric,mineralogical, chemical, microstructural and geochem-ical observations.

The results can be summarised as follows:

Fig. 8. SEM micrograph attesting the bio-activity on black limestone

� surface.

as regards the inorganic fraction: (i) the minera-logical differences between bulk black limestone and

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weathering layer reveal that chemical reactions on thesurface samples have taken place, although the new-formed minerals occur in relatively low concentra-tions; (ii) the ESEM micropictures attest thatmicrostructural changes, such as solution loss, etch-ing of crystals, de-cohesion and high porosity arepresent on the outer layer of the samples, suggestingthat thermal weathering may have played a key rolein black limestone discoloration;

� as regards the organic fraction: the bulk black

limestones are composed of a carbonaceous highlyoxidised and inert organic matter; whereas, close tothe surface of the samples, the organic matter ismainly ascribable to microorganism activity.

As a whole, our results do not provide evidences toattribute the black limestone chromatic weathering tothe oxidation processes of the original organic matter.The surfaces of the black limestone are characterised bythe occurrence of gypsum, a high amount of dolomiteand organic matter from bio-activity. Indeed, micro-cracks and calcite dissolution due to a synergic action ofchemical and mechanical weathering are observed,leading to a general roughness and de-cohesion of theblack limestone surface and causing rather diffusion oflight than reflection.

Acknowledgements

We are grateful to Dr. Monica Dapiaggi for the X-raydiffraction support; a special thanks to Dr. DavideSalvioni (Mapei S.p.A) for the ESEM analysis and Dr.Fiorenza Cella. We acknowledge Dr. Roberto Buginifor useful discussion.

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