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Black limestone used in Lombard architecture
Journal of Cultural Heritage 3 (2002) 241–249 www.elsevier.com/locate/culher
Black limestone used in Lombard architecture Nicoletta Marinoni a, Alessandro Pavese a,b,*, Roberto Bugini c, Giuseppe Di Silvestro d a
Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, 20133, Milan, Italy b National Research Council, IDPA, Section of Milan, Via Mangiagalli 34, 20133, Milan, Italy c National Research Council, Centro “Gino Bozza” per lo Studio delle Cause di Deperimento e dei Metodi di Conservazione delle Opere d’Arte - Piazza Leonardo da Vinci 32, 20133, Milan, Italy d Dipartimento di Chimica Organica e Industriale, Via Golgi 7, 20133, Milan, Italy
Abstract Black limestone samples from the quarries of Varenna (Lecco, I), Cene (Bergamo, I) and Riva di Solto (Bergamo, I) and widely used in Lombard architecture have been studied in terms of mineralogical, petrographic and chemical properties in order to provide a detailed characterisation and allow an unambiguous determination of their provenance. The inorganic and organic fractions have been separated from each other, and investigated using X-ray powder diffraction, atomic absorption, Hg-porosimetry, high performance liquid chromatography, gas chromatography, nuclear magnetic resonance and Fourier transform infrared spectroscopy. The occurrence of specific mineral phases, some binary chemical patterns (Fe/Mn, Zn/Sr, Zn/Co and Na/Cd) and the carbon chain relative molecular masses has proven to be useful markers to characterise unequivocally the materials studied. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Black limestone; Building stone; Characterisation; Inorganic fraction; Organic fraction
1. Introduction Different kinds of black limestones were used in Lombard architecture mainly for decorative purposes, owing to the vivid colour of the stone [1,2]. Outcrops of black limestone are widespread in Mesozoic rocks of Southern Alps from Lake Como to Lake Garda, and were used since Roman times for quarrying [3–5] Black limestones show similar features in terms of bulk chemical composition and the occurrence of mineral phases, so it is hard to discriminate between them. The aim of this multidisciplinary study is to find some chemical and mineralogical markers that allow one: (1) to determine the original outcrop of each quarried lithotype, in order (i) to replace eventually decayed parts of buildings and (ii) to obtain inferences on regional economic history; (2) to enhance the conservation of black limestones, exploiting the knowledge of some microscopic properties of both organic and inorganic fractions. In fact, black limestones easily decay, occurring mainly (i) as dissolutions and exfoliations due to polluted water–calcite interaction, followed by crystallisation of Ca-sulphate causing black crusts and (ii) as
* Corresponding author. E-mail address: [email protected] (A. Pavese). © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 6 - 2 0 7 4 ( 0 2 ) 0 1 2 3 3 - 5
chromatic alterations yielding a fading grey hue. In this light, preserving, replacing or at least limiting damage is of primary importance. The first stage of tackling point (1) consists of performing textural analyses by optical microscopy which produces fundamental but only qualitative results, not relevant as an input to point (2). Note that points (1) and (2) mentioned above are closely linked to each other. In such a light, a study devoted to characterise black limestones by inorganic and organic analytical techniques was undertaken. Three black limestone outcrops were considered to provide evidence for chemical–physical features helpful in discriminating unequivocally between limestones in terms of microscopic properties: Varenna (Como) [6], Cene (Bergamo) [7], Riva di Solto (Bergamo) [8]. The outcrops chosen provided materials for buildings throughout Northern Italy (Bergamo, Como, Pavia, Milano and Venezia; Refs. [2,4,5] give historical accounts accompanied by technical records on mechanical properties of the limestones in question). A combined approach based on methods commonly adopted to investigate inorganic (optical microscopy, powder diffraction, atomic absorption analyses, Hgporosimetry (macro-pores)) and organic (high performance liquid chromatography gas chromatography, FTIR, protonNMR) compounds is required. This happens as black
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Fig. 1. Map of the sampled quarries.
limestones mainly consist of micritic calcite, along with other minor mineral phases, deposited in an enclosed and shallow marine environment where the anoxic conditions allowed organic burial [9].
2. Research aim The aim of this research is to determine some diagnostic parameters appropriate for fixing the quarry provenance of different black limestones, on the basis of a mineralogical and physical–chemical characterisation. This allows a replacement of decayed parts of historical buildings with materials from the original quarries and provides information which can contribute to shed light upon the regional economic history.
3. Experimental 3.1. Sampling The sampling was carried out on lithotypes of Mesozoic formations belonging to the Lombard Triassic Series; such lithotypes are related to carbonate platform/anoxic intraplatform trough complexes (Varenna-Perledo Formation [6], Zorzino Limestone [10] and Riva di Solto Shale [8]) (Fig. 1). The Varenna-Perledo Formation, Ladinian (Middle Triassic) in age, consists of well-bedded grey-dark limestones, subdivided by thin sheets of marls and dark clays, containing scattered fossils [9] and peloids. It occurs in a bed ranging from 20 cm to 2–3 mm thick [6] and consists of troughs, related to the carbonate platform of the Esino Lario Formation in the Grigne Mountains (Prealpi Lombarde), where sedimentation occurred mostly under anaerobic to
non-aerobic conditions leading to the burial of organic matter [9]. The Zorzino Limestone and Riva di Solto Shale (mostly Norian, late Triassic), cropping out in the Southern Calcareous Alps NW of Bergamo, are black limestones deposited in anoxic to poorly oxic troughs, inter-fingering to the wide shallow carbonate complexes of the Dolomia Principale Formation [10]. The Zorzino Limestone, Rhaethian in age (late Triassic), consists of fine grained well-bedded (15–20 cm thick on average), diluted and mud-dominated turbidite [11]. It was formerly thought to represent a lagoonal environment [7], but then it was re-interpreted as an internal basin-slope deposit because of the frequent graded calcarenites and rudites in the basinal sequence [10]. The Riva di Solto Shale consists of shales and shaly marls; it occurs in thin to medium beds, sometimes alternating with grey-black limestones (mostly in the upper part of the formation) [12]. It represents the result of a sedimentation in anoxic troughs [11] where the clay minerals are related to the northern-central European domain which emerged during Carnian and Norian ages [10]. The sampled quarries, all out of use today, are located as follows: Varenna, SS n.36, near the Varenna-Bellano tunnel, on the Lake Como shore; Cene, SP of the Seriana Valley, on the left bank of river Serio; Riva di Solto, on the west shore of Lake Iseo. Six different lithotypes were homogeneously sampled for each quarry; such a sampling cannot provide exhaustive statistics, but the issues are substantiated by the small standard deviations on most of the mineralogical and chemical properties used as markers. 3.2. Sample preparation Each sample was treated in order to extract its “insoluble residue” and organic fraction. The former was obtained as
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follows: first carbonates were dissolved in hydrochloric acid; then the insoluble residue so obtained, consisting mainly of quartz and accessory phases, was separated by centrifugation and finally dried at T = 150 °C. The organic fraction was extracted from the original matrix according to the procedure below: (1) the powdered sample was treated with acetic acid and stirred for 4 d, at T = 50 °C, so as to dissolve the carbonates and the organic matter; (2) the insoluble residue was separated by filtration and discarded; the liquid fraction was then evaporated under vacuum and a solid residue (Ca-acetates and organic fraction) was obtained; (3) the solid residue was treated by methylene chloride to dissolve the organic fraction; Ca-acetates were separated by filtration and discarded; the residual liquid was then evaporated under vacuum, giving the organic fraction. The content of the organic fraction in the samples from Varenna, Cene and Riva di Solto resulted in about 5%, 8% and <1% on total weight, respectively (uncertainty ≈ 1%). 3.3. Techniques to study the inorganic fraction The inorganic fraction of the samples was characterised by the following methods. 3.3.1. Optical microscopy Observations by a Zeiss Jenepal U-polarisation microscope at 10–50X magnification, on thin sections, were carried out to define the textural features. One thin section was examined for each sample. 3.3.2. Atomic absorption Chemical analyses have been performed to determine element concentrations (Fe, Mn, Cu, Zn, Cd, Na and Sr) in bulk samples, using the atomic absorption technique. Measurements were carried out by a Perkin Elmer 2380 spectrometer with air-acetylene flame, and a hollow cathode lamp for every element investigated. The instrument was calibrated by means of standard solutions. The concentration of each element was determined as the average value of 10 independent measurements (standard deviation < 1%). 3.3.3. X-ray powder diffraction X-ray powder diffraction (XRPD) was performed on the bulk sample and the insoluble residue by means of a Philips X’Pert powder diffractometer, using CuKa radiation from a graphite monochromator. Measurements were performed at 40 kV and 40 mA.
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The isotopic analysis cannot provide reliable diagnostic parameters for an unequivocal characterisation of our samples. In fact, the isotopic composition of an element in limestones, especially of 13C, is affected by a variety of physical and chemical fractionating processes (depositional, diagenetic, post-deposition) and can also be related to the changes in the isotopic composition of the hydrosphere and atmosphere, according to the “age effect” [13]. There is, in fact, a general agreement that the C13/C12 ratio in carbonate rocks does not show any definite trend and also that the O18/O16 isotopic ratio does not support any precise correlation between the Tertiary and Permian ages [14]. Furthermore, studies on the determination of the marble provenance on the basis of the isotopic composition, d13C and d18O have revealed a large variability both in very small quarrying areas [15,16] and in single marble fragments [16]. 3.4. Techniques to study the organic fraction 3.4.1. Gel permeation chromatography (GPC) and gas chromatography (GC) GPC was carried out with a Water Instrument with six different columns, 1 × 105, 2 × 104, 2 × 103 and 500 Å, respectively. Gas chromatography was performed by a Hewlett Packard 3800 series instrument equipped with a BP5 column, and at the following experimental conditions: start temperature Ti = 150 °C, maximum temperature Tmax = 250 °C, start isotherm 0.5 min and heating rate of 5 °C min−1. 3.4.2. High performance liquid chromatography (HPLC) HPLC was carried out with a Partisil 5ODS column equipped with two different detectors set in series (a UV/visible spectrometer operating at k = 254 nm and a refractometer) so as to achieve high sensitivity even to small changes of the organic fractions. 3.4.3. Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1H NMR) FTIR was carried out on the organic fraction with a Nicolet Magna-IR system 550, in the transmission mode, over the range 300–4000 cm−1. 1H-NMR measurements were performed with a Varian Instrument operating at 200 MHz.
4. Results and discussion 4.1. Observations by optical microscopy
3.3.4. Hg-porosimetry Mercury porosimetry measurements were carried out by a porosimeter Model 2000 (Carlo Erba Instrumentation), with pressure ranging from 1 to 2000 kg cm−2, and with a Macropore Unit (Carlo Erba Instrumentation), with pressure ranging from 0 to 3.5 kg cm−2.
The samples examined do not show apparent differences: they are mudstone [17] composed of dominant calcite of micritic size (Fig. 2). Accessory minerals occur such as quartz, present in anhedral grains, and clay minerals, in a few tiny and randomly strewn flakes.
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Fig. 2. Micritic limestone with calcitic veinlets cross-cutting the sthylolites in the black limestone (Varenna thin section n. V4).
The petrographic study on thin sections with an optical microscope evidences the presence of styloliths parallel to the bedding (Fig. 3) and due to packing of limestone, which is apparent in some levels rich in bioclasts. The samples are cross-cut by some veins (Fig. 2), ranging from a few millimeter to 1 cm, filled by euhedral calcite crystals forming a drusy fabric. The organic matter occurs both inter-granular and parallel to the bedding.
Fig. 3. Packing in the black limestone (Varenna thin section n. V2).
4.2. Qualitative and quantitative phase analyses by XRPD The X-ray diffraction patterns of the insoluble residue (Table 1) of the samples allow one to characterise the different black limestones on the basis of the mineral phases present. In particular, only in the lithotypes sampled in Varenna and Riva di Solto pyrite occurs in association with low quartz, dolomite, feldspars and clay minerals. The presence of FeS2 suggests that the sedimentation of the
N. Marinoni et al. / Journal of Cultural Heritage 3 (2002) 241–249 Table 1 Phases detected by X-ray diffraction in the residue insoluble of the black limestone Locality Varenna
Cene
Riva di Solto
Sample Symbol V1 V2 V3 V4 V5 V6 C1 C2 C3 C4 C5 C6 R1 R2 R3 R4 R5 R6
Quartz
Insoluble residue Feldspar Clays Pyrite
+ + + + + + + + + + + + + + − + + +
+ + + + + + − + − − − − + + + + + +
+ + + + + + + + + − + + + + + + + +
+ + + + + + − − − − − − + + + + + +
limestone took place in a shallow water intra-platform basin, mostly under anaerobic conditions [9]. The content of low quartz was estimated by the doping method [18], and the results are presented in Table 2. They indicate that the mass percentage of low quartz can be used as a marker of provenance, at the 15r level of confidence, in the case of samples from Varenna and from Cene, and at the 5r level, for the sample from Riva di Solto. 4.3. Chemical analyses Chemical analyses (results presented in Table 3) have been carried out to determine the concentrations of Fe, Mn, Cd, Co, Cu, Zn and Sr, which are known to substitute Ca in Table 2 Content of low quartz in the black limestone Locality
Sample
Varenna
V1 V2 V3 V4 V5 V6 Average C1 C2 C3 C4 C5 C6 Average R1 R2 R3 R4 R5 R6 Average
Cene
Riva di Solto
Symbol
SiO2(%) 5.2(2) 4.8(4) 5.4(2) 3.5(5) 5.1(2) 4.6(7) 4.8(2) 3.9(3) 2.5(7) 4.3(2) 3.5(5) 3.2(4) 4.2(7) 3.6(3) 1.2(4) 1.5(3) 2.1(3) 1.8(5) 1.4(3) 1.2(5) 1.5(3)
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calcite-like structures [19]. Na content has been measured as well: sodium occurs in clays and plagioclases, which have been observed by optical microscopy in two (Cene, Riva di Solto) out of 18 thin sections only. The use of binary diagrams (Figs. 4–7) allows one to discriminate between the limestones under study. In Fig. 4, the samples from Varenna tend to exhibit a higher content in Fe and Mn than the ones from Cene and Riva di Solto, and Fe and Mn show a quasi-linear correlation. In Fig. 5, the Co/Zn binary diagram brings to light three well-marked regions wherein the samples from Varenna, Cene and Riva di Solto lie and can be differentiated from one another. No well-defined correlation is revealed from the analysis of [Fig. 5, which suggests that the contours reported do mark compositional regions but do not allow one to speculate upon possible replacement mechanisms. The Sr/Zn diagram (Fig. 6) gives a further characterisation of the different limestones, with features similar to those of Fig. 5. Three compositional regions can be contoured to locate the Sr/Zn ratio content of the limestones from Varenna, Cene and Riva di Solto. In Fig. 7, the content of Na is plotted vs. Cd, and discriminates the Varenna samples from the others because of the sodium content presumably due to the occurrence of plagioclases. 4.4. Porosity The results of the investigations based on Hg porosimetry show that the different lithotypes have very similar features, in terms of porosity. In fact, the average measured porosity in the samples quarried in Varenna and Cene is, respectively, about 2.7% and 2.5%, whereas in Riva di Solto, an average value as large as 3.2% has been determined. The macro-pore size distributions of the samples under study are very similar to one another, and give diameters ranging from 600 to 700 Å. The micritic grain of the black limestones and their very dense structure are consistent with small pore radii and low porosity. 4.5. Macromolecular species The different GPC chromatograms prove that molecules with high relative molar mass are absent from the organic fraction since the components show low hydrodynamic volumes (high elution times). The gas chromatograms indicate that the organic fractions extracted from the Varenna and Riva di Solto samples bear macromolecule species with similar weights, but with different relative abundance. In fact, they show the main peaks at quite similar retention times, i.e. 16.30 and 16.82 min for the samples from Varenna and Riva di Solto, respectively, but with different relative areas, i.e. 33.1% and 4.9%. In addition, these samples reveal signals associated with accessory components with different values of retention time and relative abundance (in the former: 14.14,
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Table 3 Elemental contents by atomic absorption technique on the bulk limestone samples Locality
Sample
Varenna
V1 V2 V3 V4 V5 V6 C1 C2 C3 C4 C5 C6 R1 R2 R3 R4 R5 R6
Cene
Riva di Solto
Symbol
Fe (ppm)
Mn (ppm)
Cu (ppm)
Zn (ppm)
Cd (ppm)
Na (ppm)
Co (ppm)
Sr (ppm)
1206 2488 2522 1878 1638 1984 1408 1512 1131 1204 1083 1390 1057 930 970 1298 954 804
43 73 75 42 36 53 32 46 32 45 35 32 34 30 32 25 17 22
28 16 24 22 18 12 42 61 32 22 18 24 38 46 25 29 22 21
80 99 109 95 73 100 137 126 117 91 122 169 21 30 27 32 14 38
9 9 8 10 8 7 11 7 6 4 11 9 9 12 9 11 8 7
45064 52818 41368 27474 32862 0 3187 3238 4797 5112 1590 2549 2717 3745 1296 1413 1848 1233
80 71 72 67 59 0 47 42 47 46 38 45 72 79 53 56 62 46
3846 3839 4972 8266 3047 0 2364 1180 1980 2047 1957 1845 1337 6987 3516 2253 2044 5476
Fig. 4. Fe vs. Mn. Colour and symbol as in Table 3.
Fig. 5. Zn vs. Co. Colour and symbol as in Table 3.
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Fig. 6. Zn vs. Sr. Colour and symbol as in Table 3.
15.93 and 16.16 min; in the latter: 2.03, 13.65, and 20.57 min). The black limestone from Cene, instead, gives a gas chromatogram which has two main peaks at 7.02 and 9.58 min with a relative abundance of 19.4% and 18.9%, respectively, associated with other peaks which show a retention time of between 5 and 14 min (for instance 5.08, 11.08, 14.106 min). Since there is a linear correlation between the retention time and the number of carbon atoms in a standard mixture with known relative molar mass, we can compare the retention times of the main detected components with those of used standards. As a first approximation, the organic matter in all samples mainly consists of macromolecules of mass ranging from C13 to C25. In particular, the organic
Fig. 7. Cd vs. Na. Colour and symbol as in Table 3.
fraction in the samples from Varenna and Riva di Solto shows apparent peaks corresponding roughly to a weight of C23, against a mass ranging from C15 to C18 observed for the limestone from Cene. In addition, the samples from Riva di Solto show significant peaks at low retention times too (for example 2.5 min), suggesting that the organic matter extracted is also characterised by hydrocarbon chains shorter than C13. In Figs. 8 and 9, the UV absorption curve and refractive index curve as a function of the retention times of HPLC are displayed, respectively. The UV absorption signal makes it evident that in the lithotypes of Varenna and Cene, the organic matter gives rise to three main absorption peaks, occurring at similar retention times. In the case of the sample from Riva di Solto, we observe only one component
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Fig. 8. UV (k = 254 nm) absorption spectrum (instrument units) of the organic fraction, as a function of the retention time (min) by HPLC.
with a retention time comparable with the ones mentioned above. Note that this difference cannot be attributed to a loss of resolution, as tests on standard compounds have proved. Likewise, the refractive index curves exhibit similar shapes for samples from Varenna and Cene, whereas significant differences occur in the case of the sample from Riva di Solto.
The FTIR and 1H-NMR spectra did not provide evidence for significant differences between the samples from Varenna, Cene and Riva di Solto. Note, however, that fossil fuels are inherently complex and contain, in general, aromatic, aliphatic and heteroatomic moieties, and the interpretation of FTIR, and especially of NMR spectra, is therefore very difficult [20]. The FTIR and 1H-NMR spectra
Fig. 9. Refractive index of the organic fraction with respect to CH2Cl2 (instrument units), as a function of the retention time (min) by HPLC.
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are not reported for the sake of brevity. The FTIR spectra show the characteristic bands of aliphatic groups. The proton NMR measurements reveal: (i) signals of the aliphatic groups between 1 and 3 ppm; (ii) peaks ranging from 3 to 4.5 ppm which may be attributed to the presence of a hydrogen atom located close to some functional group such as OH and NH2; (iii) peaks from 6.5 to 8 ppm due to H interacting with an aromatic ring.
5. Conclusions On the basis of the results discussed above, the following conclusions can be drawn: From an optical point of view: „ the different lithotypes are quite similar: they are mudstone composed mainly of calcite and have the same fabric, characterised by the effect of the packing (for instance, sthylolites). From a chemical point of view: „ [Fe/Mn] binary diagram in the case of the Varenna samples exhibits a quasi-linear correlation between Fe and Mn; „ [Zn/Co] and [Zn/Sr] binary diagrams show three different regions, one for each limestone. In particular: the samples from Riva di Solto bear the smallest content of Zn, but rather scattered values of Co and Sr; the samples from Varenna exhibit a larger content of Co and of Sr with respect to the samples from Cene; „ Na occurs in significant amounts in the samples from Varenna only. From a mineralogical point of view: „ pyrite is observed in the samples from Riva di Solto and Varenna, whereas it is absent from the samples from Cene; „ low quartz occurs in significantly different amounts in the limestones from Varenna and Riva di Solto (4.8% and 1.5%, respectively). From the organic fraction point of view: • the average content in terms of organic fraction in the samples from Varenna, Cene and Riva di Solto is about 5%, 8% and smaller than 1%, respectively; • the samples from Riva di Solto show C-chains with relative molar mass slightly larger than those from Varenna and Cene. In full: (1) The limestone from Varenna is characterised by (i) quasi-linear correlation between Fe and Mn, (ii) Na content, (iii) occurrence of pyrite, (iv) larger than 4% content of low quartz. (2) The limestone from Riva di Solto is defined by (i) low Zn content, (ii) occurrence of pyrite, (iii) low amount of quartz, (iv) low organic content (<1% by weight).
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(3) The limestone from Cene is defined by (i) the absence of pyrite, (ii) hydrocarbon chains with relative molar mass between C15 and C18.
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P. Casati, Il Trias in Lombardia (Studi geologici e paleontologici). VI. Osservazioni stratigrafiche sull’infraretico delle Prealpi Bergamasche, Riv. Ital. Paleont. 70 (1964) 447–465.
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[12] M. Gaetani, R. Gianotti, F. Jadoul, G. Ciarapica, S. Cirilli, A. Lualdi, L. Passeri, M. Pellegrini, G. Tannoia, Carbonifero superiore, Permiano e Triassico nell’area Lariana, Mem. Soc. Gel. It. 32 (1986) 5–48. [13] P.M. Jeffery, W. Compston, D. Greenhalgh, J. De Latter, On the carbon-13 abundance of limestone and coals, Geochimica et Cosmochimica Acta 7 (1955) 255–286. [14] J. Veizer, J. Hoefs, The nature of 18O/16O and 13C/12C secular trends in sedimentary carbonate rocks, Geochimica et Cosmochimica Acta 40 (1976) 133–149. [15] C. Arnoldi, E. Azzaro, M. Barbieri, P. Tucci, Petrographic and geochemical features of the “Cipollino Verde” marble from the Apuan Alps (northern Tuscany, Italy) and archaeometric implications, Per. Mineral. 68 (1999) 145–162. [16] K. Germann, G. Holzmann, F.J. Winkler, Determination of marble provenance: limits of isotopic analysis, Archaeometry 22 (1980) 99–106. [17] A. E. Adams, W. S. Mackenzie, C. Guilford, Atlante delle Rocce Sedimentarie al microscopio, Zanichelli, Bologna, 1988. [18] Zevin, Kimmel, Quantitative X-ray Diffractometry, Springer, Berlin, 1995. [19] J. Veizer, Trace elements and isotopes in sedimentary rocks, in: Paul H. Ribbe (Ed.), Carbonates: Mineralogy and Chemistry, Reviews in Mineralogy, Virginia, 1983. [20] L.D. Field, Analytical NMR, John Wiley & Sons, Australia, 1989.
Black limestone used in Lombard architecture Nicoletta Marinoni a, Alessandro Pavese a,b,*, Roberto Bugini c, Giuseppe Di Silvestro d a
Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, 20133, Milan, Italy b National Research Council, IDPA, Section of Milan, Via Mangiagalli 34, 20133, Milan, Italy c National Research Council, Centro “Gino Bozza” per lo Studio delle Cause di Deperimento e dei Metodi di Conservazione delle Opere d’Arte - Piazza Leonardo da Vinci 32, 20133, Milan, Italy d Dipartimento di Chimica Organica e Industriale, Via Golgi 7, 20133, Milan, Italy
Abstract Black limestone samples from the quarries of Varenna (Lecco, I), Cene (Bergamo, I) and Riva di Solto (Bergamo, I) and widely used in Lombard architecture have been studied in terms of mineralogical, petrographic and chemical properties in order to provide a detailed characterisation and allow an unambiguous determination of their provenance. The inorganic and organic fractions have been separated from each other, and investigated using X-ray powder diffraction, atomic absorption, Hg-porosimetry, high performance liquid chromatography, gas chromatography, nuclear magnetic resonance and Fourier transform infrared spectroscopy. The occurrence of specific mineral phases, some binary chemical patterns (Fe/Mn, Zn/Sr, Zn/Co and Na/Cd) and the carbon chain relative molecular masses has proven to be useful markers to characterise unequivocally the materials studied. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Black limestone; Building stone; Characterisation; Inorganic fraction; Organic fraction
1. Introduction Different kinds of black limestones were used in Lombard architecture mainly for decorative purposes, owing to the vivid colour of the stone [1,2]. Outcrops of black limestone are widespread in Mesozoic rocks of Southern Alps from Lake Como to Lake Garda, and were used since Roman times for quarrying [3–5] Black limestones show similar features in terms of bulk chemical composition and the occurrence of mineral phases, so it is hard to discriminate between them. The aim of this multidisciplinary study is to find some chemical and mineralogical markers that allow one: (1) to determine the original outcrop of each quarried lithotype, in order (i) to replace eventually decayed parts of buildings and (ii) to obtain inferences on regional economic history; (2) to enhance the conservation of black limestones, exploiting the knowledge of some microscopic properties of both organic and inorganic fractions. In fact, black limestones easily decay, occurring mainly (i) as dissolutions and exfoliations due to polluted water–calcite interaction, followed by crystallisation of Ca-sulphate causing black crusts and (ii) as
* Corresponding author. E-mail address: [email protected] (A. Pavese). © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 6 - 2 0 7 4 ( 0 2 ) 0 1 2 3 3 - 5
chromatic alterations yielding a fading grey hue. In this light, preserving, replacing or at least limiting damage is of primary importance. The first stage of tackling point (1) consists of performing textural analyses by optical microscopy which produces fundamental but only qualitative results, not relevant as an input to point (2). Note that points (1) and (2) mentioned above are closely linked to each other. In such a light, a study devoted to characterise black limestones by inorganic and organic analytical techniques was undertaken. Three black limestone outcrops were considered to provide evidence for chemical–physical features helpful in discriminating unequivocally between limestones in terms of microscopic properties: Varenna (Como) [6], Cene (Bergamo) [7], Riva di Solto (Bergamo) [8]. The outcrops chosen provided materials for buildings throughout Northern Italy (Bergamo, Como, Pavia, Milano and Venezia; Refs. [2,4,5] give historical accounts accompanied by technical records on mechanical properties of the limestones in question). A combined approach based on methods commonly adopted to investigate inorganic (optical microscopy, powder diffraction, atomic absorption analyses, Hgporosimetry (macro-pores)) and organic (high performance liquid chromatography gas chromatography, FTIR, protonNMR) compounds is required. This happens as black
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Fig. 1. Map of the sampled quarries.
limestones mainly consist of micritic calcite, along with other minor mineral phases, deposited in an enclosed and shallow marine environment where the anoxic conditions allowed organic burial [9].
2. Research aim The aim of this research is to determine some diagnostic parameters appropriate for fixing the quarry provenance of different black limestones, on the basis of a mineralogical and physical–chemical characterisation. This allows a replacement of decayed parts of historical buildings with materials from the original quarries and provides information which can contribute to shed light upon the regional economic history.
3. Experimental 3.1. Sampling The sampling was carried out on lithotypes of Mesozoic formations belonging to the Lombard Triassic Series; such lithotypes are related to carbonate platform/anoxic intraplatform trough complexes (Varenna-Perledo Formation [6], Zorzino Limestone [10] and Riva di Solto Shale [8]) (Fig. 1). The Varenna-Perledo Formation, Ladinian (Middle Triassic) in age, consists of well-bedded grey-dark limestones, subdivided by thin sheets of marls and dark clays, containing scattered fossils [9] and peloids. It occurs in a bed ranging from 20 cm to 2–3 mm thick [6] and consists of troughs, related to the carbonate platform of the Esino Lario Formation in the Grigne Mountains (Prealpi Lombarde), where sedimentation occurred mostly under anaerobic to
non-aerobic conditions leading to the burial of organic matter [9]. The Zorzino Limestone and Riva di Solto Shale (mostly Norian, late Triassic), cropping out in the Southern Calcareous Alps NW of Bergamo, are black limestones deposited in anoxic to poorly oxic troughs, inter-fingering to the wide shallow carbonate complexes of the Dolomia Principale Formation [10]. The Zorzino Limestone, Rhaethian in age (late Triassic), consists of fine grained well-bedded (15–20 cm thick on average), diluted and mud-dominated turbidite [11]. It was formerly thought to represent a lagoonal environment [7], but then it was re-interpreted as an internal basin-slope deposit because of the frequent graded calcarenites and rudites in the basinal sequence [10]. The Riva di Solto Shale consists of shales and shaly marls; it occurs in thin to medium beds, sometimes alternating with grey-black limestones (mostly in the upper part of the formation) [12]. It represents the result of a sedimentation in anoxic troughs [11] where the clay minerals are related to the northern-central European domain which emerged during Carnian and Norian ages [10]. The sampled quarries, all out of use today, are located as follows: Varenna, SS n.36, near the Varenna-Bellano tunnel, on the Lake Como shore; Cene, SP of the Seriana Valley, on the left bank of river Serio; Riva di Solto, on the west shore of Lake Iseo. Six different lithotypes were homogeneously sampled for each quarry; such a sampling cannot provide exhaustive statistics, but the issues are substantiated by the small standard deviations on most of the mineralogical and chemical properties used as markers. 3.2. Sample preparation Each sample was treated in order to extract its “insoluble residue” and organic fraction. The former was obtained as
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follows: first carbonates were dissolved in hydrochloric acid; then the insoluble residue so obtained, consisting mainly of quartz and accessory phases, was separated by centrifugation and finally dried at T = 150 °C. The organic fraction was extracted from the original matrix according to the procedure below: (1) the powdered sample was treated with acetic acid and stirred for 4 d, at T = 50 °C, so as to dissolve the carbonates and the organic matter; (2) the insoluble residue was separated by filtration and discarded; the liquid fraction was then evaporated under vacuum and a solid residue (Ca-acetates and organic fraction) was obtained; (3) the solid residue was treated by methylene chloride to dissolve the organic fraction; Ca-acetates were separated by filtration and discarded; the residual liquid was then evaporated under vacuum, giving the organic fraction. The content of the organic fraction in the samples from Varenna, Cene and Riva di Solto resulted in about 5%, 8% and <1% on total weight, respectively (uncertainty ≈ 1%). 3.3. Techniques to study the inorganic fraction The inorganic fraction of the samples was characterised by the following methods. 3.3.1. Optical microscopy Observations by a Zeiss Jenepal U-polarisation microscope at 10–50X magnification, on thin sections, were carried out to define the textural features. One thin section was examined for each sample. 3.3.2. Atomic absorption Chemical analyses have been performed to determine element concentrations (Fe, Mn, Cu, Zn, Cd, Na and Sr) in bulk samples, using the atomic absorption technique. Measurements were carried out by a Perkin Elmer 2380 spectrometer with air-acetylene flame, and a hollow cathode lamp for every element investigated. The instrument was calibrated by means of standard solutions. The concentration of each element was determined as the average value of 10 independent measurements (standard deviation < 1%). 3.3.3. X-ray powder diffraction X-ray powder diffraction (XRPD) was performed on the bulk sample and the insoluble residue by means of a Philips X’Pert powder diffractometer, using CuKa radiation from a graphite monochromator. Measurements were performed at 40 kV and 40 mA.
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The isotopic analysis cannot provide reliable diagnostic parameters for an unequivocal characterisation of our samples. In fact, the isotopic composition of an element in limestones, especially of 13C, is affected by a variety of physical and chemical fractionating processes (depositional, diagenetic, post-deposition) and can also be related to the changes in the isotopic composition of the hydrosphere and atmosphere, according to the “age effect” [13]. There is, in fact, a general agreement that the C13/C12 ratio in carbonate rocks does not show any definite trend and also that the O18/O16 isotopic ratio does not support any precise correlation between the Tertiary and Permian ages [14]. Furthermore, studies on the determination of the marble provenance on the basis of the isotopic composition, d13C and d18O have revealed a large variability both in very small quarrying areas [15,16] and in single marble fragments [16]. 3.4. Techniques to study the organic fraction 3.4.1. Gel permeation chromatography (GPC) and gas chromatography (GC) GPC was carried out with a Water Instrument with six different columns, 1 × 105, 2 × 104, 2 × 103 and 500 Å, respectively. Gas chromatography was performed by a Hewlett Packard 3800 series instrument equipped with a BP5 column, and at the following experimental conditions: start temperature Ti = 150 °C, maximum temperature Tmax = 250 °C, start isotherm 0.5 min and heating rate of 5 °C min−1. 3.4.2. High performance liquid chromatography (HPLC) HPLC was carried out with a Partisil 5ODS column equipped with two different detectors set in series (a UV/visible spectrometer operating at k = 254 nm and a refractometer) so as to achieve high sensitivity even to small changes of the organic fractions. 3.4.3. Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1H NMR) FTIR was carried out on the organic fraction with a Nicolet Magna-IR system 550, in the transmission mode, over the range 300–4000 cm−1. 1H-NMR measurements were performed with a Varian Instrument operating at 200 MHz.
4. Results and discussion 4.1. Observations by optical microscopy
3.3.4. Hg-porosimetry Mercury porosimetry measurements were carried out by a porosimeter Model 2000 (Carlo Erba Instrumentation), with pressure ranging from 1 to 2000 kg cm−2, and with a Macropore Unit (Carlo Erba Instrumentation), with pressure ranging from 0 to 3.5 kg cm−2.
The samples examined do not show apparent differences: they are mudstone [17] composed of dominant calcite of micritic size (Fig. 2). Accessory minerals occur such as quartz, present in anhedral grains, and clay minerals, in a few tiny and randomly strewn flakes.
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Fig. 2. Micritic limestone with calcitic veinlets cross-cutting the sthylolites in the black limestone (Varenna thin section n. V4).
The petrographic study on thin sections with an optical microscope evidences the presence of styloliths parallel to the bedding (Fig. 3) and due to packing of limestone, which is apparent in some levels rich in bioclasts. The samples are cross-cut by some veins (Fig. 2), ranging from a few millimeter to 1 cm, filled by euhedral calcite crystals forming a drusy fabric. The organic matter occurs both inter-granular and parallel to the bedding.
Fig. 3. Packing in the black limestone (Varenna thin section n. V2).
4.2. Qualitative and quantitative phase analyses by XRPD The X-ray diffraction patterns of the insoluble residue (Table 1) of the samples allow one to characterise the different black limestones on the basis of the mineral phases present. In particular, only in the lithotypes sampled in Varenna and Riva di Solto pyrite occurs in association with low quartz, dolomite, feldspars and clay minerals. The presence of FeS2 suggests that the sedimentation of the
N. Marinoni et al. / Journal of Cultural Heritage 3 (2002) 241–249 Table 1 Phases detected by X-ray diffraction in the residue insoluble of the black limestone Locality Varenna
Cene
Riva di Solto
Sample Symbol V1 V2 V3 V4 V5 V6 C1 C2 C3 C4 C5 C6 R1 R2 R3 R4 R5 R6
Quartz
Insoluble residue Feldspar Clays Pyrite
+ + + + + + + + + + + + + + − + + +
+ + + + + + − + − − − − + + + + + +
+ + + + + + + + + − + + + + + + + +
+ + + + + + − − − − − − + + + + + +
limestone took place in a shallow water intra-platform basin, mostly under anaerobic conditions [9]. The content of low quartz was estimated by the doping method [18], and the results are presented in Table 2. They indicate that the mass percentage of low quartz can be used as a marker of provenance, at the 15r level of confidence, in the case of samples from Varenna and from Cene, and at the 5r level, for the sample from Riva di Solto. 4.3. Chemical analyses Chemical analyses (results presented in Table 3) have been carried out to determine the concentrations of Fe, Mn, Cd, Co, Cu, Zn and Sr, which are known to substitute Ca in Table 2 Content of low quartz in the black limestone Locality
Sample
Varenna
V1 V2 V3 V4 V5 V6 Average C1 C2 C3 C4 C5 C6 Average R1 R2 R3 R4 R5 R6 Average
Cene
Riva di Solto
Symbol
SiO2(%) 5.2(2) 4.8(4) 5.4(2) 3.5(5) 5.1(2) 4.6(7) 4.8(2) 3.9(3) 2.5(7) 4.3(2) 3.5(5) 3.2(4) 4.2(7) 3.6(3) 1.2(4) 1.5(3) 2.1(3) 1.8(5) 1.4(3) 1.2(5) 1.5(3)
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calcite-like structures [19]. Na content has been measured as well: sodium occurs in clays and plagioclases, which have been observed by optical microscopy in two (Cene, Riva di Solto) out of 18 thin sections only. The use of binary diagrams (Figs. 4–7) allows one to discriminate between the limestones under study. In Fig. 4, the samples from Varenna tend to exhibit a higher content in Fe and Mn than the ones from Cene and Riva di Solto, and Fe and Mn show a quasi-linear correlation. In Fig. 5, the Co/Zn binary diagram brings to light three well-marked regions wherein the samples from Varenna, Cene and Riva di Solto lie and can be differentiated from one another. No well-defined correlation is revealed from the analysis of [Fig. 5, which suggests that the contours reported do mark compositional regions but do not allow one to speculate upon possible replacement mechanisms. The Sr/Zn diagram (Fig. 6) gives a further characterisation of the different limestones, with features similar to those of Fig. 5. Three compositional regions can be contoured to locate the Sr/Zn ratio content of the limestones from Varenna, Cene and Riva di Solto. In Fig. 7, the content of Na is plotted vs. Cd, and discriminates the Varenna samples from the others because of the sodium content presumably due to the occurrence of plagioclases. 4.4. Porosity The results of the investigations based on Hg porosimetry show that the different lithotypes have very similar features, in terms of porosity. In fact, the average measured porosity in the samples quarried in Varenna and Cene is, respectively, about 2.7% and 2.5%, whereas in Riva di Solto, an average value as large as 3.2% has been determined. The macro-pore size distributions of the samples under study are very similar to one another, and give diameters ranging from 600 to 700 Å. The micritic grain of the black limestones and their very dense structure are consistent with small pore radii and low porosity. 4.5. Macromolecular species The different GPC chromatograms prove that molecules with high relative molar mass are absent from the organic fraction since the components show low hydrodynamic volumes (high elution times). The gas chromatograms indicate that the organic fractions extracted from the Varenna and Riva di Solto samples bear macromolecule species with similar weights, but with different relative abundance. In fact, they show the main peaks at quite similar retention times, i.e. 16.30 and 16.82 min for the samples from Varenna and Riva di Solto, respectively, but with different relative areas, i.e. 33.1% and 4.9%. In addition, these samples reveal signals associated with accessory components with different values of retention time and relative abundance (in the former: 14.14,
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Table 3 Elemental contents by atomic absorption technique on the bulk limestone samples Locality
Sample
Varenna
V1 V2 V3 V4 V5 V6 C1 C2 C3 C4 C5 C6 R1 R2 R3 R4 R5 R6
Cene
Riva di Solto
Symbol
Fe (ppm)
Mn (ppm)
Cu (ppm)
Zn (ppm)
Cd (ppm)
Na (ppm)
Co (ppm)
Sr (ppm)
1206 2488 2522 1878 1638 1984 1408 1512 1131 1204 1083 1390 1057 930 970 1298 954 804
43 73 75 42 36 53 32 46 32 45 35 32 34 30 32 25 17 22
28 16 24 22 18 12 42 61 32 22 18 24 38 46 25 29 22 21
80 99 109 95 73 100 137 126 117 91 122 169 21 30 27 32 14 38
9 9 8 10 8 7 11 7 6 4 11 9 9 12 9 11 8 7
45064 52818 41368 27474 32862 0 3187 3238 4797 5112 1590 2549 2717 3745 1296 1413 1848 1233
80 71 72 67 59 0 47 42 47 46 38 45 72 79 53 56 62 46
3846 3839 4972 8266 3047 0 2364 1180 1980 2047 1957 1845 1337 6987 3516 2253 2044 5476
Fig. 4. Fe vs. Mn. Colour and symbol as in Table 3.
Fig. 5. Zn vs. Co. Colour and symbol as in Table 3.
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Fig. 6. Zn vs. Sr. Colour and symbol as in Table 3.
15.93 and 16.16 min; in the latter: 2.03, 13.65, and 20.57 min). The black limestone from Cene, instead, gives a gas chromatogram which has two main peaks at 7.02 and 9.58 min with a relative abundance of 19.4% and 18.9%, respectively, associated with other peaks which show a retention time of between 5 and 14 min (for instance 5.08, 11.08, 14.106 min). Since there is a linear correlation between the retention time and the number of carbon atoms in a standard mixture with known relative molar mass, we can compare the retention times of the main detected components with those of used standards. As a first approximation, the organic matter in all samples mainly consists of macromolecules of mass ranging from C13 to C25. In particular, the organic
Fig. 7. Cd vs. Na. Colour and symbol as in Table 3.
fraction in the samples from Varenna and Riva di Solto shows apparent peaks corresponding roughly to a weight of C23, against a mass ranging from C15 to C18 observed for the limestone from Cene. In addition, the samples from Riva di Solto show significant peaks at low retention times too (for example 2.5 min), suggesting that the organic matter extracted is also characterised by hydrocarbon chains shorter than C13. In Figs. 8 and 9, the UV absorption curve and refractive index curve as a function of the retention times of HPLC are displayed, respectively. The UV absorption signal makes it evident that in the lithotypes of Varenna and Cene, the organic matter gives rise to three main absorption peaks, occurring at similar retention times. In the case of the sample from Riva di Solto, we observe only one component
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Fig. 8. UV (k = 254 nm) absorption spectrum (instrument units) of the organic fraction, as a function of the retention time (min) by HPLC.
with a retention time comparable with the ones mentioned above. Note that this difference cannot be attributed to a loss of resolution, as tests on standard compounds have proved. Likewise, the refractive index curves exhibit similar shapes for samples from Varenna and Cene, whereas significant differences occur in the case of the sample from Riva di Solto.
The FTIR and 1H-NMR spectra did not provide evidence for significant differences between the samples from Varenna, Cene and Riva di Solto. Note, however, that fossil fuels are inherently complex and contain, in general, aromatic, aliphatic and heteroatomic moieties, and the interpretation of FTIR, and especially of NMR spectra, is therefore very difficult [20]. The FTIR and 1H-NMR spectra
Fig. 9. Refractive index of the organic fraction with respect to CH2Cl2 (instrument units), as a function of the retention time (min) by HPLC.
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are not reported for the sake of brevity. The FTIR spectra show the characteristic bands of aliphatic groups. The proton NMR measurements reveal: (i) signals of the aliphatic groups between 1 and 3 ppm; (ii) peaks ranging from 3 to 4.5 ppm which may be attributed to the presence of a hydrogen atom located close to some functional group such as OH and NH2; (iii) peaks from 6.5 to 8 ppm due to H interacting with an aromatic ring.
5. Conclusions On the basis of the results discussed above, the following conclusions can be drawn: From an optical point of view: „ the different lithotypes are quite similar: they are mudstone composed mainly of calcite and have the same fabric, characterised by the effect of the packing (for instance, sthylolites). From a chemical point of view: „ [Fe/Mn] binary diagram in the case of the Varenna samples exhibits a quasi-linear correlation between Fe and Mn; „ [Zn/Co] and [Zn/Sr] binary diagrams show three different regions, one for each limestone. In particular: the samples from Riva di Solto bear the smallest content of Zn, but rather scattered values of Co and Sr; the samples from Varenna exhibit a larger content of Co and of Sr with respect to the samples from Cene; „ Na occurs in significant amounts in the samples from Varenna only. From a mineralogical point of view: „ pyrite is observed in the samples from Riva di Solto and Varenna, whereas it is absent from the samples from Cene; „ low quartz occurs in significantly different amounts in the limestones from Varenna and Riva di Solto (4.8% and 1.5%, respectively). From the organic fraction point of view: • the average content in terms of organic fraction in the samples from Varenna, Cene and Riva di Solto is about 5%, 8% and smaller than 1%, respectively; • the samples from Riva di Solto show C-chains with relative molar mass slightly larger than those from Varenna and Cene. In full: (1) The limestone from Varenna is characterised by (i) quasi-linear correlation between Fe and Mn, (ii) Na content, (iii) occurrence of pyrite, (iv) larger than 4% content of low quartz. (2) The limestone from Riva di Solto is defined by (i) low Zn content, (ii) occurrence of pyrite, (iii) low amount of quartz, (iv) low organic content (<1% by weight).
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(3) The limestone from Cene is defined by (i) the absence of pyrite, (ii) hydrocarbon chains with relative molar mass between C15 and C18.
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