Twelve lithologies have been chosen among the limestones commonly used in the built heritage of northeastern Italy (Fig. 1): Vicenza Stone (Nanto and Costozza variety), Istria Stone (Orsera variety), Chiampo Stone (Paglierino and Ondagata variety), Red and Brown Verona Stone, Pink and White Asiago Stone, Aurisina Stone, Botticino Stone and Carrara marble. These stones come from different geological formations outcropping in northeastern Italy, Croatia (Istra Stone), and in the Tuscany region (Italy) (Fig. 2). A summary of the main geological outlines and lithological classification of all the considered rock types is reported in Table 1. In this study, the Carrara marble is used as a reference material to compare the grain-size distribution obtained by Optical Microscopy and by EBSD (see section 4.2).
Vicenza Stones (Nanto and Costozza varieties, NA and CO)
Vicenza Stone (Pietra Tenera, or “Soft Stone”, of Vicenza) is a general name used to indicate various biocalcarenitic limestones quarried in the Berici Hills, extending over an area of about 165 km2 south of Vicenza, characterized by light colors with ivory, light grey, straw yellow, warm light brown hues. Lithostratigraphically, Vicenza Stone belongs to the early to middle Eocene Nummulitic Limestone Formation and the Oligocene Castelgomberto Limestone Formation (Cattaneo et al. 1976; Benchiarin 2007; Cappellaro et al. 2012). They derive their popular local name of “Soft Stone” from their soft texture, which makes them easy to cut and carve, favored by the high porosity and the water content when freshly extracted. Visible macropores (> 200 µm, according to the nomenclature reported in Siegesmund and Dürrast 2011) are abundant and the content in large bioclasts is high. The typical weathering forms consist in dissolution and pulverization, accompanied by an often-important biological colonization; humidity marks and black crusts are often present. Due to its high specific surface determined by porosity and the presence of thin clay layers containing swelling micas such as montmorillonite, Vicenza stone is liable to deteriorate (Marchesini et al. 1972; Cattaneo et al. 1976; Fassina and Cherido 1985; Ginevra et al. 1999;Benchiarin 2007; Benchiarin et al. 2012; Di Benedetto et al. 2015).
In the present work, the Nanto and the Costozza varieties (samples NA and CO, respectively) have been considered.
Nanto Stone is a type of Vicenza Stone quarried near the town of Nanto, along the eastern margin of the Berici Hills, from the layers placed at the bottom of the Nummulitic Limestone Formation (Massari et al. 1976; Benchiarin 2007). This variety is also known with the commercial name of Giallo Dorato (i.e. “Golden Yellow”) due to its warm yellowish-brown hue. Its texture is characterized by isoriented large biolcasts (mainly macroforaminifera) in a golden sand-sized matrix rich in planktonic foraminifera.
Costozza Stone is the traditional light ivory variety of White Vicenza Stone. It is a packstone rich in encrusting coralline algae, large foraminifera, bryozoans and echinids with variable amounts of miliolids, which create its thin and flowery texture. It is quarried from the stratigraphically higher layers of the Castelgomberto Limestone Formation, which includes back-reef tidal channel deposits of Oligocene age.
Vicenza Stone was largely used in historical masonry and in statuary, as admirably testified by the Olympic Theatre, the Palladian Villas and the Lodges of the Basilica Palladiana in Vicenza (all part of the UNESCO World Heritage Site “City of Vicenza and the Palladian Villas of the Veneto”), the 78 statues located in Prato della Valle square in Padua (Braga 2004; Cornale and Rosanò 1994) (Fig. 3), the Cathedral of Modena and the Ghirlandina (belonging to the UNESCO World Heritage Site “Cathedral, Torre Civica and Piazza Grande, Modena”). This stone was also used in the Mysterious Baths by De Chirico in Milan, and in numerous 20th century buildings, both for internal and external cladding, internal floors and staircases, and other building components. Thanks to its fire-resistance, it has been traditionally used in fireplaces (Rodolico 1953).
Aurisina Stone (AU)
Aurisina Stone (sample AU) is a general term used to indicate several varieties of biomicritic and fossiliferous limestones (from grainstone to wackestone) quarried in the Aurisina quarrying district since the Roman Age (e.g. from the top portion of the so-called “Roman Quarry”; Maritan et al. 2003), some hundred meters from the coastline, about 10 km north of Trieste. These limestones outcrop in the Classical Karst Region of northeastern Italy and southwestern Slovenia, and belong to the carbonate sequences of the northwesternmost part of the External Dinarides. Stratigraphically, the Aurisina limestones belong to the late Cretaceous Sežana (late Turonian – early Santonian) and Lipica (Santonian-Campanian) Formations (Cucchi et al. 1987; Cucchi et al. 2015; Jurkovšek et al. 2016), the latter including the most economilally relevant layers, constituted by massive beds with homogeneous texture of biomicrites and biosparites with abundant rudist bioclasts, deposited in a shallow-water platform from low-energy littoral and lagoonal to high-energy open shelf environments.
Aurisina Stone is a rock that has been intensively used since Roman times to realize buildings, infrastructures and artifacts (e.g. statues, funerary inscriptions, etc.) in many cities of the Regio X Venetia et Histria (e.g. Aquileia, Concordia Sagittaria, Oderzo, Altino, Padua, Verona, Brescia, Cremona, Piacenza, Mantua), Regio VIII Aemilia (e.g. Modena, Reggio-Emilia, Ravenna, Rimini, Fano) and Regio XI Transpadana (e.g. Milan, Pavia) (Maritan et al. 2003; Bugini and Folli 2014; Previato 2018) (Fig. 4). The present-day exploitation mainly concerns the following three varieties: Fiorito (large rudist shells, rudstone-floatstone), Granitello (coarse-sand shell fragments, grainstone-packstone), Roman stone (fine shell fragments, packstone-wackestone). This study considers the Granitello variety, which is a light grey limestone with irregular white spots, characterized by the presence of abundant rudists bioclasts.
Istria Stone (Orsera variety, OR)
The Istrian peninsula is part of a vast mainly Mesozoic carbonate platform with limestones ranging in age from late Jurassic to Eocene, and structurally forming an anticline, the core of which outcrops in mid-western Istria, where the Kirmenjak unit (late Tithonian) transgressively lies on the late Kimmeridgian – early Tithonian Rovinj bauxite (Durn et al. 2003). The Kirmenjak unit consists in three members: a basal member of clay, breccia and mudstone, followed by a second member of thick bedded stylolitised mudstone and a third member made of fenestral mudstone. Stylolitised mudstones were deposited in a low-energy subtidal to low intertidal environment, forming a 65 m thick course which provided the best quality and most valuable “Istrian limestone”. Although thousands of quarries are known from the Istrian peninsula, the most representative material used in the Venetian architecture derives from this course, which was thouroughly exploited in the two largest known quarries near Vrsar (Orsera) and Rovinj (Lazzarini 2012). In the present study we consider the Orsera variety (Kirmenjak stone, from the Montraker quarry) (sample OR), characterized by a salt white – pale grey color, very compact texture, low porosity, and conchoidal fracture, with compressive strength and density approaching those of marble (Geometrante et al. 2000).
The use of the Istria Stone in Roman times is limited to Istria, with rare examples not further than Aquileia (Lazzarini 2012). It is unknown when the Istria Stone was first used in Venice, but quarrying activity was already well established in the 13th century when numerous statues were realized using this material (Lazzarini 2012). Although Istria Stone was initially used mainly for statuary, in 1307, the Venetian Senate established that Istria Stone should be the only building material used in Venetian buildings as well as in all the Venetian territories in the Venetian hinterland, in southern Italy and Greece. It has been calculated that about 80% of stone employed in Venice buildings derived from Istria region (Dunda and Kujundžić 2004). The technical features of this stone, consisting in its high durability, extremely low porosity and water absorption, high compressive strength, and resistance to the Venetian salt-rich environment, made this stone the ideal building material to be used for the foundations directly on top of the timber boards surmounting the timber piles. This stone layer ended up with a continuous course of stones called “cadene” above the average sea level (Foraboschi 2017), with the stylolitic layers oriented horizontally truncating the capillary water paths, and creating a multilayer moisture barrier (Šimunić Buršić et al. 2007; Lazzarini 2012). In addition, like any other compact limestone, Istrian Stone is difficult to be carved (even more than marble) because it tends to splinter easily and presents conchoidal fracture, especially when worked parallel to the joints; on the other hand, this stone can be easily polished (Lazzarini 2012). In the city of Venice (UNESCO World Heritage Site “Venice and its Lagoon”) numerous buildings are completely covered with Istria Stone, such as the Doge’s Palace (14th century), the Rialto Bridge and the Bridge of Sighs (end of the 16th century), while a modern example is that of Santa Lucia railway station, built in the first half of the 20th century.
Orsera limestone was also used in Dubrovnik (Croatia) together with the varieties quarried in Vrnik and Korčula (Crnković and Jovičić 1993), for the Basilica Euprasiana in Poreč, Croatia (6th century), for the Mausoleum of Theodoric in Ravenna (6th century) (all UNESCO World Heritage sites), for the Cathedral of Fermo (Marche region, central Italy; reconstructed at the beginning of the 13th century), for the Tomb of Dante in Ravenna (a neoclassical monument built in the second half of the 18th century), for the Monument to the Fallen of the First World War in Ancona, for the ancient “kažuni”, traditional stone cottages of the Istrian peninsula that served as shelter for shepherds (Fig. 4), just to mention some remarkable examples (Dunda and Kujundžić 2004). The use of the Istria Stone has been documented also in Turin, where some ornamental elements, and interior and exterior facings of the San Vincenzo block of via Roma are made of this stone (Borghi et al. 2015).
Chiampo Stones (Ondagata and Paglierino varieties, ON and PA)
Chiampo Limestone refers to a compact facies of the Nummulitic Limestone Formation (early to middle Eocene), which is ~ 30 mt thick and crops out in the Venetian Prealps, more specifically in the area of the eastern Lessini Mountains, along both the flanks of the Chiampo Valley (Papazzoni and Sirotti 1995; Matteucci and Russo 2005), intercalated among late Paleocene – Eocene mafic tuffs, some of which containing an important fauna of three-dimensionally preserved siliceous sponges (Frisone et al. 2020).The commercial names Chiampo Marble or Chiampo Stone refer to different varieties of this sedimentary rocks which generally consist in rudstones rich in macroforaminifera, calcareous algae, rare scleractinians, echinoids, and molluscs, with light brownish-grey to pale pink color. This study considers the Paglierino and Ondagata varieties. Chiampo Stone was largely appreciated by the Italian architect Vincenzo Scamozzi (1548–1616) because of its whiteness, granularity (similar to that of Vicenza stone), strength and compactness.
The Ondagata variety (sample ON) is characterized by a pinkish hue and evident weavy stratification. It can be perfectly polished even if the stone texture is not uniform as it is often composed of an aggregate of fossil remains, especially Nummulites and Discocyclina. Currently the most precious type is the light brownish-grey Paglierino variety (sample PA), which is compact and homogeneous, with barely visible stratification. Chiampo limestone has been used mainly for small decorative elements (stairs, wells) but also for monuments like the two monolithic columns in the “Piazza dei Signori” city square of Vicenza (Rodolico 1953). Between the 1920s and the 1950s the Chiampo quarries have been extensively cultivated for the production of the headstones used in the 1st World War CWGC (Commonwealth War Graves Commission) cemeteries in Italy, the Niguarda Hospital in Milan, the Exchange Palace (Palazzo della Nuova Borsa Valori) in Genoa, the Courthouse and the Finance Palace in Bolzano, the “Palazzo Grande” in Livorno, the Shrine of the Báb and the International Archives, two of the buildings in the Bahá’í World Centre in Haifa (Israel) (Fig. 5).
Verona Stones (Red Verona and Brown Verona marble, RV and BV)
Red and Brown Verona Marbles (samples RV and BV, respectively) are commercial names used to indicate limestones with different reddish hues pertaining to the Rosso Ammonitico Veronese (RAV) Formation (middle – late Jurassic), which crops out extensively in the Venetian Prealps. This work considers both the Red and Brown Verona varieties. RAV is a very distinctive lithostratigraphic unit in the Mesozoic deposits of the Trento Plateau, a submarine structural high characterized by very low sedimentation rates (Winterer and Bosellini 1981; Martire et al. 2006; Lukeneder 2010, 2011). Sometimes, due to its high request, the stone is also quarried from the Scaglia Rossa formation, which is less valuable. Petrographically, the Verona Stone is a nodular ammonite-bearing limestone characterized by the juxtaposition of light colored nodules and anastomosing darker layers richer in clay and crossed by dissolution seams. Grain size is fine, and the color is variable from red to yellow or white. The biogenic component is prevalent, with skeletal rests mainly of planktonic and bethic foraminifera, radiolarians, calcispheres, and thin-shelled bivalved (Korus 2005; Benchiarin 2007; Martire et al. 2006).
This type of stone, also called “Veronese marble” or “Verona Broccatello limestone”, was well known by Leonardo da Vinci, who reported that “Truovasi nelle montagne di Verona la sua pietra rossa mista di tutti i nichi convertiti in essa pietra” (In the mountains of Verona there is a red stone mixed with all the shells converted in the same type of stone). This stone has been widely used as ornamental and dimension stone since ancient times. For example, in Verona the Roman amphitheatre known as Arena, Ponte Pietra (Stone Bridge), and the Roman Theatre have been erected using this stone. In Venice, the Red Verona Stone has been used for architectural details such as capitals, columns, floor slabs, architraves, cornices and doorframes, such as the internal floor and architectural elements of the portals on the western façade of St. Mark’s Basilica, and well curbs such as in the courtyard of Ca’ d’Oro (early 15th century). In association with Istrian Stone, Red Verona Stone provided the characteristic two-toned external ornamentation of the Doge’s Palace and many other buildings in Venice. The use of architectural elements in Red Verona Stone are common in many of the major cathedrals of Northern Italy such as in those of Cremona, Parma and Bologna. The lion sculptures supporting columns at the entrance of the Basilica of Santa Maria Maggiore in Bergamo (Fig. 5) have been also realized with Red Verona Stone. In the last century, its diffusion in the global market allowed the Red Verona Stone (sometimes as Rosso Ammonitico Stone) being enumerated among the stones of interest for urban geoturism itineraries, contributing to the geodiversity in urban cultural spaces in cities from other continents (e.g. in Rio de Janeiro; Polck et al., 2020).
Asiago Stones (Pink and White varieties, RO and BI)
The typical lithofacies of the Asiago Stone pertains to the Maiolica Formation (also referred as Biancone Formation in older literature), formed in the early Cretaceous on the Trento Plateau, and stratigraphically lying on the Rosso Ammonitico Formation. It consists in pelagic thin-bedded white micritic limestone containing nodules and lenses of cherts, rich in microfossils of radiolaria, ostracods, echinoderms, sponge spiculae and foraminifera (Martire 1996; Lukeneder 2010, 2011). Stylolites are common parallel to layering. Macroscopically, some facies are similar to Red Verona, but normally color is paler and texture is less nodular. This research considers both the Pink and White Asiago Stone varieties (samples RO and BI, respectively).
The stone is more frequently used for small decorative elements in residential buildings of the Veneto Region (windowsill, doorframe, tiling), but there are examples of use for “noble purposes” like the new altar in the Basilica of Saint Anthony in Padua, where the Pink Asiago is coupled with Orsera stone, and the Asiago War Memorial (Fig. 5).
Botticino Stone (BO)
Botticino Stone (sample BO) is an ornamental stone quarried in the Botticino, Rezzato and Mazzano area, east of Brescia (northen Italy), mostly from open-pit sites. It consists in partially dolomitized stylolitic compact micritic limestones forming thick ivory, grey or cream colored beds. Lithostratigraphically, Botticino Stone belongs to the Corna Formation (early Jurassic), and comprises carbonate deposits formed in a subtidal platform environment with episodes of temporary emersion marked by up to centimetric illite-rich layers influencing cultivation operations. These limestones show a variety of textures ranging from desiccated mudstone to floatstone and wackestone containing oncolites, dasyclad and red algae, and other bioclasts which are generally hard to determine due to the intense dolomitization (Schirolli 1997; Di Battistini et al. 2005; Vernia et al. 2005; Borghi et al. 2015; Masetti et al. 2017). Quarrying activity in the area also involved limestones now formalized in the overlying “Ercinite di Rezzato” and “Corso Rosso di Botticino” stratigraphical units, representing the progressive drowning of the Corna platform, with increasingly pelagic and deep-sea facies, consisting in cream-colored and then reddish mudstones and wackestones containing peloids, crinoids, echinoids and increasing amounts of sponge spicules, with frequent chert nodules (Schirolli 2007).
The variety considers in this study is that of the Classical Botticino Marble (in commercial names limestones are commonly called marbles, although strictly incorrect geologically speaking), the most diffuse type. This stone has been used since Roman times, especially in Brescia (in antiquity Brixia), where the quarries are located. Here, Botticino stone was used in the construction of the Roman Temple of the Emperor Vespasian (Capitolium of Brixia) and the Theater (1st century), the Church of Our Ladie of Miracles (15th century), the Loggia Palace (15th century), and the New Cathedral (17th -19th century) (Clerici and Meda 2005). At the end of 19th century the use of this building stone began to spread in the whole Italy and abroad (Fig. 6): the Vittoriano monument in Rome, the Brussels-North railway station, the Victory Monument in Bolzano, the Palace of the Commercial Italian Bank in Milan, the White House in Washington, the Grand Central Terminal and the basement of the Statue of Liberty in New York, the Chamber of Commerce in Osaka, the Red Sea Hospital in Jeddah, the One International Place in the Financial District of Boston, are all realized using this stone.
Carrara marble (M)
White Carrara marble is the only metamorphic rock considered in this research. We used this stone as a reference material in order to evaluate the reliability of determining the grain-size distribution by optical microscopy (OM), by comparing with results obtained by electron backscattered diffraction (EBSD).
Carrara marble has a large number of commercial varieties. These rocks derive from original Mesozoic carbonate sequences of the Tuscan Nappe which underwent low-grade metamorphism, and now they outcrop in the Apuan Alps (Carmignani et al. 1978; Meccheri et al. 2007; Borghi et al. 2015). The Apuan marbles are the most famous Italian marbles. Although they have homogeneous chemical and mineralogical composition, different Apuan white marbles show different behavior when exposed to external environment (Cantisani et al. 2009).
White Carrara marbles have a pearly white color, fine to medium grain size; their color is homogeneous or spotted with small patches and gray veins, irregularly distributed and due to microcrystalline pyrite. Carrara marble has been extensively used since the Roman Age. Important monuments of Ancient Rome as well as Renaissance sculptures are made of this material, such as the Pantheon, Trajan’s (113 CE) and Marcus Aurelius’ Columns (180–192 CE), Michelangelo’s Pietà (1499 CE) and David (1501–1504 CE) (Fig. 6). The importance of these monuments and the availability of raw material made this stone a primary choice for relevant monuments and buildings all over the world. The Marble Arch in London (19th century), innumerable sarcophagi of nobles around Europe, the Cathedral of Manila, the Opera House in Oslo, the Finland Hall in Helsinki, the Prem Mandir Hindu Temple in Vrindavan (India), the Sheikh Zayed Grand Mosque in Abu Dhabi, Harvard Medical School in Boston and various other buildings in the USA and Canada, the fountain Bassin de Madrid in Paris are on the long list of prominent buildings which largely utilized this marble. As other stones, Carrara marble has been largely used also in the territories close to the quarry district (i.e. the Apuan district and Tuscany). For example, the Cathedrals of Massa and Carrara as well as the Ducal Palace of Massa are entirely decorated with Carrara marble cladding; in the whole Tuscany, churches are often decorated combining Carrara marble with other colored ornamental stones (e.g. the Cathedral, the Baptistery and the Giotto’s bell tower in Florence).