NERO DI COLONNATA (“BLACK CARRARA MARBLE”): PETROGRAPHIC AND STABLE ISOTOPE COMPARISONS WITH FAMOUS WHITE CARRARA MARBLE AND ONE CHINESE IMPORT (Invited Presentation)

Carrara marble was formed by Tertiary metamorphism of Triassic to Early Jurassic limestones, and has been quarried from the Colonnata, Miseglia and Torano valleys in the Apuan Alps, near the town of Carrara in Tuscany, Italy since Roman times. This marble is used extensively for architectural and decorative purposes, and its finest white variety is highly prized as statuary stone. Michelangelo’s “David,” for example, was made of Carrara statuario. Consequently, White Carrara marble is one of the best known and most famous stones in the world.

Less is known, however, about associated, but rarer Nero di Colonnata (“Black Carrara marble”), quarried in the Colonnata Valley and used locally as a popular building and decorative stone in part due to its visual contrast with White Carrara. In the 20^th century Nero was also used for some sculptures (e.g., “Cavallino” by Arturo Dazzi, at the Fine Arts Academy in Carrara). Petrographically it is recrystallized peloidal packstone to grainstone with a very different texture from mosaics of calcite crystals found in typical marble.

Stable isotope study included a comparison with white, including statuario, and gray (bardiglio) marbles from 7 quarries from all three valleys in the Carrara region. Their carbon isotope values cluster around 2 ‰ VPDB, and most of their oxygen isotope values range between -2 and -8 ‰ VPDB, with some samples reaching -15 ‰. Some samples of Nero di Colonnata (δ¹⁸O = -2.7 to -6.9 ‰ and δ¹³C around 2 ‰), overlap with the Carrara marble, likely indicating similarities in their protoliths, but other samples have δ¹³C values as high as 5.2 ‰, forming a distinct compositional field that reflects differences in levels of recrystallization between these black limestones vs true marbles.

Comparison also was made with one example of Chinese “black marble,” obtained from a local studio in Carrara and presumably imported as a less expensive and more abundant option than the local black stones. Isotopic composition (δ¹³C = 3.4 ‰; δ¹⁸O = -6.4 ‰) of this fine grained fossiliferous (foraminifera and echinoderms) wackestone to packstone of unknown age and location, is completely different from the local Carrara examples. This allows for easy petrographic and geochemical distinction between these stones and demonstrates the usefulness of these techniques for their characterization.