Pelagosite

A variety of Aragonite

A variety of pisolitic aragonite

It consists of hard, shiny, dark coatings on rocky substratum, produced by the action of
cyanobacteria where the supralittoral rock is frequently wetted by sea aerosol.

Originally described from Palagruža Island (Pelagosa Island), Splitsko-dalmatinska Co., Croatia, as a hard, shiny, tar-looking mineral encrusting the dolomite cliffs immediately above tide line by Carlo Marchesetti (C. de Marchesetti) in 1876. One year later, it was named “pelagosite” by Michele Stossich (1877).
In 1878 the French chemist Stanislas Cloëz published a chemical analysis of the blackish varnish-like mineral encrusting the dolomite cliff of Cap Ferrat, Alpes-Maritimes, France, which was previously observed at this locality and at the nearby Balzi Rossi/Baoussé-Roussé, Liguria, Italy, by A. de Chambrun de Rosemont (1874); it turned out to be calcium carbonate with small amount of organic matter. The presence of a similar substance on littoral rocks was also observed in Corsica by Descloizeaux, in Algeria and at Réunion Island by Vélain (Cloëz, 1878). Gustav Tschermak (1878) in his short note on pelagosite reported the data published by Cloëz and provided the description of a large specimen from Pelagosa Island, sent him by Ludwig Karl Moser, a naturalist of Trieste [to be noted that in Palache et al. (1951) the Tschermak’s work has been erroneously attributed to Moser]. Other reports on such a mineral from various other coastal localities were published in the years that followed.
The first reports on pelagosite were followed by 50 years of debate about the nature and origin of this mineral until 1926, when Ettore Onorato published a comprehensive paper on the morphological, mineralogical, physical, chemical, and biological characteristics of pelagosite from the Tremiti islands, Puglia, Italy, which are located 75 km SW of Palagruža. Pelagosite turned out to be pisolitic aragonite, which is produced by cyanobacteria where the littoral rock is frequently wetted by sea aerosol.
In a recent interdisciplinary study on pelagosite sampled at Palagruža (Montanari et al., 2007) X-ray diffraction confirmed that pelagosite is pure aragonite and optical microscopy confirmed the pisolitic structure of the mineral, which appears as a rythmic dark-light microlaminar arrangement similar to tree concentric rings. Thicker laminae contain submicron-size inclusions of organic matter, including fragments of cyanobacterial cells. By assuming that these laminae
represent yearly accretions (just like the rings in a tree trunk), Montanari et al. (2007) determined that their arrangement bears cyclicity with frequencies comparable to those of the periodic climate changes controlled by El Niño and the North Atlantic Oscillation (i.e., dry-wet/cold-warm meteorologic conditions alternating approximately every 3, 8, and 12 years).
In a recent microbiological study of pelagosite samples from Palagruža, Macalady et al. (2008) confirmed Onorato’s conclusion of a biogenic origin for this aragonite and, through DNA analysis of the organic fraction, they determined the presence of cells of different species of cyanobacteria.