Sulfur, Aragonite

Cozzo Disi Mine, Casteltermini, Agrigento Province, Sicily, Italy

6,8×5,5×3,2 cm.

What happened 5/6 millions years ago ?

The Messinian salinity crisis was a geological event during which the Mediterranean Sea went into a cycle of nearly complete desiccation (drying-up) throughout the latter part of the Messinian age of the Miocene epoch, from 6 to 5 million years ago. It ended with the Zanclean flood, when the Atlantic reclaimed the basin. During this dramatic event, selective sedimentation of dissolved salts began in order of time and chemical properties : first were Carbonatic deposit (CaCO₃), second were Chalky deposit (CaSO₄) and for last Sodium/Potassium Chloride (NaCl – KCl) deposits, all presents and well represented in Sicily. Between the two deposits of Carbonate and Gypsum, a series of anaerobic bacteria or/and the presence of hydrocarbons have fixed the Sulphur, as element, following this sequence of chemical reactions :

a)  2CaSO4 + 2CH4 (or anaerob.bact.) – -> Ca (HS)2 + CaCO3 + CO2 + 3H2O

b)  Ca (HS)2 + CO2 + H2O – – > CaCO3 + 2 H2S

c)  3H2S + CaSO4 – – > 4S + Ca (OH)2 + H2O

d)  the real consequence was : Gypsum – – > Calcite + Sulfur.

This specimen is not only amazing but also a perfect witness of these dramatic geological events.