Mineralogy of Miocene phosphatic nodules in SE Sicily (Italy)

Abstract

This paper describes the geochemistry and petrography of three phosphatic nodular deposits in Ragusa Province, south-east Sicily (Italy). Phosphate nodules of late Burdigalian age are dispersed in a soft, friable packstone matrix within the Irminio Member of the Ragusa Formation. Mineralogical analyses revealed large amounts of calcite (64 to 89 wt.%) and smaller quantities of carbonate-fluorapatite (CFA). P2O5 content was less than 18%. Microtextural observations demonstrated that phosphate precipitation occurred in the microenvironments in the sediment in confined spaces (i.e. cavities, perforations) and not in the shells of microorganisms. The crystals were all of a similar size (~1 mm), presented imperfections, and the{0001}CFA and the{1010}CFA forms were clearly apparent. These hexagonal prisms showed reduced growth along their c axis. Our data suggest that bacteria were involved in the precipitation of CFA.