THE NATURE OF FERROMANGANESE CONCRETIONS FROM THE EASTERN GULF-OF-FINLAND, BALTIC SEA

As a result of favorable environmental conditions, ferromanganese conc retions in the eastern part of the Gulf of Finland achieve the highest levels of abundance and variability found in shallow-water environmen ts. The abundance of the Mn-rich spheroidal concretions concentrated i n the most prospective fields in the Russian sector of the Gulf of Fin land reaches 50 kg/m(2), and the total weight of these concretions is calculated to be about 6 million tonnes. This weight is considered to be relatively constant with time because of the cyclic processes of gr owth and dissolution of the concretions. The spheroidal concretions co ntain on average 33.8% MnO2, 20.2% Fe2O3, and 2.7% P2O5. The concentra tions of Cu, Ni, Zn, Mo, etc., in the concretions are near the regiona l background levels. The contents of these elements have increased dur ing the last few decades as a result of the anthropogenic input of the metals into the Baltic. The growth rate of the spheroidal concretions is calculated to be about 0.03-0.08 mm/yr. The technology of extracti on of Mn from the Mn-rich spheroidal concretions has been developed by the Hypronikel Institute in St. Petersburg. This will permit the comm ercial recovery of these concretions in the future.