PRIMA SEGNALAZIONE DI TODOROKITEE BIRNESSITE IN UN FILONCELLO PRESSO IL MONTE CIRCEO, LAZIO MERIDIONALE

Abstract

A small vein containing Mn minerals in the Cretaceous limestone of Mt. Circeo, Southern Latium, was analyzed optically, by X-ray diffraction, and by an energy dispersion electron microprobe. The Mn mineralization found in the pores of the dark limestone which makes up the vein con­stitutes a cryptocrystalline mass which cannot be determined optically. However, the saccharoidal texture of the limestone near the contact and the presence in it of Ca­bearing dolomite idioblasts indicate recrystallization at relatively high temperatures. X-ray data disclose the presence of the higher peaks of todorokite ( d = 9. 7 4A and 4.84A) and birnessite (d=7.23A and 3.61A) in addition to the calcite peaks. Finally, the microprobe analyses give variable values for each spot, the Mn and Ca contents being lower and higher respectively than those reported in the lite­rature for both todorokite and birnessite. Relatively high Ba and Cu contents suggest a hydrotermal origin, probably re­lated to the volcanic activity of Southern Latium.