Distribuzione dello stilpnomelano nel Paleozoico del Brianzonese ligure

Cerro, Antonio ; Gianotti, Renato ; Vanossi, Mario ; Veniale, Fernando

Atti della Accademia nazionale dei Lincei. Rendiconti della Classe di scienze fisiche, matematiche e naturali, Série 8, Tome 50 (1971) no. 3, pp. 345-355

Résumé

Over two thousand thin sections of crystalline, igneous, volcanic and sedimentary epimetamorphic rocks from the paleozoic basement of the ligurian Briançonnais have been examined. About 2% of them contain needles, flakes, and sheaf-like aggregates; their pleochroism ranges from brown to yellowish in most cases (ferri-stilpnomelane, always associated with K-feldspar bearing rocks and often growing on K-feldspar crystals), or, more rarely, from green to yellow-greenish or colorless (ferro-stilpnomelane, in rocks without any obvious K-feldspar). The first one also forms on hornblende, biotite and chloritoid; the second one on sericite, chlorite and cryptocrystalline argillitic groundmass. The stilpnomelane is nearly always set within microfractures and cleavage fissures of the host minerals, or along their borders. The needles lie across foliation, thus indicating their post-tectonic crystallization. Special chemical conditions, such as availability of Fe and K and relatively low concentration of Mg and Al, together with locally favourable conditions of mobility, often connected with cataclastic texture of the rock, seem to be overall needed for the formation of stilpnomelane. However, its geographic distribution suggests that the zoneographic position of the rock at time of development of stilpnomelane might perhaps have facilitated or prevented its segregation.