(Umbers in pillow lava from the Mineoka tectonic belt, Boso peninsula.

Abstract

Umber-like rock was found in close association with Palaeogene pillow lavas in the Mineoka mountains, Boso peninsula, Japan. The material is dark brown and amorphous in appearance and highly fragile. XRD and DTA studies showed that the rock contained poorly-crystalline goethite with minor pyrolusite. Electron probe microanalysis gave SiO2 9.83, TiO2 0.21, Al2O3 3.55, total Fe as Fe2O3 46.80, total Mn as MnO2 20.89, MgO 0.93, CaO 0.88, Na2O 0.35, K2O 0.48, H2O+ or - 13.60, = 97.52.M.Ha.