Weathered Biotite from Ono-Niimachi, Fukushima Prefecture

Abstract

Weathered biotite in Cretaceous hornblende-biotite granodiorite from Ono-Niimachi, Fukushima Prefecture, Northeast Japan was examined by X-ray diffraction and EPMA. Specimens 1 and 2 taken from the upper part of thick weathering profile consist of interstratification of biotite and Al-rich dioctahedral vermiculite. EPMA shows that these two specimens are composed of stacking of thin layers which each have different proportion of the components. Kaolinite is found along the cleavage plane of original biotite or partly replacing it. These two specimens are characterized by Al enrichment during lateritic weathering.
Specimens 3, 4 and 5 were taken from the lower part of weathering profile where fresh granodiorite remains as corestones. Specimen 5 from a corestone is fresh biotite with minor amount of K-vermiculite. Specimen 4 from weakly weathered granodiorite consists of biotite and K-vermiculite. Specimen 3 from saprolite is interstratification of biotite, K-vermiculite and trioctahedral vermiculite and also shows layered structure.