R1: Characterization and description of minerals (Joint Session with The Gemmological Society of Japan)

[R1P-13] Formation process of secondary minerals in hydrothermally altered basalt from Kouragahana, Shimane Peninsula, Japan

[Presentation award entry]

*Shuichi Ito¹, Mariko Nagashima¹ (1. Yamaguchi Univ.)

Keywords:prehnite, pumpellyite, ferriprehnite, babingtonite, Kouragahana

Secondary minerals such as prehnite (pr), pumpellyite (pp), babingtonite (bb), and thomsonite (tm) occur in the hydrothermally altered basalt (pillow lava) distributed to Kouragahana, Shimane Peninsula, Japan. There are two types (A, B) of occurrence: pp and pr associated in druses within the pillow (A), and pp, bb and tm associated in interspace between the pillows (B). In A, pr crystallized after formation of pp due to the reaction of primary minerals with hydrothermal fluid. In contrast, Fe-rich pr, bb, and tm are formed by local seawater-hydrothermal fluid interactions (B).