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

[R1-06] Occurrence and genesis of Ca-rich nepheline and associated minerals in the Kajishiyama basanite, Tsuyama, Okayama Prefecture, SW Japan

[Presentation award entry]

*Keiya Yoneoka¹, Maki Hamada¹, Syoji Arai¹ (1. Kanazawa Uni. Nat.)

Keywords:Nepheline, Basanite, Apatite, Calcic

Nepheline is a feldspathoid mineral, and the ideal formula is shown as K₂Na₆Al₈Si₈O₃₂ (Z = 1). We found Ca-rich nepheline containing Ca up to 0.467 apfu from Kajishiyama basanite. Coexisted apatite was crystallized before crystallization of nepheline, and there is a negative correlation between P content of whole rock composition and Ca content of nepheline. Therefore, it is possible that in low P basanite magma, the amount of crystalized apatite is few, residual melt becomes high Ca, and Ca-rich nepheline is formed.