JpGU-AGU Joint Meeting 2017

講演情報

[JJ] ポスター発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG73] [JJ] 岩石・鉱物・資源

2017年5月22日(月) 10:45 〜 12:15 ポスター会場 (国際展示場 7ホール)

コンビーナ:齊藤 哲(愛媛大学大学院理工学研究科)、門馬 綱一(独立行政法人国立科学博物館)、野崎 達生(海洋研究開発機構海底資源研究開発センター)、土谷 信高(岩手大学教育学部地学教室)

[SCG73-P02] Babingtonite with epitaxial hedenbergite whiskers

*永嶌 真理子1浜根 大輔2 (1.山口大学大学院創成科学研究科、2.東京大学物性研究所)

キーワード:バビントン石、TEM、エピタキシャル成長

Babingtonite, Ca2Fe2+Fe3+[Si5O14(OH)], is a hydrous pyroxenoid group mineral consisting of chains built from five twisted SiO4 tetrahedra. It typically occurs in hydrothermally altered zeolite-dominant veins and cavities in basic igneous rocks, and in skarn deposits. Clinopyroxene with diopside-hedenbergite component, Ca(Mg, Fe2+)Si2O6, also occurs in skarns. Although it does commonly not coexist with babingtonite, clinopyroxene whiskers rarely grow on babingtonite. However, their relationship has not been confirmed directly because of the difficulty of simultaneous determination of atomic arrangement in both minerals. In this study, the platy babingtonite (dark green in color) overgrown by clinopyroxene whiskers from two different localities were investigated: (1) Arvigo, Grisons, Switzerland and (2) Kreimbach/Kaulbach, Kaiserslautern, Germany, using transmission electron microscope (TEM) to understand their relationship and formation process. In the both specimens, the boundary between babingtonite and hedenbergite was sharply defined. The relationship of babingtonite (Bab) and hedenbergite (Hd) was determined as Bab[100]//Hd[112] in the Arvigo and Bab[-100]//Hd[1-12] in the Kreimbach/Kaulbach specimen. Diffractions derived from Bab(031) and Hd(02-1) in the Arvigo, and Bab(031) and Hd(021) in the Kreimbach/Kaulbach sample were observed at the identical position. Their topological relationship can be explained by their crystal structures. The reciprocity between babingtonite and hedenbergite is governed by the direction of the SiO4-tetrahedral chain, and the configuration of octahedra and meets the requirement to associate both structures. A five-periodic chain of babingtonite harmoniously transitions into a two-periodic chain of hedenbergite. The octahedral cluster consisting four octahedra of babingtonite continuously transforms to the octahedral chain in hedenbergite. Therefore, hedenbergite is apparently an epitaxial phase grown on {010} of the platy babingtonite basal. The occurrences of babingtonite with epitaxial hedenbergite imply that they are essentially formed within the babingtonite stability field. At the initial stage, babingtonite formed under high fO2 and water saturated condition. Subsequently, slight fluctuation, in particular in fO2, possibly promoted the nucleation of hedenbergite crystals on {010} of pre-existing babingtonite, leading to the epitaxial relation. Hedenbergite whiskers tend to be getting shorter from the centre to the edge of the babingtonite plate in both Arvigo and Kreimbach/Kaulbach specimens, suggesting that the crystal growth of babingtonite continued during hedenbergite formation.