山口県長門市向津具半島大津玄武岩に産する晶洞鉱物

Abstract

Occurrence, assemblages and chemical compositions of zeolite and associated amygdale minerals in Otsu alkali basalt from Mukatsuku Peninsula, Nagato, Yamaguchi, Japan, were studied to find formation process of amygdale minerals. Cavities with amygdale zeolite and associated minerals distribute in the Otsu alkali basalt near Cape Kawashiri at about 25 m level (outcrops O4, L1 and L2) and at 45 m level (outcrops O1 and O2) from the bottom of the Otsu alkali basalt. Chabazite-Na occurs in cavities of all outcrops. Phillipsite-Ca, levyne-Ca and erionite occur only in cavities of upper level outcrop O2, whereas they do not occur in the cavities of lower level O4, L1 and L2 outcrops. The erionite grows as the epitaxial crystal on the surface of levyne-Ca. Heulandite-Ca occurs only in L2 outcrop. Calcite is a common amygdale mineral. Barite and quartz occur in cavities of the lower level outcrops. Chabazite-Ca occurs rarely as inclusions in chabazite-Na in the cavity of outcrop O4. In the cavities of outcrops O2, O4 and L2, crystallization trends from the wall to center are phillipsite-Ca to chabazite-Na, chabazite-Ca to chabazite-Na, and heulandite-Ca to chabazite-Na, respectively, whereas, in the cavities of the outcrops O1 and L1, only chabazite-Na occurs. Zeolites formed in early stage tend to rich in divalent exchangeable cations, such as Ca and Mg, and Si. In contrast, zeolites crystallized in the later stage tend to be low in Si and high in monovalent cations. The zeolites and assemblages of amygdale minerals at lower 25 m level outcrops suggest higher crystallization temperature than those at higher outcrops.