Phase relation in the NaAlSiO₄–SiO₂–H₂O system for the hydrothermal precipitation of jadeite, albite, natrolite, and analcime in jadeitite of the Itoigawa–Omi area, Japan

摘要

Jadeitite from the Itoigawa–Omi area in the Hida–Gaien belt is hydrothermal in origin, occurring as tectonic blocks in a serpentinite mélange. Most jadeitite shows bimineralic mineralogy essentially composed of jadeite and albite without quartz. It sometimes has veins and cavities filled with zeolite–bearing assemblages of natrolite–jadeite and analcime–jadeite. In veins and cavities, jadeite often shows euhedral shapes in natrolite and analcime matrices and accompanies Sr–Ti–Zr–bearing new minerals such as itoigawaite, rengeite, and matsubaraite. Phase relation in the NaAlSiO_(4)–SiO_(2)–H_(2)O system has been analyzed based on the Schreinemakers’ rule to explain the hydrothermal origin of these jadeitites and the euhedral form of jadeite. The albite– and natrolite–jadeitites were precipitated from a hydrothermal fluid in the pressure–temperature field surrounded by the following four reactions: 1) albite = jadeite + quartz, 2) natrolite = nepheline + jadeite + 2 water, 3) natrolite + albite = 3 jadeite + 2 water, and 4) analcime = jadeite + water. Jadeite and analcite seem to be in equilibrium because of their euhedral shapes, but never crystallize from a fluid phase in the NaAlSiO_(4)–SiO_(2)–H_(2)O system. To explain the presence of euhedral jadeite in an analcime matrix, we propose two possible interpretations: 1) that the introduction of evolved, multicomponent, hydrothermal fluid becomes the fluid–analcime–jadeite triangle and appears in a pseudo–ternary system and 2) that hydrothermal fluid was present in an amount insufficient to form a water–saturated, analcime–bearing assemblage.