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

Abstract

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 NaAlSiO4-SiO2-H2O 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 NaAlSiO4-SiO2-H2O 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.