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 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 natrolitejadeitites 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.

Gem sparkles deep: Preface of the special issue on ‘Jadeite and jadeitite’

“…They confirm the authors' preliminary results of timing of jadeite formation at~520 Ma, as well as constraining a protolith age of~560 Ma. Goto et al (2017) propose phase relations of lowtemperature hydrothermal jadeitites based on Schreinemakers' rule. They apply these results to explain the hydrothermal precipitations of jadeitite in the Itoigawa jadeite.…”

“…The JAMS invited two review articles on Japanese jadeitites (Tsujimori and Harlow, 2017;Nishiyama et al, 2017) and requested another four review articles (Tsujimori, 2017;Miyajima, 2017;Hirajima, 2017;Ohtani et al, 2017) from experts with JAMS membership. In addition, in response to a call for papers to the Japanese geoscience community, five research articles Goto et al, 2017;Fukuyama et al, 2017;Takahashi et al, 2017;Sakamaki, 2017) were contributed from the authors and authors' groups conducting research on jadeite and jadeitite. The eleven papers in this issue cover the fields of mineralogy, petrology, geochemistry, geochronology, mineral physics, and planetary science from different perspectives.…”