Optical characters, chemical composition and formation mechanism of colorless sodic and sodic-calcic amphiboles in piemontite-quartz schists from high P-low T Sanbagawa metamorphic belt in central Shikoku, Japan

Abstract

Colorless sodic and sodic-calcic amphiboles occur in piemontite-quartz schists in the Asemi-gawa (garnet zone) and Besshi (albite-biotite zone) areas of high P-low T Sanbagawa metamorphic belt in central Shikoku, Japan. Colorless sodic amphiboles in the Asemi-gawa area are glaucophane while colorless sodic-calcic amphiboles from the Besshi area are barroisite and magnesiokatophorite. Colorless sodic and sodic-calcic amphiboles are richer in Mg and poorer in Fe in comparison with the colored sodic and sodic-calcic amphiboles from the epidote-quartz schists of the same metamorphic grade. Optical characters of the glaucophanes from the Asemi-gawa area are identified by their colorless and lack of pleochroism in the α, β and γ directions; nγ ┴ Z = 13, 2V(α) = 20 and β = Y. Optical characters of magnesiokatophorite from the Besshi area are: :colorless and without pleochroism in the α, β and γ directions; nγ ┴ Z =17, 2V(α) = 25 and β =Y. Chemical composition of the coexisting minerals with colorless sodic and sodic-calcic amphiboles have shown a very high oxygen fugacity condition in which iron exists only as Fe3+ and occurs mostly in hematite.  Therefore, such condition was responsible for formation of sodic and sodic-calcic amphiboles which are very rich in Mg.

Keywords

References

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  • Receive Date: 02 July 2025
  • Revise Date: 04 February 2026
  • Accept Date: 02 July 2025

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