Sector-zoned aegirine in Sanbagawa quartz schist from the western Kii Peninsula, central Japan

Abstract

Sector-zoned aegirine was found in a specimen of Sanbagawa quartz schist from the Iimori region of the western Kii Peninsula, central Japan. The schist consists of quartz, aegirine, albite, amphibole (magnesioriebeckite to manganocummingtonite), garnet (spessartine), rhodochrosite, pyrophanite, jacobsite, and apatite. Aegirine occurs as subhedral to euhedral prismatic crystals less than 250 μm long. Three sectors in the zoning are distinguished on the basis of backscattered electron images. The [001] sector is more enriched in Ca and Mg (as CaMgSi2O6) and depleted in Al (as NaAlSi2O6) than the {110} and {100} sectors, and the {110} sector is more enriched in Fe3+ (NaFe3+Si2O6) and depleted in Al than the {100} sector. The sector-zoned aegirine is mantled by XJd-rich aegirine, and there is an outermost rim of XJd-poor aegirine. These textural relationships suggest that after rapid non-equilibrium crystallization, the XJd-rich mantle as well as the XJd-poor outermost rim formed under conditions of equilibrium with the surrounding minerals.