Abstract
Sector-zoned manganoan aegirine-augites from the Kamisugai mine, Ehime Prefecture, have been investigated by an optical microscopy and an electron probe microanalyser. Sector-zoned crystal elongates along c-axis and is composed of the {100} and {010} sectors. The cores of the {100} sectors are rich in johannsenite and diopside molecules, while the rims become poorer in johannsenite molecule. The cores of the {010} sectores are rich in acmite and diopside molecules, and the rims become richer in acmite molecule. These zonal structures of each sector are discussed in reference to the crystallization temperature and oxygen fugacity in particular. These factors control the solubilities of johannsenite and diopside molecules to acmite molecule. The sectoral fractionation of the molecules during crystallization can be explained by the difference of the lattice energies of each molecule and that of the magnitude of misfit between Si-O tetrahedron and M-O polyhedron in the structure.
