اسپسارتین گارنت با ساختار ماکل نوری متقارن پیدا شده در یک پیه مونتیت کوارتز شیست در ناحیه آسمی گاوا در نوار دگرگونی سانباگوا در شیکوکوی مرکزی ژاپن

رOpticully sector· twinned ga rn et was found in a piemontite quartz sch ist in Ihe Asemi.gawa area from the Sanbagnwa metamorphic helt. The Sanbagawa metamorphic hell wa.~ formed during a regional Cretaceous intermediate high pressure type of metamorphism which is present through Southwest Japan. The garnet contains abundant inclusions of piemontite, quartz, hematite, braunite, talc and sodie amphibole. It is almost homogenous spessartine garnet (Xsps = 0.85). Piemontite occurs botb in matrix and as inclusion in garnet and albite and it usually shows distinct zonation in which core of the grain contains lower Fe3+ and higher Mn3+ . and AJ--contents than those of the rim. The garnet is .. nisotropic and composed of several parts or sectors visible in crossed polars. They show ditTerent patterns such as octahedral, hexahedral and tetrahedral in which optical orientation and ext inction position are roughly symmet rica l. A rhombododecahedral model ga rnet was used to interpret the geometry of the optically sector structure and show thitl sectors with symmetrical Miller's indices have similar optical properties. Textural relation and sys tematic partitioning of Mn3 + , Fe3+ and Al among garnet, picmontite and bntunite indicate that gal net formalion is due to the picmontitc und braunite reaction