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