orderdsorder structure and the internal texture of stilbite
- Mzumro Axrzuru, ND
Optical textures in birefringent grossular-andradite garnets were studied from the standpoint of crystal growth. Grossular from Eden Hill, vermont, shows one-to-one correlation between growth surface features consisting of well-defined pyramidal hillocks and the internal texture. Grossular from Fujikaramari, Japan, consists of growth sectors with fine lamellae which apparpntly correspond to grofih steps. Andradite from Kamihogi, Japan, is composed of many triangular sectors which have the character of so-called lineage structure. The three specimens show sector twins whose planes are parallel, normal, or inclined to the  aiis in a thin section parallel to the (110) growth plane. The texture can be interpreted by a general mechanism of formation of order-disorder growth sectors suggested by Akizuki (1981a). That is, various degrees of Fe3+/Al ordering in the octahedral cation sites were produced on the side faces of surface steps during growth.