Lamellar texture and optical anomaly in andradite from the Kamaishi mine, Japan

Detailed investigations of optical anomalies and surface texture of birefringent grossular-andradite from Kamaishi mine, Japan, were made by optical polarizing microscopy, electron-probe microanalysis (EPMA) and back-scattered electron imaging from the standpoint of crystal growth. This garnet shows one-to-one correlation between surface features and the internal texture. The average chemical composition measured by EPMA near rim is Adr 78.6 Grs 21.1 Sps 0.2 and near core is Adr 57.0 Grs 42.2 Sps 0.7 . The surface features correspond to internal texture observed under crossed polarizers. The triangular sectors in rhombic blocks under cross polarizers are symmetric with [001] axis and show sector twins. Scanning electron microscopy (SEM) showed a pyramidal shape of rhombohedral growth hillocks consistent with spiral growth mechanism. The vicinal faces of blocks show extinction angles of 31–40° with respect to the crystallographic direction [001]. This andradite is optically biaxial with 2V z of about (−) 70° in both {110} sectors. The principal direction X is almost normal to (110) and coincides with the growth direction [110]. The positions of Y and Z rotate by up to 19° from the crystallographic direction [001] and [110] respectively, around X . The value of X varies slightly from sector to sector; however, a 2-fold axis is parallel to [110]. The symmetry determined optically for the garnet is monoclinic or triclinic.