• journal_title：European Journal of Mineralogy
• Contributor：Manzoor A. Badar ; Shanawer Niaz ; Safdar Hussain ; Mizuhiko Akizuki
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2013-01-01
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2013/0025-2234
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：25
• issue：1
• firstpage：53

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_78.6Grs_21.1Sps_0.2 and near core is Adr_57.0Grs_42.2Sps_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 [1̅10] 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.